如何循环并打印 c 中的结构数组？

Question

I am writing a function separate from main that reads in an array of structs and prints each array element's struct elements. My problem however is the condition statement for looping though this array to print each element. sizeof(a)/sizeof(a[]) just doesnt work in the function because it is equal to zero within the prototype. Is there something i am missing? I've tried pointer arithmetic and it does not work either.

void printNames( person a[]){

    int i;

    for(i = 0;i<sizeof(a)/sizeof(a[0]);i++)
        printf("Name: %s | Age: %d\n", a[i].name, a[i].age);

}

int main(int argc, char *argv[]){

    if (argc == 1 || argc%2 == 0){
        printf("Invalid arguments.\n Usage: %s name1 age1 name2 age2 ...", argv[0]);
        return 0;
    }

    printf("You have entered %d person(s) into the program.\n", argc/2);
    person people[argc/2];

    int i;

    for (i = 0; i<argc/2;i++){
        strcpy(people[i].name, argv[i*2+1]);
        people[i].age = atoi(argv[i*2+2]);

    if(people[i].age <= 0){
        printf("Invalid age <= 0. Try again.");
        return 0;
    }


}

Answer 1

a becomes a pointer when passed as parameter to a function, so it cannot figure out the size of the array just from having this pointer, you have to inform the function about its size:

void printNames(person *a, size_t num)
{
    int i;
    for (i = 0; i < num; i++)
        printf("Name: %s | Age: %d\n", a[i].name, a[i].age);
}

Then call printNames from main like this:

printNames(people, sizeof(people)/sizeof(*people));

Answer 2

In C, array parameters are decayed into pointers. So, the expression sizeof(a)/sizeof(a[0]) becomes sizeof(int *)/sizeof(int) which results in 1 (assuming the size of int and int * is 4) and hence the for loop within the printNames() is executed only once regardless of the size of the array.

Therefore, sizeof shouldn't be used to obtain the number of elements in these cases. Instead, we can pass an additional parameter for array size to printNames() .

Consider the following C program:

#include<stdio.h>
void func(int array[])  
{
  int i;   
  int arraySize = sizeof(array)/sizeof(array[0]); /* arraySize <-- 1<-- 4/4 */
  for (i = 0; i < arraySize; i++) 
  {  
    array[i] = i; 
  }
}

int main()
{
  int i;  
  int array[4] = {0, 0 ,0, 0};
  func(array);

  for(i = 0; i < sizeof(arr)/sizeof(arr[0]); i++)
    printf(" %d " ,arr[i]);

  getchar();  
  return 0;
}    

Output: 0 0 0 0 on a Intel Architecture-32 machine.

So the corrected program is:

#include<stdio.h>
void func(int array[], size_t arraySize)  
{
  int i;   
  for (i = 0; i < arraySize; i++) 
  {  
    array[i] = i;  
  }
}

int main()
{
  int i;  
  int array[4] = {0, 0 ,0, 0};
  func(array, 4);

  for(i = 0; i < sizeof(array)/sizeof(array[0]); i++)
    printf(" %d ", array[i]);

  getchar();  
  return 0;
}