Useful String Libraries - <ctype.h> and <string.h>

Table of contents

1. Useful String Libraries - <ctype.h> and <string.h>
   1. ctype.h — Character Classification & Conversion
      1. Mocking Spongebob Example
   2. string.h — String Manipulation
      1. strnlen()
      2. strncpy()
      3. strchr()

Whilst handling strings in C is not necessarily the most user friendly, there are still quite a few helper functions available. Getting to grips with a few of them will help with the Lab activities and your Command Line project.

ctype.h — Character Classification & Conversion

These functions help you work with individual characters, especially when checking or changing their type. A full explanation of all the functions can be found at tutorials point.

Function	Description	Example
toupper(c)	Converts a lowercase letter to uppercase	'a' → 'A'
tolower(c)	Converts an uppercase letter to lowercase	'Z' → 'z'
isalpha(c)	Checks if c is a letter (A–Z or a–z)	'b' → true
isdigit(c)	Checks if c is a digit (0–9)	'5' → true

Mocking Spongebob Example

If we wanted to check the case of only the letters to write ALTerNaTIG cApS we can use functions from the <ctype.h> library.

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

int main(void) {
    char input[] = "hello world!";
    for (int i = 0; input[i] != '\0'; i++) {
        if (isalpha(input[i])) {
            // Alternate case: even index → uppercase, odd index → lowercase
            if (i % 2 == 0) {
                input[i] = toupper(input[i]);
            } else {
                input[i] = tolower(input[i]);
            }
        }
    }
    printf("SpOnGeBoB: %s\n", input);
    return 0;
}

Gives the result SpOnGeBoB: HeLlO WoRlD!

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string.h — String Manipulation

These functions help you work with strings (arrays of characters). A full list of the library functions with examples can be found on tutorials point.

Like with using scanf to read strings, we need to be careful when using somme of the functions in <string.h> as there are some functions with can easily cause buffer overflow because they do not have fixed maximum characters. Often there are safer versions which require specified limits, these normally have an n in their name i.e. strlen (unsafe) vs strnlen (safe).

strnlen()

• Measures the length of a string up to a maximum number of characters.
• Prevents reading past the buffer.

size_t len = strnlen(buffer, sizeof(buffer));

strncpy()

• Copies a string up to a specified number of characters.
• Safer than strcpy() but may not null-terminate the result.

strncpy(dest, src, sizeof(dest) - 1);
dest[sizeof(dest) - 1] = '\0'; // Ensure null termination

strchr()

• Finds the first occurrence of a character in a string.
• Useful for removing line endings or searching for delimiters.

buffer[strcspn(buffer, "\n")] = '\0';

or if you might have Windows line endings \n\r

buffer[strcspn(buffer, "\r\n")] = '\0';

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