readfile

How do you read n bytes from a file and put them into a vector using iterators?

半腔热情 — Fri, 19 Feb 2021 07:30:23 +0000

How do you read n bytes from a file and put them into a vector<uint8_t> using iterators? 半腔热情 2021-02-19 15:30:23

问题

Based on this this question:

How to read a binary file into a vector of unsigned chars

In the answer they have:

std::vector<BYTE> readFile(const char* filename)
{
    // open the file:
    std::basic_ifstream<BYTE> file(filename, std::ios::binary);

    // read the data:
    return std::vector<BYTE>((std::istreambuf_iterator<BYTE>(file)),
                              std::istreambuf_iterator<BYTE>());
}

Which reads the entire file into the vector.

What I want to do is read (for example) 100 bytes at a time in the vector, then do stuff, and then read the next 100 bytes into the vector (clear the vector between). I don't see how to specify how much of the file to read (i.e. how to setup the iterators). Is that even possible?

I am trying to avoid having to write my own code loop to copy each byte at a time.

回答1:

You can use ifstream::read for that.

std::vector<BYTE> v(100);
while ( file.read(reinterpret_cast<char*>(v.data()), 100) )
{
   // Find out how many characters were actually read.
   auto count = file.gcount();

   // Use v up to count BTYEs.
}

回答2:

You could write a function to:

void readFile( const std::string &fileName, size_t chunk, std::function<void(const std::vector<BYTE>&)> proc )
{
    std::ifstream f( fileName );
    std::vector<BYTE> v(chunk);
    while( f.read( v.data(), v.size() ) ) {
        v.resize( f.gcount() );
        proc( v );
        v.resize( chunk );
    }
}

then usage is simple:

void process( const std::vector<BYTE> &v ) { ... }

readFile( "foobar", 100, process ); // call process for every 100 bytes of data

or you can use lambda etc for callback.

回答3:

Or you can write your own function for that:

template<typename Data>
std::istreambuf_iterator<Data> readChunk(std::istreambuf_iterator<Data>& curr, std::vector<Data>& vec, size_t chunk = 100) {
    for (int i = 0; curr != std::istreambuf_iterator<Data>() && i < chunk; ++i, ++curr) {
        vec.emplace_back(*curr);
    }
    return curr;
}

and use it as:

std::ifstream file("test.cpp");
std::vector<BYTE> v;
std::istreambuf_iterator<BYTE> curr(file);
readChunk<BYTE>(curr, v);

And you can call this function again.

来源：https://stackoverflow.com/questions/50491833/how-do-you-read-n-bytes-from-a-file-and-put-them-into-a-vectoruint8-t-using-it

标签

c++

vector

readfile

ifstream

How do you read n bytes from a file and put them into a vector using iterators?

喜你入骨 — Fri, 19 Feb 2021 07:25:07 +0000

How do you read n bytes from a file and put them into a vector<uint8_t> using iterators? 喜你入骨 2021-02-19 15:25:07

问题

Based on this this question:

How to read a binary file into a vector of unsigned chars

In the answer they have:

std::vector<BYTE> readFile(const char* filename)
{
    // open the file:
    std::basic_ifstream<BYTE> file(filename, std::ios::binary);

    // read the data:
    return std::vector<BYTE>((std::istreambuf_iterator<BYTE>(file)),
                              std::istreambuf_iterator<BYTE>());
}

Which reads the entire file into the vector.

I am trying to avoid having to write my own code loop to copy each byte at a time.

回答1:

You can use ifstream::read for that.

std::vector<BYTE> v(100);
while ( file.read(reinterpret_cast<char*>(v.data()), 100) )
{
   // Find out how many characters were actually read.
   auto count = file.gcount();

   // Use v up to count BTYEs.
}

回答2:

You could write a function to:

void readFile( const std::string &fileName, size_t chunk, std::function<void(const std::vector<BYTE>&)> proc )
{
    std::ifstream f( fileName );
    std::vector<BYTE> v(chunk);
    while( f.read( v.data(), v.size() ) ) {
        v.resize( f.gcount() );
        proc( v );
        v.resize( chunk );
    }
}

then usage is simple:

void process( const std::vector<BYTE> &v ) { ... }

readFile( "foobar", 100, process ); // call process for every 100 bytes of data

or you can use lambda etc for callback.

