Characters and text

A char is a one-byte number. Which character each number means is fixed by a table: ASCII for the first 128 values. 'A' is 65, 'a' is 97, '0' is 48, a space is 32 and the newline '\n' is 10.

Letters are numbers

#include <stdio.h>

int main(void) {
    char c = 'g';
    printf("'%c' is %d\n", c, c);
    printf("position in the alphabet: %d\n", c - 'a');
    printf("uppercase: %c\n", c - 32);
    char d = '7';
    printf("digit value: %d\n", d - '0');
    return 0;
}

Because letters are in order, you can do arithmetic on them:

• c - 'a' is a letter's position in the alphabet (0 for a);
• c - 32 turns a lowercase ASCII letter into uppercase ('a' - 'A' is 32);
• d - '0' turns a digit character into its number: '7' - '0' is 7.

This is how toupper, atoi and the Caesar cipher work.

Beyond ASCII: Unicode and UTF-8

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

int main(void) {
    char word[] = "অ";
    printf("bytes: %zu\n", strlen(word));
    for (int i = 0; word[i] != '\0'; i++) printf("%d ", (unsigned char)word[i]);
    printf("\n");
    return 0;
}

ASCII has no Bangla letters, no accents and no emoji. Unicode gives every character in every script a number (a code point): অ is U+0985. UTF-8 stores those numbers in 1 to 4 bytes: ASCII characters keep their single byte, and others use 2, 3 or 4. So strlen counts bytes, not letters: "অ" has length 3. Watch the array in the visualizer: three bytes for one letter.

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