ctype.h
- C course
- Intermediate
- Lesson 86/116
- 5 min
Learn character classification with ctype.h.
ctype.h
The <ctype.h> header provides character classification and conversion functions: isalpha (is letter), isdigit (is digit), toupper (to uppercase) and tolower (to lowercase).
These functions take an int (the numeric value of a char) and return an int. isalpha('A') is true (nonzero), isdigit('5') is true, isdigit('a') is false (0). toupper('a') returns 'A'; if already uppercase it stays the same. tolower does the reverse. For non-ASCII characters these functions depend on the locale; the default C locale handles only ASCII letters. To uppercase a string, we apply toupper to each character in a loop.
Uppercasing a string
To uppercase all letters of a string, we set up a loop and apply toupper to each character. The loop runs until the null character.
Definition: char s[] = "merhaba"; for (int i = 0; s[i]; i++) s[i] = toupper(s[i]);. Here the s[i] condition stops the loop when s[i] is null (0). Each step toupper uppercases the letter and writes it back. Result: MERHABA. This technique is common for text normalization; for example, if you want to compare user input in uppercase, you first uppercase it. The same method with tolower works for lowercase. Modifying character arrays in place saves memory.
Practice Uppercase a string
In main declare char s[] = "merhaba";. In for (int i = 0; s[i]; i++) do s[i] = toupper(s[i]);. After the loop printf("%s\n", s);.
Starter code
#include <stdio.h>
#include <ctype.h>
// main: char s[] = "merhaba";
// in a for loop uppercase each letter with toupper
// print with printf
int main(void) {
// write here
return 0;
}Sample solution
#include <stdio.h>
#include <ctype.h>
int main(void) {
char s[] = "merhaba";
for (int i = 0; s[i]; i++) {
s[i] = toupper(s[i]);
}
printf("%s\n", s);
return 0;
}Expected output
MERHABA
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 ctype functions
| Term | Meaning |
|---|---|
isalpha |
is letter |
isdigit |
is digit |
toupper |
to uppercase |
tolower |
to lowercase |
2 Convert to uppercase
s[i] = ____(s[i]);s[i] = toupper(s[i]);Explanation: toupper converts a letter to uppercase.
3 What does toupper('a') return?
- 0
- 'a'
- 'A' Correct answer
- 1
Explanation: toupper converts a lowercase letter to uppercase.