回答3:

Or you can write your own function for that:

template<typename Data>
std::istreambuf_iterator<Data> readChunk(std::istreambuf_iterator<Data>& curr, std::vector<Data>& vec, size_t chunk = 100) {
    for (int i = 0; curr != std::istreambuf_iterator<Data>() && i < chunk; ++i, ++curr) {
        vec.emplace_back(*curr);
    }
    return curr;
}

and use it as:

std::ifstream file("test.cpp");
std::vector<BYTE> v;
std::istreambuf_iterator<BYTE> curr(file);
readChunk<BYTE>(curr, v);

And you can call this function again.

来源：https://stackoverflow.com/questions/50491833/how-do-you-read-n-bytes-from-a-file-and-put-them-into-a-vectoruint8-t-using-it

标签

How do you read n bytes from a file and put them into a vector using iterators?

百般思念 — Fri, 19 Feb 2021 07:24:39 +0000

How do you read n bytes from a file and put them into a vector<uint8_t> using iterators? 百般思念 2021-02-19 15:24:39

问题

Based on this this question:

How to read a binary file into a vector of unsigned chars

In the answer they have:

std::vector<BYTE> readFile(const char* filename)
{
    // open the file:
    std::basic_ifstream<BYTE> file(filename, std::ios::binary);

    // read the data:
    return std::vector<BYTE>((std::istreambuf_iterator<BYTE>(file)),
                              std::istreambuf_iterator<BYTE>());
}

Which reads the entire file into the vector.

I am trying to avoid having to write my own code loop to copy each byte at a time.

回答1:

You can use ifstream::read for that.

std::vector<BYTE> v(100);
while ( file.read(reinterpret_cast<char*>(v.data()), 100) )
{
   // Find out how many characters were actually read.
   auto count = file.gcount();

   // Use v up to count BTYEs.
}

回答2:

You could write a function to:

void readFile( const std::string &fileName, size_t chunk, std::function<void(const std::vector<BYTE>&)> proc )
{
    std::ifstream f( fileName );
    std::vector<BYTE> v(chunk);
    while( f.read( v.data(), v.size() ) ) {
        v.resize( f.gcount() );
        proc( v );
        v.resize( chunk );
    }
}

then usage is simple:

void process( const std::vector<BYTE> &v ) { ... }

readFile( "foobar", 100, process ); // call process for every 100 bytes of data

or you can use lambda etc for callback.

回答3:

Or you can write your own function for that:

template<typename Data>
std::istreambuf_iterator<Data> readChunk(std::istreambuf_iterator<Data>& curr, std::vector<Data>& vec, size_t chunk = 100) {
    for (int i = 0; curr != std::istreambuf_iterator<Data>() && i < chunk; ++i, ++curr) {
        vec.emplace_back(*curr);
    }
    return curr;
}

and use it as:

std::ifstream file("test.cpp");
std::vector<BYTE> v;
std::istreambuf_iterator<BYTE> curr(file);
readChunk<BYTE>(curr, v);

And you can call this function again.

来源：https://stackoverflow.com/questions/50491833/how-do-you-read-n-bytes-from-a-file-and-put-them-into-a-vectoruint8-t-using-it

标签

Fortran read mixed text and numbers

不打扰是莪最后的温柔 — Thu, 18 Feb 2021 19:44:04 +0000

Fortran read mixed text and numbers 不打扰是莪最后的温柔 2021-02-19 03:44:04

问题

I am using Fortran 90 to read a file that contains data in the following format

number# 125 var1= 2 var2= 1 var3: 4
        .
        .
        .
        .
number# 234 var1= 3 var2= 5 var3: 1

I tried the following command and works fine

read (2,*)  tempstr , my_param(1), tempstr , my_param(2), tempstr , my_param(3)

Problem is when the numbers become larger and there is no space between string and number, i.e. the data looks as following:

number# 125 var1= 2 var2=124 var3: 4

I tried

     read (2,512)  my_param(1), my_param(2), my_param(3)

512 format('number#', i, 'var1=', i, 'var2=', i, 'var3:', i)

It reads all number as zero

I can't switch to some other language. The data set is huge, so I can't pre-process it. Also, the delimiters are not the same every time. Can someone please help with the problem?

Thanks in advance

回答1:

While I still stand with my original answer, particularly because the input data is already so close to what a namelist file would look like, let's assume that you really can't make any preprocessing of the data beforehand.

