Why is the value of the pointer pointing at the 0th index of an array different from the actual 0th index of the array in C?

Question

Python programmer here. I know just the basics about C but I was given the task of taking n entries in an array then creating a function that takes an array pointer as its argument, and then accessing the elements of the array that the pointer is pointing at.

A simplified version of my initial code was as follows:

#include<stdio.h>

void func(int *pArr, int size){
    for (int j = 0; j < size; j++){
        printf("%d ",*(pArr+j));
    }
}

int main(){
    int n;
    printf("Enter the number of elements you want in your list: ");
    scanf("%d", &n);
    int a[n];
    for (int i = 1; i <= n; i++){
        printf("Enter element no.%d: ", i);
        scanf("%d", &a[i]);
    }
    func(&a[0], n);
    return 0;
}

Here my thought process was, since pArr is pointing at the 0th index of a, *(pArr+j) when j = 0 should just return *pArr which should just be the 0th index of a.

But to my surprise, the following is the output I got for a test run where a[5] = {1, 3, 9, 8, 2}

863014944 1 3 8 9

The program prints out a garbage value for *(pArr+j) when j = 0 but works fine after that. So I modified the code by initializing "j" with a value of 1 instead of 0 and changing the "<" to "<=" and surprisingly the program did run perfectly fine after those changes.

This is what puzzles me.

If I'm declaring a pointer *pArr pointing at the 0th index of an array a, i.e., *pArr = &a[0]; then shouldn't printing *(pArr+1) return the 1st index? Why is it that my program returns the 0th index instead? Why is *(pArr+0) not equal to a[0] when pArr is pointing at a[0]?

Answer 1

func is correct, but you didn't populate a correctly.

Think of array indexes as offsets, which is to say how far they are from the start the array.

• The index/offset of the first element of an array is 0.
• The index/offset of the second element of an array is 1.
• ⋮
• The index/offset of the last element of an array with n elements is n-1.

So this is what you are building:

int a[5]
  0   1   2   3   4
+---+---+---+---+---+
| ? | 1 | 3 | 9 | 8 | 2
+---+---+---+---+---+

• You never assign a value to a[0].
• You write a value beyond the end of the array. (Undefined behaviour!)
• Every value is in the wrong element (box).

Replace

for (int i = 1; i <= n; i++)  // 1 (inclusive) to n (inclusive)

with

for (int i = 0; i < n; i++)   // 0 (inclusive) to n (exclusive)