islower : <ctype.h> library function

islower function checks whether a character is lowercase alphabet(a-z) or not.

Function prototype of islower

int islower(int c);
Function islower takes a character as input in form of an integer. When we pass a value of data type char to islower function, corresponding ASCII value of that character is passed.

Return value of islower

If passed character is a lowercase character, then islower function returns non-zero(true) integer otherwise 0(false).

C program to show the use of islower function

The following program is to check whether a character is lowercase alphabet or not.

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

int main(){
    char string[] = "Aa.1";
    int index = 0;
    
    while(string[index] != '\0'){
        if(islower(string[index])){
            printf("'%c' is a lowercase character\n",
                string[index]);
        } else {
            printf("'%c' is not a lowercase character\n",
                string[index]);
        }
        index++;
    }
    
    getch();
    return 0;
}

Program Output
'A' is not a lowercase character
'a' is a lowercase character
'.' is not a lowercase character
'1' is not a lowercase character