The next best thing is to read in the whole line into a character(len=<enough>) variable, then extract the values out of that with String Manipulation. Something like this:

program mixed2
    implicit none
    integer :: num, val1, val2, val3
    character(len=50) :: line
    integer :: io_stat

    open(unit=100, file='data.dat', action='READ', status='OLD')
    do
        read(100, '(A)', iostat=io_stat) line
        if (io_stat /= 0) exit
        call get_values(line, num, val1, val2, val3)
        print *, num, val1, val2, val3
    end do
    close(100)

    contains

        subroutine get_values(line, n, v1, v2, v3)
            implicit none
            character(len=*), intent(in) :: line
            integer, intent(out) :: n, v1, v2, v3
            integer :: idx

            ! Search for "number#"
            idx = index(line, 'number#') + len('number#')

            ! Get the integer after that word
            read(line(idx:idx+3), '(I4)') n

            idx = index(line, 'var1') + len('var1=')
            read(line(idx:idx+3), '(I4)') v1

            idx = index(line, 'var2') + len('var3=')
            read(line(idx:idx+3), '(I4)') v2

            idx = index(line, 'var3') + len('var3:')
            read(line(idx:idx+3), '(I4)') v3
        end subroutine get_values
end program mixed2

Please note that I have not included any error/sanity checking. I'll leave that up to you.

回答2:

First up, 720 thousand lines is not too much for pre-processing. Tools like sed and awk work mostly on a line-by-line basis, so they scale really well.

What I have actually done was to convert the data in such a way that I could use namelists:

$ cat preprocess.sed

# Add commas between values
# Space followed by letter -> insert comma
s/ \([[:alpha:]]\)/ , \1/g

# "number" is a key word in Fortran, so replace it with num
s/number/num/g

# Replace all possible data delimitors with the equals character
s/[#:]/=/g

# add the '&mydata' namelist descriptor to the beginning
s/^/\&mydata /1

# add the namelist closing "/" character to the end of the line:
s,$,/,1

$ sed -f preprocess.sed < data.dat > data.nml

Check that the data was correctly preprocessed:

$ tail -3 data.dat
number#1997 var1=114 var2=130 var3:127
number#1998 var1=164 var2=192 var3: 86
number#1999 var1=101 var2= 48 var3:120

$ tail -3 data.nml
&mydata num=1997 , var1=114 , var2=130 , var3=127/
&mydata num=1998 , var1=164 , var2=192 , var3= 86/
&mydata num=1999 , var1=101 , var2= 48 , var3=120/

Then you can read it with this fortran program:

program read_mixed
    implicit none
    integer :: num, var1, var2, var3
    integer :: io_stat
    namelist /mydata/ num, var1, var2, var3

    open(unit=100, file='data.nml', status='old', action='read')
    do
        read(100, nml=mydata, iostat=io_stat)
        if (io_stat /= 0) exit
        print *, num, var1, var2, var3
    end do
    close(100)
end program read_mixed

来源：https://stackoverflow.com/questions/38133083/fortran-read-mixed-text-and-numbers

标签

fortran

readfile

fortran90

How to read file in Java Script line by line from hardcoded path and name of the file?

女生的网名这么多〃 — Mon, 08 Feb 2021 04:09:42 +0000

How to read file in Java Script line by line from hardcoded path and name of the file? 女生的网名这么多〃 2021-02-08 12:09:42

问题

I would like to read file in Java Script. The best it would be to read line by line, but there is also possibility to read the whole file at once. Generally on the web there is quite a lot of implementations, but I would like to read a file in very simple way by entering, hardcoded path and file name in the Java Script code, not but any buttons or something like this. The pseudo code below:

<!DOCTYPE html>
<html>
<body>
    <script type="text/javascript">
        var file = FileReader("home/test.txt");//hardcoded path and name of the file
        var listOfLines = [];
        var line = file.readLine();
        while(line != eof) {
            listOfLines.push(file.readLine());
            file.readLine();
        }
    </script>
</body>
</html>

Is there such possibility to do something like this. Thank you.

回答1:

That would be a pretty big security hole, if your browser could simply read arbityry files from your filesystem. Think, every banner on the web could read configuation files of your OS, and stuff like that.

also check this question/answer: How to set a value to a file input in HTML?
slightly different question, same problem.

