fill_n（）不允许更改变量值

Question

I am using fill_n() function to initialize my array values, but then I cannot change the values of the array, it is always the initial value.

Can someone explain me why is that?

#include<iostream>
#include<ctime>


int main(){

    //Matrix 
    int m[3][3] = {{1,0,0},{0,1,0},{0,0,1}};
    int v[3] = {5,6,7}; 
    int result[3];// = {0,0,0};
    int n = 0, i, j;

    std::fill_n(result,sizeof(result),0);

    clock_t time = clock();

    while(n<1000){
        n++;
        for(i=0;i<3;i++){
            for(j=0;j<3;j++){
                result[j] = result[j]+(v[i]*m[j][i]); //changing the values here.
            }
        }
    }

    time = clock() - time;

    for(i=0;i<3;i++){
    std::cout<<result[i]<<"\n";
    }

    std::cout<<"Execution time: "<<((float)time)/CLOCKS_PER_SEC<<"\n";

    return 0;
}

The output of the code is always 0.

Answer 1

By std::fill_n(result,sizeof(result),0); you are asking to fill the first sizeof(result) elements by 0.

However, only 3 elements are available in result . Thus you wrote out-of-bound and led to undefined behavior. You should write std::fill_n(result, sizeof(result) / sizeof(result[0]), 0); instead.

Note that sizeof(result) returns size of result in bytes , not in elements .

Answer 2

sizeof(result) returns the number of bytes held by the variable result . Since result is an array of 3 int s, its size in memory is 3 * sizeof(int) , which (unless sizeof(int) is 1 ) much greater than 3 . Thus you are writing pass array bounds and incurring undefined behavior.

The actual way to calculate the size would be to divide the entire size of result by its element type. That is, sizeof(result) / sizeof(int) .

If you want to initialize each element to 0 , a simpler way to do it would be to value-initialize the array:

int result[3]{}; // or = {} (pre C++11)

Answer 3

Number of elements (or size) of an array of type type would be sizeof( array ) / sizeof( type ). Second argument required for fill_n is the number of elements to fill a container (in your case, an array of type int) and not the size of an array in bytes. Naturally, sizeof( array ) returns sizeof( type ) times number of elements.