Character Functions

The library ctype.h has functions that work with one character. Two of them change it:

• toupper(c) gives the uppercase letter: toupper('a') is 'A';
• tolower(c) gives the lowercase letter: tolower('Q') is 'q'.

Other characters stay as they are: toupper('5') is '5', and toupper('A') is 'A'.

#include <ctype.h>
#include <stdio.h>

int main(void) {
    char name[] = "eva";
    name[0] = toupper(name[0]);
    printf("%s\n", name);
    return 0;
}

This prints:

Eva

toupper doesn't change the character you give it. It gives you a new character, and you have to store it: name[0] = toupper(name[0]);. A line with only toupper(name[0]); compiles, but the name stays eva, because the new character is thrown away. (The result is a number of type int, the code of the character. When you store it in a char or print it with %c, it's a character again.)

Other functions answer a question about a character:

• isupper(c): is it an uppercase letter?
• isalpha(c): is it a letter?
• isdigit(c): is it a digit, from '0' to '9'?

The answer is a number: 0 means no, and any other number means yes. So use them directly as a condition, as you know from the lesson on conditions: if (isdigit(c)). This loop counts the digits in a code:

char code[] = "R2D2";
int digits = 0;
for (int i = 0; code[i] != '\0'; i++) {
    if (isdigit(code[i])) {
        digits++;
    }
}
printf("Digits: %d\n", digits);

This prints:

Digits: 2

These functions only know the letters of the English alphabet, without accents. One more reason to keep the strings you go through character by character without accents.