But if the client gives your app the file you should process, that's something different.

//use fetch() to get the file content
function fetchFile(file){
  const {name, lastModified, size, type} = file;
  return fetch(URL.createObjectURL(file))
    .then(response => response.text())
    .then(text => ({name, lastModified, size, type, text}));
}

//use a FileReader to get the content
function readFile(file){
  const {name, lastModified, size, type} = file;
  return new Promise((resolve, reject) => {
    const reader = new FileReader();
    reader.onload = () => resolve(reader.result);
    reader.onerror = reject;
    reader.readAsText(file);
  })
  .then(text => ({name, lastModified, size, type, text}));
}


let input = document.querySelector('input');
input.onchange = function(){
  const promises = [...input.files].map(fetchFile);  //fetch
  //const promises = [...input.files].map(readFile);  //FileReader

  Promise.all(promises)
    .then(files => {
      console.log(files);
    })
}

<input type="file" />

This is just a quick snippet. You'd have to check wether there are further security measures/obstacles due to the fact that a web page is accessing local files; but since this snippet is working, I'm confident that you're good.

回答2:

FileReader is working well and it is well supported, see:

https://caniuse.com/#search=FileReader

even IE has a partial support.

But it can read ONLY file returned from the user. You cannot read any file with an hardcoded path from the developer. Of course that is for security reasons, the javascript engine in the browser runs in a sandbox.

PS Also, to read big csv files from javascript, I suggest this library, that I used many times with success:

https://www.papaparse.com/

reference:

https://www.w3.org/TR/FileAPI/

https://www.w3.org/TR/FileAPI/#dfn-filereader

https://developer.mozilla.org/it/docs/Web/API/FileReader

来源：https://stackoverflow.com/questions/48994679/how-to-read-file-in-java-script-line-by-line-from-hardcoded-path-and-name-of-the

标签

Implementing a ReadFile with a timeout inducing too much CPU ussage

一个人想着一个人 — Fri, 29 Jan 2021 03:21:15 +0000

Implementing a ReadFile with a timeout inducing too much CPU ussage 一个人想着一个人 2021-01-29 11:21:15

问题

I use kernel32.dll ReadFile to read the HID output of a connected device. I wanted to create a timeout because this call is a blocking method and if it hangs for any reason the main loop in my application also hangs.

    [DllImport("kernel32.dll", SetLastError = true)]
    static internal extern bool ReadFile(IntPtr hFile, [Out] byte[] lpBuffer, uint nNumberOfBytesToRead, out uint lpNumberOfBytesRead, IntPtr lpOverlapped);

    public static HidLibrary.HidDeviceData.ReadStatus FastReadWithTimeout(this HidLibrary.HidDevice device, byte[] inputBuffer, int timeout)
    {
        return FastReadWithTimeoutWorker(device, inputBuffer, timeout).GetAwaiter().GetResult();
    }

    private static async Task<HidLibrary.HidDeviceData.ReadStatus> FastReadWithTimeoutWorker(HidLibrary.HidDevice device, byte[] inputBuffer, int timeout)
    {
        var task = Task.Run(async () => {
            try
            {
                uint bytesRead;
                if (NativeMethods.ReadFile(device.Handle, inputBuffer, (uint)inputBuffer.Length, out bytesRead, IntPtr.Zero))
                {
                    return HidLibrary.HidDeviceData.ReadStatus.Success;
                }
                else
                {
                    return HidLibrary.HidDeviceData.ReadStatus.NoDataRead;
                }
            }
            catch (Exception)
            {
                return HidLibrary.HidDeviceData.ReadStatus.ReadError;
            }
        });
        if (await Task.WhenAny(task, Task.Delay(timeout)) == task)
        {
            return task.Result;
        }
        else
        {
            device.CancelIO();
            return HidLibrary.HidDeviceData.ReadStatus.WaitTimedOut;
        }
    }

The issue is my current method of doing this causing more CPU overhead than I can allow. Is there something wrong with the way I am doing this or is there a better way to implement a timeout in a native method?

I also just discovered SetCommTimeouts(), but am having trouble finding a working example of it, documentation is spotty.

