C islower() function

C islower(int ch)

The islower() function is used to check whether a character is lowercase alphabet (a-z) or not. The function is defined in the ctype.h header file.

Note: Letter case is the distinction between the letters that are in larger uppercase or capitals (or more formally majuscule) and smaller lowercase (or more formally minuscule) in the written representation of certain languages.


int islower( int arg );

islower() Parameters:

Name Description Required /Optional
ch ch is a character of class lower in the current locale. Required

Return value from islower()

  • The islower() function returns non-zero if ch is a lowercase letter; otherwise, returns 0.

Example: C islower() function

#include <stdio.h>
#include <ctype.h>
int main()
    char ch;
    ch = 'y';
    printf("\nIf %c is lower character or not? %d", ch, islower(ch));
    ch = 'Y';
    printf("\nIf %c is lower character or not? %d", ch, islower(ch));
    ch = 'A';
    printf("\nIf %c is lower character or not? %d", ch, islower(ch));


If y is lower character or not? 2
If Y is lower character or not? 0
If A is lower character or not? 0

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What's an object file in C?

An object file is the real output from the compilation phase. It's mostly machine code, but has info that allows a linker to see what symbols are in it as well as symbols it requires in order to work. (For reference, "symbols" are basically names of global objects, functions, etc.)

A linker takes all these object files and combines them to form one executable (assuming that it can, i.e.: that there aren't any duplicate or undefined symbols). A lot of compilers will do this for you (read: they run the linker on their own) if you don't tell them to "just compile" using command-line options. (-c is a common "just compile; don't link" option.)

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