C Function to Convert Lowercase to Uppercase in a String – Simple Code Example

String manipulation is an essential part of programming. In this post, you’ll learn how to write a C function to convert all lowercase characters in a string to their uppercase equivalents using basic character operations.

---

Problem Statement

Write a function that:

• Accepts a character string as an argument.
• Scans each character in the string.
• Converts all lowercase letters (a-z) to their uppercase equivalents (A-Z).

---

Concept Behind the Logic

In C, characters are stored using ASCII values:

• The ASCII value of 'a' is 97 and 'A' is 65.
• So the difference between lowercase and uppercase is 32.

You can convert a lowercase letter to uppercase by subtracting 32 from its ASCII value.

---

C Function to Convert Lowercase to Uppercase

#include <stdio.h>

void toUpperCase(char *str) {
    int i = 0;
    while (str[i] != '\0') {
        if (str[i] >= 'a' && str[i] <= 'z') {
            str[i] = str[i] - 32;
        }
        i++;
    }
}

int main() {
    char text[100];

    printf("Enter a string: ");
    fgets(text, sizeof(text), stdin);

    toUpperCase(text);

    printf("Uppercase string: %s", text);

    return 0;
}

---

Sample Output

Enter a string: Hello World
Uppercase string: HELLO WORLD

---

Explanation

• The function toUpperCase() takes a character pointer.
• It iterates through each character in the string.
• If the character is between 'a' and 'z', it is converted by subtracting 32.
• The updated string is then printed in main().

---

Key Points

• You don’t need any special library to convert characters manually.
• For more robust code, use toupper() from <ctype.h>, but in this example we use ASCII arithmetic for clarity.

---

Final Thoughts

This is a classic string manipulation task in C. Practicing this helps in:

• Understanding pointer usage in functions.
• Learning ASCII-based operations.
• Strengthening your grasp on string handling.

Now go ahead, tweak the function, and try writing a lowercase-to-uppercase converter for user input with more validations!