来源：https://stackoverflow.com/questions/53362883/implementing-a-readfile-with-a-timeout-inducing-too-much-cpu-ussage

标签

readfile

Python read .txt File -> list

ぃ、小莉子 — Thu, 28 Jan 2021 06:38:03 +0000

Python read .txt File -> list ぃ、小莉子 2021-01-28 14:38:03

问题

I have a .txt File and I want to get the values in a list. The format of the txt file should be:

value0,timestamp0
value1,timestamp1
...
...
...

In the end I want to get a list with

[[value0,timestamp0],[value1,timestamp1],.....]

I know it's easy to get these values by

direction = []
for line in open(filename):
    direction,t = line.strip().split(',')
    direction = float(direction)
    t = long(t)
    direction.append([direction,t])
return  direction

But I have a big problem: When creating the data I forgot to insert a "\n" in each row.

Thats why I have this format:

value0, timestamp0value1,timestamp1value2,timestamp2value3.....

Every timestamp has exactly 13 characters.

Is there a way to get these data in a list as I want it? Would be very much work get the data again.

Thanks Max

回答1:

I coded a quickie using your example, and not using 13 but len("timestamp") so you can adapt

instr = "value,timestampvalue2,timestampvalue3,timestampvalue4,timestamp"

previous_i = 0
for i,c in enumerate(instr):
    if c==",":
        next_i = i+len("timestamp")+1
        print(instr[previous_i:next_i])
        previous_i = next_i

output is descrambled:

value,timestamp
value2,timestamp
value3,timestamp
value4,timestamp

回答2:

import re
input = "value0,0123456789012value1,0123456789012value2,0123456789012value3"

for (line, value, timestamp) in re.findall("(([^,]+),(.{13}))", input):
    print value, timestamp

回答3:

You will have to strip the last , but you can insert a comma after every 13 chars following a comma:

import re
s = "-0.1351197,1466615025472-0.25672746,1466615025501-0.3661744,1466615025531-0.4646‌7665,1466615025561-0.5533287,1466615025591-0.63311553,1466615025621-0.7049236,146‌6615025652-0.7695509,1466615025681-1.7158673,1466615025711-1.6896278,146661502574‌1-1.65375,1466615025772-1.6092329,1466615025801"

print(re.sub("(?<=,)(.{13})",r"\1"+",", s))

Which will give you:

-0.1351197,1466615025472,-0.25672746,1466615025501,-0.3661744,1466615025531,-0.4646‌7665,1466615025561,-0.5533287,1466615025591,-0.63311553,1466615025621,-0.7049236,146‌6615025652-0.7695509,1466615025681,-1.7158673,1466615025711,-1.6896278,146661502574‌1-1.65375,1466615025772,-1.6092329,1466615025801,

回答4:

I think you could do something like this:

direction = []
for line in open(filename):
    list = line.split(',')
    v = list[0]
    for s in list[1:]:
        t = s[:13]
        direction.append([float(v), long(t)])
        v = s[13:]

If you're using python 3.X, then the long function no longer exists -- use int.

来源：https://stackoverflow.com/questions/37976716/python-read-txt-file-list

标签

Reading the dynamic bitset written data from file cannot read the correct data

白昼怎懂夜的黑 — Wed, 27 Jan 2021 12:57:35 +0000

Reading the dynamic bitset written data from file cannot read the correct data 白昼怎懂夜的黑 2021-01-27 20:57:35

问题

So I have a vector which has three numbers. 65, 66, and 67. I am converting these numbers from int to binary and appending them in a string. the string becomes 100000110000101000011 (65, 66, 67 respectively). I am writing this data into a file through dynamic_bitset library. I have BitOperations class which does the reading and writing into file work. When I read the data from file instead of giving the above bits it gives me these 001100010100001000001 bits.

Here is my BitOperations class:

#include <iostream>
#include <boost/dynamic_bitset.hpp>
#include <fstream>
#include <streambuf>
#include "Utility.h"
using namespace std;
using namespace boost;

template <typename T>
class BitOperations {
private:
    T data;
    int size;
    dynamic_bitset<unsigned char> Bits;
    string fName;
    int bitSize;

public:
    BitOperations(dynamic_bitset<unsigned char> b){
        Bits = b;
        size = b.size();
    }

    BitOperations(dynamic_bitset<unsigned char> b, string fName){
        Bits = b;
        this->fName = fName;
        size = b.size();
    }

    BitOperations(T data, string fName, int bitSize){
        this->data = data;
        this->fName = fName;
        this->bitSize = bitSize;
    }

