struct array initialization in C

问题:

Here is part of my code. I would like to initialize only the arraylist[0] as arraylist[0].x = 0 and arraylist[0].y = 0. I do not need to initialize the rest of the struct array. How can I do it? Thank you.

#include <stdio.h>
struct example {
    int x;
    int y;
};
struct example arraylist[40];

int main(int argc, char *argv[]){
    printf("%d\n %d\n", arraylist[0].x, arraylist[0].y);
    return 0;
}

回答1:

You can initialise any particular element of the struct array.

For example:

struct example arraylist[40] = { [0]={0,0}}; //sets 0th element of struct

struct example arraylist[40] = { [5]={0,0}}; //sets 6th element of struct

This is called Designated Initializers which used to be a GNU extension before C99 adapted it and is also supported in standard C since C99.

回答2:

Since you are talking about a variable in file scope, here, you don't have to do anything, since such variables are always initialized by 0 if you don't provide an explicit initializer.

回答3:

In C, once you initialize part of a struct / array, you initialize the rest of it with 0.

You should have no problem with that, as you should not access uninitialized variables in first place, and their value is not defined.

回答4:

In C all static and extern variables are initialized to 0 unless explicitly initialized otherwise.