Integer array to string

Question

This function creates takes an int * buffer and creates a neatly formatted string (useful for printing the contents of an array).

• Is the code easy to follow?
• Is it efficient?
• Am I allocating and freeing memory properly?
• Bonus question: is there an idiomatic way to do this in C, such as through a set of library routines?

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

char *int_array_to_string(int *arr, size_t length)
{
    char **strings = calloc(length, sizeof(char *));
    int num_chars = 0;
    for (size_t i = 0; i < length; i++)
    {
        char *num_string = calloc(32, sizeof(char)); // 32 digits should fit any int
        sprintf(num_string, "%d", arr[i]);
        strings[i] = num_string;
        num_chars += strlen(num_string);
    }
    num_chars += 2 * (length - 1); // extra bytes for comma and space chars following the numbers (except for the last number)
    num_chars += 2;                // bytes for curly braces at the beginning and end

    char *result = calloc(num_chars, sizeof(char));
    size_t i = 0;
    result[i++] = '{';
    for (size_t j = 0; j < length; j++)
    {
        char *str = strings[j];
        for (size_t k = 0; k < strlen(str); k++)
            result[i++] = str[k];
        free(str);
        result[i++] = ',';
        result[i++] = ' ';
    }
    free(strings);
    result[num_chars - 1] = '}';

    return result;
}

Answer 1

• result is not zero-terminated. You should allocate one byte more.

• To my taste, there are too many allocations. Compute the required size, and allocating once. You already allocate 32 bytes per array element, so

    char * result = malloc(length * 32 + whatever_extra_space_necessary);
  

• Now recall that sprintf returns an amount of bytes it actually printed, and remove a superfluous call to strlen:

    char * where = result;
    for (size_t i = 0; i < length; i++) {
        size_t printed = sprintf(where, "%d, ", arr[i]);
        where += printed;
    }
  

• Your code prints an unpleasant ", " after the last element of an array. If it is a conscious decision, it is all right; if it is not, consider printing the first element separately:

    print("%d", arr[0]);
    for (i = 1; ....)
        print(", %d", arr[i]);