    BitOperations(int bitSize, string fName){
        this->bitSize = bitSize;
        this->fName = fName;
    }

    void writeToFile(){
        if (data != ""){
            vector<int> bitTemp = extractIntegersFromBin(data);
            for (int i = 0; i < bitTemp.size(); i++){
                Bits.push_back(bitTemp[i]);
            }
        }
        ofstream output(fName, ios::binary| ios::app);
        ostream_iterator<char> osit(output);
        to_block_range(Bits, osit);
        cout << "File Successfully modified" << endl;
    }

    dynamic_bitset<unsigned char> readFromFile(){
        ifstream input(fName);
        stringstream strStream;
        strStream << input.rdbuf();
        T str = strStream.str();

        dynamic_bitset<unsigned char> b;
        for (int i = 0; i < str.length(); i++){
            for (int j = 0; j < bitSize; ++j){
                bool isSet = str[i] & (1 << j);
                b.push_back(isSet);
            }
        }
        return b;
    }
};

And here is the code which calls theses operations:

#include <iostream>
// #include <string.h>
#include <boost/dynamic_bitset.hpp>
#include "Utility/BitOps.h"

int main(){
    vector<int> v;
    v.push_back(65);
    v.push_back(66);
    v.push_back(67);

    stringstream ss;
    string st;
    for (int i = 0; i < v.size(); i++){
        ss = toBinary(v[i]);
        st += ss.str().c_str();
        cout << i << " )" << st << endl;
    }
    // reverse(st.begin(), st.end());
    cout << "Original: " << st << endl;

    BitOperations<string> b(st, "bits2.bin", 7);
    b.writeToFile();
    BitOperations<string>c(7, "bits2.bin");
    boost::dynamic_bitset<unsigned char> bits;
    bits = c.readFromFile();
    string s;
    
    // for (int i = 0; i < 16; i++){
        to_string(bits, s);
        // reverse(s.begin(), s.end());
    // }
    cout << "Decompressed: " << s << endl;
}

What am I doing wrong which results in incorrect behaviour?

EDIT: Here is the extractIntegersFromBin(string s) function.

vector<int> extractIntegersFromBin(string s){

    char tmp;
    vector<int> nums;

    for (int i = 0; s[i]; i++ ){
        nums.push_back(s[i] - '0');
    }

    return nums;
}

Edit 2: Here is the code for toBinary:

stringstream toBinary(int n){
    vector<int> bin, bin2;
    int i = 0;
    while (n > 0){
        bin.push_back(n % 2);
        n /= 2;
        i++;
    }

    // for (int j = i-1; j >= 0; j--){
    //     bin2.push_back(bin[j]);
    // }
    reverse(bin.begin(), bin.end());
    stringstream s;
    for (int i = 0; i < bin.size(); i++){
        s << bin[i];
    }

    return s;
}

回答1:

You are facing two different issues:

The boost function to_block_range will pad the output to the internal block size, by appending zeros at the end. In your case, the internal block size is sizeof(unsigned char)*8 == 8. So if the bit sequence you write to the file in writeToFile is not a multiple of 8, additional 0s will be written to make for a multiple of 8. So if you read the bit sequence back in with readFromFile, you have to find some way to remove the padding bits again.
There is no standard way for how to represent a bit sequence (reference). Depending on the scenario, it might be more convenient to represent the bits left-to-right or right-to-left (or some completely different order). For this reason, when you use different code pieces to print the same bit sequence and you want these code pieces to print the same result, you have to make sure that these code pieces agree on how to represent the bit sequence. If one piece of code prints left-to-right and the other right-to-left, you will get different results.

Let's discuss each issue individually:

Regarding issue 1

I understand that you want to define your own block size with the bitSize variable, on top of the internal block size of boost::dynamic_bitset. For example, in your main method, you construct BitOperations<string> c(7, "bits2.bin");. I understand that to mean that you expect the bit seqence stored in the file to have a length that is some multiple of 7.

If this understanding is correct, you can remove the padding bits that have been inserted by to_block_range by reading the file size and then rounding it down to the nearest multiple of your block size. Though you should note that you currently do not enforce this contract in the BitOperation constructor or in writeToFile (i.e. by ensuring that the data size is a multiple of 7).

