C Strings

• Strings aren’t actually strings in C, they’re pointers (of char type)

Note

String literals like “Hello World!” are NUL-terminated, but char arrays are not automatically NUL terminated.

String variables

    char *s = "Hello, World!\n";
    printf("%s\n",s);
    for (int i = 0; i < 13; i++){
        printf("%c\n", s[i]);
    }

    char a[] = "Goodbye World!";
    printf("%s\n",a);

    for (int i = 0; i < 15; i++){
        printf("%c\t", a[i]);
    }

String initializers

    char *s = "Hello, world!";
    char t[] = "hello world";

• But these 2 are subtly different.
  • (Immutable) The first one is a pointer to a string literal (i.e a pointer to the first char in a string)

    char *s = "Hello, world!";
    // this will create a pointer s point to read-only value in the op
    s[0] = 'z'; // ERROR

  • (Mutable) The second one is different, it’s a mutable copy of the string that we can change at will:

    char t[] = "hello world";
    // equivalent to
    // char t[] = {'h','e','l','l','o',' ','w','o','r','l','d','\0'};
    
    t[0] = 'z'; // No problem
    printf("%s\n", t); // "zello, again!"

String length

Note

In C, the char type is typically 8 bits ( 1 byte), the binary representation 11111111 corresponds to the maximum value for an 8-bit unsigned char. In decimal, this value is 255.

When you’re making a new language, you have basically two options for storing a string in memory:

• 1. Store the bytes of the string along with a number indicating the length of the string.
• 2. Store the bytes of the string, and mark the end of the string with a special byte called the terminator.

If you want strings longer than 255 characters, option 1 requires at least two bytes to store the length. Whereas option 2 only requires one byte to terminate the string. So a bit of savings there, so C took approach #2.

In C, a “string” is defined by 2 basics characteristics:

• A pointer to the first character in the string
• A Zero-valued byte ( or NUL character) somewhere in memory after that pointer that indicates the end of the string

char *s = "Hello!"; // Actually "Hello!\0" behind the scenes

• Getting string length:

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

int main(void){
    char *s = "Hello, World!";
    printf("The string is %zu bytes long.\n", strlen(s));
}

• Behind the scene, the strlen() basically looks like this:

int strlen(char *s) {
    int count = 0;

    while (s[count] != '\0'){
        count++;
    }
    return count;
}

String copy

• Use strcpy

int main(void){
    char *s = "Hello, World!";
    char t[100];

    strcpy(t,s);

    t[0] = 'z';
    printf("%s\n", s);
    printf("%s\n", t);

}

• Use memcpy

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


int main(){
    char s[] = "Goats!";
    char t[100];

    memcpy(t,s,7); //Copy 7 bytes - inclding the NUL terminator;
    printf("%s\n", t);
}

Array of strings

char *a[10];
//This declares an array of 10 char pointer.
//Each pointer points to a "string"