C Program to Convert Lowercase to Uppercase

You can convert lowercase to uppercase in C by checking if a character is between 'a' and 'z' and then subtracting 32 from its ASCII value, like if(c >= 'a' && c <= 'z') c = c - 32;.

Examples

Inputhello

OutputHELLO

Inputworld123

OutputWORLD123

InputaBcDeF

OutputABCDEF

How to Think About It

To convert lowercase letters to uppercase, check each character to see if it is a lowercase letter by comparing it with 'a' and 'z'. If it is, convert it by subtracting 32 from its ASCII code because uppercase letters are 32 positions before lowercase letters in ASCII. Leave other characters unchanged.

Algorithm

1

Get the input string from the user.

2

For each character in the string, check if it is between 'a' and 'z'.

3

If yes, subtract 32 from its ASCII value to convert it to uppercase.

4

If not, leave the character as it is.

5

Print the converted string.

Code

c

#include <stdio.h>

int main() {
    char str[100];
    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);

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

    printf("Uppercase string: %s", str);
    return 0;
}

Output

Enter a string: hello
Uppercase string: HELLO

Dry Run

Let's trace the input 'hello' through the code

1

Input string

str = 'hello\0'

2

Check first character 'h'

'h' is between 'a' and 'z', so convert: 'h' (ASCII 104) - 32 = 'H' (ASCII 72)

3

Check second character 'e'

'e' is lowercase, convert to 'E'

4

Check third character 'l'

'l' is lowercase, convert to 'L'

5

Check fourth character 'l'

'l' is lowercase, convert to 'L'

6

Check fifth character 'o'

'o' is lowercase, convert to 'O'

7

End of string

Stop loop at '\0'

Index	Original Char	Is Lowercase?	Converted Char
0	h	Yes	H
1	e	Yes	E
2	l	Yes	L
3	l	Yes	L
4	o	Yes	O

Why This Works

Step 1: Check if character is lowercase

The code uses if (str[i] >= 'a' && str[i] <= 'z') to find lowercase letters because ASCII codes for 'a' to 'z' are continuous.

Step 2: Convert by subtracting 32

Subtracting 32 from the ASCII value converts a lowercase letter to uppercase since uppercase letters are 32 positions before lowercase in ASCII.

Step 3: Leave other characters unchanged

Characters outside 'a' to 'z' range remain the same, so numbers and symbols are not affected.

Alternative Approaches

Using built-in function toupper()

c

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

int main() {
    char str[100];
    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);

    for (int i = 0; str[i] != '\0'; i++) {
        str[i] = toupper(str[i]);
    }

    printf("Uppercase string: %s", str);
    return 0;
}

This method is simpler and safer because it handles locale and non-alphabetic characters properly.

Manual ASCII check with function

c

#include <stdio.h>

char to_upper(char c) {
    if (c >= 'a' && c <= 'z')
        return c - 32;
    else
        return c;
}

int main() {
    char str[100];
    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);

    for (int i = 0; str[i] != '\0'; i++) {
        str[i] = to_upper(str[i]);
    }

    printf("Uppercase string: %s", str);
    return 0;
}

This approach separates logic into a function, improving readability and reusability.

Complexity: O(n) time, O(1) space

Time Complexity

The program loops through each character once, so time grows linearly with input size, making it O(n).

Space Complexity

The conversion is done in-place without extra arrays, so space complexity is O(1).

Which Approach is Fastest?

Manual ASCII subtraction and toupper() both run in O(n), but toupper() is optimized and safer for different locales.

Approach	Time	Space	Best For
Manual ASCII subtraction	O(n)	O(1)	Simple ASCII-only conversion
Using toupper()	O(n)	O(1)	Locale-aware and safer conversion
Function-based manual check	O(n)	O(1)	Readable and reusable code