In your readFromFile method, first note that the inner loop incorrectly takes the blockSize into account. So if blockSize is 7, this incorrectly only considers the first 7 bits of each block. Whereas the blocks that were written by to_block_range use the full 8 bit of each 1-byte block, since boost::dynamic_bitset does not know anything about your 7-bit block size. So this makes you miss some bits.

Here is one example for how to fix your code:

    size_t bitCount = (str.length()*8) / bitSize * bitSize;
    size_t bitsPerByte = 8;

    for (int i = 0; i < bitCount; i++) {
      size_t index = (i / bitsPerByte);
      size_t offset = (i % bitsPerByte);

      bool isSet = (str[index] & ( 1 << offset));
      b.push_back(isSet);
    }

This example first calculates how many bits should be read in total, by rounding down the file size to the nearest multiple of your block size. It then iterates over the full bytes in the input (i.e. the internal blocks that were written by boost::dynamic_bitset), until the targeted number of bits have been read. The remaining padding bits are discarded.

An alternative method would be to use boost::from_block_range. This allows you to get rid of some boiler plate code (i.e. reading the input into some string buffer):

  dynamic_bitset<unsigned char> readFromFile() {
    ifstream input{fName};

    // Get file size
    input.seekg(0, ios_base::end);
    ssize_t fileSize{input.tellg()};

    // TODO Handle error: fileSize < 0

    // Reset to beginning of file
    input.clear();
    input.seekg(0);

    // Create bitset with desired size
    size_t bitsPerByte = 8;
    size_t bitCount = (fileSize * bitsPerByte) / bitSize * bitSize;
    dynamic_bitset<unsigned char> b{bitCount};

    // TODO Handle error: fileSize != b.num_blocks() * b.bits_per_block / bitsPerByte

    // Read file into bitset
    std::istream_iterator<char> iter{input};
    boost::from_block_range(iter, {}, b);

    return b;
  }

Regarding issue 2

Once you have solved issue 1, the boost::dynamic_bitset that is written to the file by writeToFile will be the same as the one read by readFromFile. If you print both with the same method, the output will match. However, if you use different methods for printing, and these methods do not agree on the order in which to print the bits, you will get different results.

For example, in the output of your program you can now see that the "Original:" output is the same as "Decompressed:", except in reverse order:

Original: 100000110000101000011
...
Decompressed: 110000101000011000001

Again, this does not mean that readFromFile is working incorrectly, only that you are using different ways of printing the bit sequences.

The output for Original: is obtained by directly printing the 0/1 input string in main from left to right. In writeToFile, this string is then decomposed in the same order with extractIntegersFromBin and each bit is passed to the push_back method of boost::dynamic_bitset. The push_back method appends to the end of the bit sequence, meaning it will interpret each bit you pass as more significant than the previous (reference):

Effects: Increases the size of the bitset by one, and sets the value of the new most-significant bit to value.

Therefore, your input string is interpreted such that the first bit in the input string is the least significant bit (i.e. the "first" bit of the sequence), and the last bit of the input string is the most significant bit (i.e. the "last" bit of the sequence).

Whereas you construct the output for "Decompressed:" with to_string. From the documentation of this method, we can see that the least-significant bit of the bit sequence will be the last bit of the output string (reference):

Effects: Copies a representation of b into the string s. A character in the string is '1' if the corresponding bit is set, and '0' if it is not. Character position i in the string corresponds to bit position b.size() - 1 - i.

So the problem is simply that to_string (by design) prints in opposite order compared to the order in which you print the input string manually. So to fix this, you have to reverse one of these, i.e. by printing the input string by iterating over the string in reverse order, or by reversing the output of to_string.

来源：https://stackoverflow.com/questions/65547187/reading-the-dynamic-bitset-written-data-from-file-cannot-read-the-correct-data

标签

How to read local files using HTML 5 FileReader? [duplicate]

被刻印的时光ゝ — Wed, 20 Jan 2021 20:43:46 +0000

How to read local files using HTML 5 FileReader? [duplicate] 被刻印的时光ゝ 2021-01-21 04:43:46

问题

Objective

I am making an application, and I need to read a local file using JavaScript and HTML 5, without any <input> tags or user interaction whatsoever.

What I tried

On my research, I found two tutorials that are heavily cited in SO:

https://www.html5rocks.com/en/tutorials/file/dndfiles/
http://blog.teamtreehouse.com/reading-files-using-the-html5-filereader-api

However, there is a problem. Both of this tutorials require user interaction via the input tag, which is a life killer since I want to read the contents of the file automatically into a string.

