C isupper() Function

The isupper() function in C is a standard library function that checks if a given character is an uppercase letter. It is part of the C standard library (ctype.h). This function is useful for determining if a character is an uppercase alphabetic letter.

Table of Contents

  1. Introduction
  2. isupper() Function Syntax
  3. Examples
    • Checking if a Character is an Uppercase Letter
    • Using isupper() with User Input
  4. Real-World Use Case
  5. Conclusion

Introduction

The isupper() function checks if a given character is an uppercase letter, meaning it is one of the characters from 'A' to 'Z'. This function is useful in various scenarios, such as validating user input or parsing text.

isupper() Function Syntax

The syntax for the isupper() function is as follows:

#include <ctype.h>
int isupper(int c);

Parameters:

  • c: The character to be checked, which is passed as an int.

Returns:

  • The function returns a non-zero value (true) if the character is an uppercase letter; otherwise, it returns 0 (false).

Examples

Checking if a Character is an Uppercase Letter

To demonstrate how to use isupper() to check if a character is an uppercase letter, we will write a simple program.

Example

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

int main() {
    char ch = 'G';

    // Check if the character is an uppercase letter
    if (isupper(ch)) {
        printf("'%c' is an uppercase letter.\n", ch);
    } else {
        printf("'%c' is not an uppercase letter.\n", ch);
    }

    return 0;
}

Output:

'G' is an uppercase letter.

Using isupper() with User Input

This example shows how to use isupper() to check if a character provided by the user is an uppercase letter.

Example

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

int main() {
    char ch;

    // Get user input for the character
    printf("Enter a character: ");
    scanf("%c", &ch);

    // Check if the character is an uppercase letter
    if (isupper(ch)) {
        printf("'%c' is an uppercase letter.\n", ch);
    } else {
        printf("'%c' is not an uppercase letter.\n", ch);
    }

    return 0;
}

Output (example user input 'A'):

Enter a character: A
'A' is an uppercase letter.

Output (example user input 'a'):

Enter a character: a
'a' is not an uppercase letter.

Real-World Use Case

Validating Uppercase Initials

In real-world applications, the isupper() function can be used to validate that initials or other significant letters in a string are uppercase.

Example: Validating Initials

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

int main() {
    char name[100];
    int is_valid = 1;

    // Get user input for the name
    printf("Enter a name: ");
    scanf("%s", name);

    // Check if the first letter of each word is uppercase
    for (int i = 0; name[i] != '\0'; i++) {
        if (i == 0 || name[i - 1] == ' ') {
            if (!isupper(name[i])) {
                is_valid = 0;
                break;
            }
        }
    }

    // Print the result
    if (is_valid) {
        printf("The name '%s' is valid.\n", name);
    } else {
        printf("The name '%s' is invalid. Each word must start with an uppercase letter.\n", name);
    }

    return 0;
}

Output (example user input name "John Doe"):

Enter a name: John Doe
The name 'John Doe' is valid.

Output (example user input name "john doe"):

Enter a name: john doe
The name 'john doe' is invalid. Each word must start with an uppercase letter.

Conclusion

The isupper() function is essential for checking if a character is an uppercase letter in C. It is useful in various applications, particularly in fields like data validation and text processing, where it is necessary to ensure that characters belong to the uppercase alphabetic set.

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