Selecting only the first few characters in a string C++

问题

I want to select the first 8 characters of a string using C++. Right now I create a temporary string which is 8 characters long, and fill it with the first 8 characters of another string.

However, if the other string is not 8 characters long, I am left with unwanted whitespace.

string message = "        ";

const char * word = holder.c_str();

for(int i = 0; i<message.length(); i++)
    message[i] = word[i];

If word is "123456789abc", this code works correctly and message contains "12345678".

However, if word is shorter, something like "1234", message ends up being "1234 "

How can I select either the first eight characters of a string, or the entire string if it is shorter than 8 characters?

回答1:

Just use std::string::substr:

std::string str = "123456789abc";
std::string first_eight = str.substr(0, 8);

回答2:

Just call resize on the string.

回答3:

If I have understood correctly you then just write

std::string message = holder.substr( 0, 8 );

Jf you need to grab characters from a character array then you can write for example

const char *s = "Some string";

std::string message( s, std::min<size_t>( 8, std::strlen( s ) );

回答4:

Or you could use this:

#include <climits>
cin.ignore(numeric_limits<streamsize>::max(), '\n');

If the max is 8 it'll stop there. But you would have to set

const char * word = holder.c_str();

to 8. I believe that you could do that by writing

 const int SIZE = 9;
 char * word = holder.c_str();

Let me know if this works.

If they hit space at any point it would only read up to the space.

回答5:

char* messageBefore = "12345678asdfg"
int length = strlen(messageBefore);
char* messageAfter = new char[length];

for(int index = 0; index < length; index++)
{
    char beforeLetter = messageBefore[index];
    // 48 is the char code for 0 and 
    if(beforeLetter >= 48 && beforeLetter <= 57)
    {
        messageAfter[index] = beforeLetter;
    }
    else
    {
        messageAfter[index] = ' ';
    }
}

This will create a character array of the proper size and transfer over every numeric character (0-9) and replace non-numerics with spaces. This sounds like what you're looking for.

Given what other people have interpreted based on your question, you can easily modify the above approach to give you a resulting string that only contains the numeric portion.

Something like:

int length = strlen(messageBefore);
int numericLength = 0;
while(numericLength < length &&
      messageBefore[numericLength] >= 48 &&
      messageBefore[numericLength] <= 57)
{
    numericLength++;
}

Then use numericLength in the previous logic in place of length and you'll get the first bunch of numeric characters.

Hope this helps!

来源：https://stackoverflow.com/questions/34097048/selecting-only-the-first-few-characters-in-a-string-c

标签

c++

string

c-strings