Code

So far I managed to get the following code:

let readFile = function(path) {
    let reader = new FileReader();

    reader.onload = function(e) {
        var text = reader.result;
        console.log(text);
    };

    // Read in the image file as a data URL.
    reader.readAsText(MissingFileHandle);
};

But as you can see, I am missing an important step, I am missing MissingFileHandle. My idea would be to pass a path to this method, and so the method would read the file locally as text and print it into the console, but I am unable to achieve this.

Question

Given a relative path, how can I read the contents of a file using HTML 5 without using <input> tags?

回答1:

The HTML5 fileReader facility does allow you to process local files, but these MUST be selected by the user, you cannot go rooting about the users disk looking for files.

Is it possible to load a file with JS/HTML5 FileReader on non served page?

How to open a local disk file with Javascript?

How to set a value to a file input in HTML?

Javascript read file without using input

These links help you to find answer.

回答2:

This Can do a trick.

HTML

        <h1>Text File Reader</h1>
        <div>
            Select a text file: 
            <input type="file" id="fileInput">
        </div>
        <pre id="fileDisplayArea"><pre>

    </div>

window.onload = function() {
        var fileInput = document.getElementById('fileInput');
        var fileDisplayArea = document.getElementById('fileDisplayArea');

        fileInput.addEventListener('change', function(e) {
            var file = fileInput.files[0];
            var textType = /text.*/;

            if (file.type.match(textType)) {
                var reader = new FileReader();

                reader.onload = function(e) {
                    fileDisplayArea.innerText = reader.result;
                }

                reader.readAsText(file);    
            } else {
                fileDisplayArea.innerText = "File not supported!"
            }
        });
}

来源：https://stackoverflow.com/questions/41849745/how-to-read-local-files-using-html-5-filereader

标签

javascript

html

readfile

How to read local files using HTML 5 FileReader? [duplicate]

喜你入骨 — Wed, 20 Jan 2021 20:42:26 +0000

How to read local files using HTML 5 FileReader? [duplicate] 喜你入骨 2021-01-21 04:42:26

问题

Objective

I am making an application, and I need to read a local file using JavaScript and HTML 5, without any <input> tags or user interaction whatsoever.

What I tried

On my research, I found two tutorials that are heavily cited in SO:

https://www.html5rocks.com/en/tutorials/file/dndfiles/
http://blog.teamtreehouse.com/reading-files-using-the-html5-filereader-api

However, there is a problem. Both of this tutorials require user interaction via the input tag, which is a life killer since I want to read the contents of the file automatically into a string.

Code

So far I managed to get the following code:

let readFile = function(path) {
    let reader = new FileReader();

    reader.onload = function(e) {
        var text = reader.result;
        console.log(text);
    };

    // Read in the image file as a data URL.
    reader.readAsText(MissingFileHandle);
};

Question

Given a relative path, how can I read the contents of a file using HTML 5 without using <input> tags?

回答1:

The HTML5 fileReader facility does allow you to process local files, but these MUST be selected by the user, you cannot go rooting about the users disk looking for files.

Is it possible to load a file with JS/HTML5 FileReader on non served page?

How to open a local disk file with Javascript?

How to set a value to a file input in HTML?

Javascript read file without using input

These links help you to find answer.

回答2:

This Can do a trick.

HTML

        <h1>Text File Reader</h1>
        <div>
            Select a text file: 
            <input type="file" id="fileInput">
        </div>
        <pre id="fileDisplayArea"><pre>

    </div>

window.onload = function() {
        var fileInput = document.getElementById('fileInput');
        var fileDisplayArea = document.getElementById('fileDisplayArea');

        fileInput.addEventListener('change', function(e) {
            var file = fileInput.files[0];
            var textType = /text.*/;

            if (file.type.match(textType)) {
                var reader = new FileReader();

                reader.onload = function(e) {
                    fileDisplayArea.innerText = reader.result;
                }

                reader.readAsText(file);    
            } else {
                fileDisplayArea.innerText = "File not supported!"
            }
        });
}

来源：https://stackoverflow.com/questions/41849745/how-to-read-local-files-using-html-5-filereader

标签

javascript

html

readfile