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In the above code, we have included the iostream.h header file. This allows us to use all the
functions defined in iostream.h within our program. A header file can be included using either
angle brackets (< >) or double quotes (" ").
Angle brackets (< >) instruct the compiler to search for the specified header file in the
compiler’s include directory.
Double quotes (" ") instruct the compiler to search for the header file in the current directory
before looking in the compiler’s include directory.
Let us use some of the library functions to understand the concepts better.
Functions of ctype.h Header File
The ctype.h header file contains the definitions of character handling functions. You need to
include this header file in your program to use the character functions. Some of the commonly
used functions of the ctype.h header file are:
Function Name Use Example Output
isalpha(char c) It returns true if the character c is an isalpha(‘B’) True
uppercase or a lowercase letter. Otherwise
returns false.
islower(char c) It returns true if the character c is a lowercase islower(‘B’) False
letter. Otherwise returns false.
isupper(char c) It returns true if the character c is an isupper(‘B’) True
uppercase letter. Otherwise returns false.
isdigit(char c) It returns true if the character c is a digit isdigit(‘9’) True
from 0 through 9. Otherwise returns false.
isalnum(char c) It returns true if character c is a digit from isdigit(‘$’) False
0 through 9 or a letter either uppercase or
lowercase. Otherwise returns false.
tolower(char c) It converts the character c to lowercase if tlower(‘B’) b
it is in uppercase. Nothing will change if
the character is a lowercase letter or a non-
alphabetic character.
toupper(char c) It converts the character c to uppercase if it toupper(‘b’) B
is in lowercase. Nothing will change if the
character is an uppercase letter or a non-
alphabetic character.
Program 1: To use character handling functions.
#include<iostream.h>
#include<ctype.h>
#include<conio.h>
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Header Files and Library Functions

