What are Strings in C if not Simply Char Arrays?

Question

I'm new to the C programming language, and I was under the impression that strings are just arrays of characters. However, when I tried the following code below (among some other tests):

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

int main(int argc, char **argv)
{
    char apple1[] = { 'a', 'p', 'p', 'l', 'e', '\0' };
    char *apple2 = "apple";
    char apple3[] = "apple";

    printf("%i\n", apple1 == apple2); // 0
    printf("%i\n", apple2 == apple3); // 0
    printf("%i\n", apple3 == apple1); // 0
    printf("%i\n", "apple" == apple1); // 0
    printf("%i\n", "apple" == apple2); // 1
    printf("%i\n", "apple" == apple3); // 0
    printf("%i\n", !strcmp(apple1, apple2)); // 1

    for (size_t i = 0; i < strlen(apple) + 1; i++)
    {
        printf("%i", apple1[i] == apple2[i]);
    } // 111111

    return 0;
}

I got some unexpected results. Is there any reason for these, at least for me, counterintuitive results? Thank you very much.

`I got some unexpected results` Which ones are unexpected and why? — tkausl, Mar 13 '22 at 19:38
A "string" is just a character array with a terminating 0. Comparing them with `==` is comparing their memory address, not the contents. Use `strcmp` to compare the contents for equality. — Retired Ninja, Mar 13 '22 at 19:38
@RetiredNinja Thanks for the answer. Is there any reason why apple2 == "apple" returns 1? — William Ryman, Mar 13 '22 at 19:40
@tkausl At least based on my current understanding of things (that all strings are just arrays), the results I am getting seem to be contradicting each other. That is mainly what confuses me. — William Ryman, Mar 13 '22 at 19:41

Vlad from Moscow · Accepted Answer · 2022-03-13T20:24:48.117

In these declarations

char apple1[] = { 'a', 'p', 'p', 'l', 'e', '\0' };
char *apple2 = "apple";
char apple3[] = "apple";

there are declared two arrays apple1 and apple3 that contain the string "apple".

In this declaration

char *apple2 = "apple";

there is declared a pointer to the string literal "apple".

In these calls of printf

printf("%i\n", apple1 == apple2); // 0
printf("%i\n", apple2 == apple3); // 0
printf("%i\n", apple3 == apple1); // 0
printf("%i\n", "apple" == apple1); // 0
printf("%i\n", "apple" == apple2); // 1
printf("%i\n", "apple" == apple3); // 0

there are compared addresses of first characters of different arrays that occupy different extents of memory. Arrays used in expressions with rare exceptions are converted to pointers to their first elements. So the result of the expressions is the integer value 0 except this call

printf("%i\n", "apple" == apple2); // 1

because in this case there are compared pointers to the same string literal (its first character) because it seems the compiler allocated one character array to store the string literal "apple" used in this call and in this declaration of a pointer

char *apple2 = "apple";

You can represent the above declaration and the call of printf the following way

char *apple2 = &"apple"[0];
//...
printf("%i\n", &"apple"[0] == apple2); // 1

However in general even if you will write for example

printf("%i\n", "apple" == "apple");

then the output can be either 0 or 1 depending on how the compiler stores identical string-literals: either as different character arrays or as one character array (it depends on compiler options).

To compare character arrays that contain strings you need to use standard C string function strcmp.

What are Strings in C if not Simply Char Arrays?

1 Answers1