C Programming – How to convert a string to lowercase or uppercase?

Sometimes we may need to convert a string to either lowercase or uppercase depending on the program requirement. We have a set of functions in C Library to convert a string to either lowercase or uppercase.

Note that, these functions are not standard library functions, and you may or may not see these functions in your version of C.

Convert a string to lowercase

By using strlwr() function in C, we can convert a string to lowercase; which means all of the string characters are in lowercase. The syntax of the function looks like below;

char *strlwr(char *str);

It takes a string as an argument and converts the string to a lowercase string. Observe that, it will convert the original string to a lowercase string; and returns the address of the string.

Convert a string to uppercase

C provides strupr() function to convert a string to uppercase. The syntax of the function looks like below;

char *strupr(char *str);

This function takes a single argument, string as an argument, converts the string to uppercase, and returns the address of the string. Observe that, this function converts the original string to uppercase.

Here is a working example where you can see the string is converted to lowercase and uppercase.

#include <stdio.h>
#include <string.h>

int main()
{
    const char str[] = "Hello World!";
    
    printf("Lowercase: %s\n", strlwr(str));
    printf("Uppercase: %s\n", strupr(str));

    return 0;
}

Our own functions to convert strings to either lowercase or uppercase

As mentioned above, strlwr() & strupr() functions are non-standard C library functions and if you do not find these functions in your version of C, you can simply write your own functions to convert a string to lowercase or uppercase with the help of tolower() & toupper() character conversion functions.

tolower() function is to convert a character to lowercase and toupper() function is used to convert a character to uppercase. By using these functions, now we are going to implement our own versions of strlwr() and strupr() functions to convert a string to either lowercase or uppercase.

Here is the working code;

char* str2lower(char *str)
{
	if (str == 0)
		return (char*)0;

	int len = strlen(str);
	for (int i = 0; i < len; i++)
	{
		str[i] = tolower(str[i]);
	}

	return str;
}

char* str2upper(char* str)
{
	if (str == 0)
		return (char*)0;

	int len = strlen(str);
	for (int i = 0; i < len; i++)
	{
		str[i] = toupper(str[i]);
	}

	return str;
}

//Malin