Weird behavior with C array

June 24, 2022

I have two arrays A and B both with the same elements { 1, 2, 3, 4 } but after doing A[B[i]] = A[i] + 1 array B is getting populated with different number but in reality it should be unchanged.

#include <stdio.h>

void arrayPrint(int *arr, int size) {
    for (int i = 0; i < size; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");
}

int main() {
    int A[4] = { 1, 2, 3, 4 };
    int B[4] = { 1, 2, 3, 4 };

    int n = 3;

    printf("Before \n");

    printf("Array A \n");
    arrayPrint(A, 4);

    printf("Array B \n");
    arrayPrint(B, 4);

    for (int i = 0; i <= n; i++) {
        if (A[i] == B[i]) {
            A[B[i]] = A[i] + 1;
        }
    }

    printf("\nAfter \n");

    printf("Array A \n");
    arrayPrint(A, 4);

    printf("Array B \n");
    arrayPrint(B, 4);

    return 0;
}

Output is:

Before
Array A
1 2 3 4
Array B
1 2 3 4

After
Array A
1 2 3 4
Array B
5 2 3 4

but it should be:

Before
Array A
1 2 3 4
Array B
1 2 3 4

After
Array A
1 2 3 4
Array B
1 2 3 4

>Solution :

For B[i] equal to 4 the expression A[B[i]] access memory beyond the array A because the valid range of indices for the array A is [0, 3].

So it seems the compiler placed the array B at once after the array A and the first element of the array B was changed in this statement

A[B[i]] = A[i] + 1;

for i equal to 3.

Instead of your for loop

for (int i = 0; i <= n; i++) {
    if (A[i] == B[i]) {
        A[B[i]] = A[i] + 1;
    }
}

you could write for example

for (int i = 0; i <= n; i++) {
    if ( i != 3 && A[i] == B[i]) {
        A[B[i]] = A[i] + 1;
    }
}

Or more precisely

const size_t N = sizeof( A ) / sizeof( *A );

for (int i = 0; i <= n; i++) {
    if ( i != N - 1 && A[i] == B[i]) {
        A[B[i]] = A[i] + 1;
    }
}