Print the array elements

Question

I have the following code:

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

struct sArray {
    int *arr;
    int index; // ultimo indice riempito 
    int prev_index; // penultimo indice
    int lenght;
};

typedef struct sArray arr;  // structArray with type arr

void init(arr *a, int initSize) {
    a->arr = malloc(initSize * sizeof(int));
    a->lenght = initSize;
    a->index = 0;
    a->prev_index = 0;
}

void insert(arr *a, int value) {
    int index = a->index;
    int length = a->lenght;
    int prev_index = a->prev_index;
    if (index < length) {
        a->arr[index] = value;
    } else {
        a->arr = malloc(1 * sizeof(int));
        a->arr[index] = value;
        a->lenght += 1;
    }
    a->index += 1;
    if (index > 0){
       a->prev_index += 1; 
    };
}

int main(void) {
    arr a;

    // init array with 10 elements
    init(&a, 10);

    // try to add 20 elements
    for (int i = 0; i < 20; i++) {
        insert(&a, i);
        printf("Valore: %d\n", a.arr[i]);
    }
    printf("Lenght a: %d\n", a.lenght);
    // the left operand of -> must be a pointer;
    // the . operator is used if it is not a pointer

    for (int i = 0; i < 20; i++) { // PROBLEM HERE
        printf("Valore: %d\n", a.arr[i]);
    }

    return 0;
}

I don't understand why I can't print the array's elements (a.arr[i]) inside the second for loop in main function. In particular, the elements are correctly printed inside the first loop in which there is a insert() function, but in the second loop the elements are printed totally random, while I expect that the elements should be from 0 to 19.

Answer 1

Your allocation in insert() is wrong.

        a->arr = malloc(1 * sizeof(int));

allocate a buffer for only one new element.

Instead of this, you have to

• Allocate enough size (can hold a->lenght + 1 ints)
• Store previously added data

You can do this via realloc() like this:

        a->arr = realloc(a->arr, (a->lenght + 1) * sizeof(int));