vector

问题 Why vector[n] = val doesn't give segmentation fault or changes the vector data, right before reserving an empty vector. Check this example: #include <iostream> #include <vector> int main() { std::vector<int> temp; temp.reserve(8); temp[0] = 1; temp[3] = 3; //why no attribution??? temp[7] = 1; temp[8] = 3; //why no segmentation fault??? std::cout << temp.size(); for(auto&a: temp){ //because the attribution didn't work, no loop needed std::cout << a; } return 0; } Also, why the operator []

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,

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,

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,

问题 I'm trying to complete the activity at the bottom of this page, where I need to print the index of each element as well as the value. I'm starting from the code use std::fmt; // Import the `fmt` module. // Define a structure named `List` containing a `Vec`. struct List(Vec<i32>); impl fmt::Display for List { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { // Extract the value using tuple indexing // and create a reference to `vec`. let vec = &self.0; write!(f, "[")?; // Iterate over

问题 I'm trying to complete the activity at the bottom of this page, where I need to print the index of each element as well as the value. I'm starting from the code use std::fmt; // Import the `fmt` module. // Define a structure named `List` containing a `Vec`. struct List(Vec<i32>); impl fmt::Display for List { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { // Extract the value using tuple indexing // and create a reference to `vec`. let vec = &self.0; write!(f, "[")?; // Iterate over

问题 I have a dataframe in which one column contains numeric vectors. I want to filter rows based on a condition involving that column. This is a simplified example. df <- data.frame(id = LETTERS[1:3], name=c("Alice", "Bob", "Carol")) mylist=list(c(1,2,3), c(4,5), c(1,3,4)) df$numvecs <- mylist df # id name numvecs # 1 A Alice 1, 2, 3 # 2 B Bob 4, 5 # 3 C Carol 1, 3, 4 I can use something like mapply e.g. mapply(function(x,y) x=="B" & 4 %in% y, df$id, df$numvecs) which correctly returns TRUE for

问题 I need to erase all elements from a vector which fulfill a certain criteria. My first approach would be to loop through the vector and call vector::erase on all elements which fulfill the criteria. As far as I understand, vector::erase has a bad performance for this use case, because it removes the item from the underlying array, and moves the rest of the vector forward by one element (or more if you erase a range of elements). When you remove multiple elements, the rear elements will be

问题 It's been a while since I last visited arrays (I've been working with vectors recently) and I need to convert an 2D vector back into a 2D array because of a library I am using accepts the paramaters of type double array where the accessors of this array is foo[i][j] for example. Here is my code: double** setupHMM(vector<vector<double> > &vals, int N, int M) { double** temp; temp = new double[N][M]; for(unsigned i=0; (i < N); i++) { for(unsigned j=0; (j < M); j++) { temp[i][j] = vals[i][j]; }

问题 It's been a while since I last visited arrays (I've been working with vectors recently) and I need to convert an 2D vector back into a 2D array because of a library I am using accepts the paramaters of type double array where the accessors of this array is foo[i][j] for example. Here is my code: double** setupHMM(vector<vector<double> > &vals, int N, int M) { double** temp; temp = new double[N][M]; for(unsigned i=0; (i < N); i++) { for(unsigned j=0; (j < M); j++) { temp[i][j] = vals[i][j]; }