In C, the **`tolower()` function** is used to convert uppercase letters to lowercase.

When an uppercase letter is passed into the `tolower()` function, it converts it into lowercase.

However, when a lowercase letter is passed into the `tolower()` function, it returns the same letter.

>Note: In order to use this function, `ctype.h` needs to be included.

# Syntax
The function takes a letter as a parameter and returns it in lowercase.

The prototype of the `tolower()` function is:



> Note: The character is passed in and returned as int, i.e., the ASCII value of the character moves around.

# Code
Let’s start with the most basic example of converting an uppercase letter into lowercase using this function.

Take a look at the code below:

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

int main() 
{ 
	char ch; 

	// letter to convert to lowercase
	ch = 'B'; 

	printf("%c in lowercase is represented as %c", 
		ch, tolower(ch)); 

	return 0; 
} 

from diagrams import Diagram
from diagrams.aws.compute import EC2
from diagrams.aws.database import RDS
from diagrams.aws.network import ELB

with Diagram("Web Service", show=False):
    ELB("lb") >> EC2("web") >> RDS("userdb")

Now, let's look at how to convert words and sentences to lowercase. However, we can't just give all the words as input because `tolower()` *can only take one character at a time*.

So, to do this, we need to use a loop over the word to individually convert each letter to lowercase.




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

int main() 
{ 
    // counter for the loop
    int i = 0;

    // word to convert to uppercase
    char word[] = "eDuCaTiVE.Io\n"; 
    char chr; 

    // Loop
    while (word[i]) { 
        chr = word[i]; 
        printf("%c", tolower(chr)); 
        i++; 
    } 
  
    return 0; 
} 

In the code above, we pass each letter of the word to the `tolower()` function to get and display its lowercase form.

What is the tolower() function in C?

The tolower() function in C converts uppercase letters to lowercase while leaving lowercase letters unchanged.