使用C中的递归打印整数数组的逆序

Question

I am not able to understand how I am getting a garbage value in output . Can someone explain me the flow . Thank you .

#include <stdio.h>

void rev(int *ptr){

  if(*ptr) {
    rev(ptr+1);
    printf("%d\n",*ptr);

  }
}

int main(){      

  int arr[]={4,2,3,1,5};  
  rev(arr);  
  return 0;  
}

output -

32764   
5
1
3 
2  
4     

Answer 1

Your relying on if (*ptr) being 0 to block the recursion is not going to end well. Arrays in C are not terminated with zero automatically. Formally, the behaviour of your program is undefined as you will attempt to read memory past the end of the array.

The normal way of dealing with this is to either pass the array length, or use a special value to signal the end of the array.

Answer 2

Your base condition is not correct. You recurse until a 0 is found in the array but it doesn't have one.

You have perhaps (incorrectly) assumed that there's a 0 after the end of the array. But this is not true in C. Your program has undefined behaviour as per C standard.

If you need to print the entire array, then pass the size as well:

void rev(int *ptr, size_t size){    
    if(size) {
        rev(ptr+1, --size);
        printf("%d\n",*ptr);
    }
}

int main(){

    int arr[]={4,2,3,1,5};
    rev(arr, sizeof arr/sizeof arr[0]);
    return 0;
}