How to use scanf() correctly when reading input into an array using pointers?

Question

For some reason, reading input using scanf() won't work this way:

int main() {
    int arr1_size = 0;

    // Reading array size
    scanf("%d", &arr1_size);

    int* arr1 = (int*) malloc(sizeof(int) * arr1_size);

    // Reading and printing array values
    for(int i = 0; i < arr1_size; i++) {
        scanf("%d", arr1);
        arr1++;
    }
    for(int i = 0; i < arr1_size; i++) {
        printf("%d", arr1[i]);
    }

    return 0;
}

What could be the issue?

You're losing the starting address of arr1 here: scanf("%d", arr1); arr1++;. I think more correctly, scanf is working as expected, printf is not. — Fiddling Bits
– Fiddling Bits, Commented Jun 1, 2020 at 17:44
Use scanf("%d", arr1 + i); to preserve the pointer from malloc. Then printf will work correctly. — Weather Vane
– Weather Vane, Commented Jun 1, 2020 at 17:46
Before issuing malloc it would be better to go on only if(arr1_size > 0) — Roberto Caboni
– Roberto Caboni, Commented Jun 1, 2020 at 17:50

pifor · Accepted Answer · 2020-06-01 17:59:36Z

1

Here is a slightly improved version of your program:

#include <stdio.h>
#include <stdlib.h>


int main() {
    int arr1_size = 0;

    printf("Hello,\nPlease enter the size of the first array:");
    scanf("%d", &arr1_size);

    int *arr1 = (int*) malloc(sizeof(int) * arr1_size);

    printf("\nPlease enter arr1 of size %d:\n", arr1_size);
    for(int i = 0; i < arr1_size; i++) {
        scanf("%d", arr1 + i);
    }
    printf("\nArray contents:\n");    
    for(int i = 0; i < arr1_size; i++) {
        printf("%d \n", arr1[i]);
    }

    return 0;
}

Execution:

./scan
Hello,
Please enter the size of the first array:3

Please enter arr1 of size 3:
1
2
3

Array contents:
1 
2 
3

answered Jun 1, 2020 at 17:59

pifor

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2 Comments

Hanik Over a year ago

Thanks! Could you please explain what exactly went wrong? I understand that after my scan arr1 didn't point to the initial address anymore but doesn't printing with indices fix this? Did the scan I used work at all?

pifor Over a year ago

As already said, when entering data for an array with scanf you need to use the adress of the "ith" array element (arr1 + i) when entering the "ith" data item. In addition I have modified printf so that each array item is printed on its own line: it's easier to see what is printed (for a small array of course).

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