Character Functions in C++

Created On: 05 November 2022
Character Functions in C++
Character functions are those in-built functions present in the C++ programming language that help us to process characters in a string. These functions are present in the <cctype> header file. These functions can be classified into two types: Classification functions and Transformation functions. 

Below is the list of Character Functions with their usage in programming:
Classification Function Definition
isalnum(ch) This function returns true if the character ch is a letter or a digit.
isalpha(ch) This function returns true if the character ch is a letter.
iscntrl(ch) This function returns true if the character ch is a control character. (Ex: \t, \n, \b, \r)
isdigit(ch) This function returns true if the character ch is a digit.
isgraph(ch) This function returns true if the character ch is not a space but a printable value.
islower(ch) This function returns true if the character ch is a lowercase letter.
isupper(ch) This function returns true if the character ch is an uppercase letter.
isprint(ch) This function returns true if the character ch is a printable character.
ispunct(ch) This function returns true if the character ch is a punctuation character. (Ex: !, #, $, @)
isspace(ch) This function returns true if the character ch is whitespace.
isblank(ch) This function returns true if the character ch is blank.
isxdigit(ch) This function returns true if the character ch is a hexadecimal digit.
Transformation Function Definition
tolower(ch) This function is used to change character ch to lowercase if it is an uppercase letter otherwise leave it unchanged.
toupper(ch) This function is used to change character ch to uppercase if it is a lowercase letter otherwise leave it unchanged.

These in-built character functions take a single character as a parameter and return a value that can either be a boolean type value if it is a Classification function or a character type value if it is a Transformation function.

Let's discuss each character function with C++ example code:
1. isalnum(): This function returns true if the character is an alphabet or a number else it will return false. It is known as alphanumeric and includes characters from a-z, A-Z, and 0-9.

Example code:
/*C++ program to check isalnum() function*/
#include<iostream>
#include<cctype>
using namespace std;

int main(){
  char ch[10] = "12Abc@";

  //working of isalnum() function
  for(int i = 0; i < 6; i++){
    if(isalnum(ch[i]))
      cout<<ch[i]<<" is an alphanumeric character."<<endl;
    else
      cout<<ch[i]<<" is not an alphanumeric character."<<endl;  
  }
  return 0;
}
Output: 
1 is an alphanumeric character.
2 is an alphanumeric character.
A is an alphanumeric character.
b is an alphanumeric character.
c is an alphanumeric character.
@ is not an alphanumeric character.

2. isalpha(): This function returns true if the character is an alphabet else it returns false. It includes characters from a-z and A-Z.

Example code:
/*C++ program to check isalpha() function*/
#include<iostream>
#include<cctype>
using namespace std;

int main(){
  char ch[10] = "1Ab@";

  //working of isalpha() function
  for(int i = 0; i < 4; i++){
    if(isalpha(ch[i]))
      cout<<ch[i]<<" is an alphabet character."<<endl;
    else
      cout<<ch[i]<<" is not an alphabet character."<<endl;  
  }
  return 0;
}
Output:
1 is not an alphabet character.
A is an alphabet character.
b is an alphabet character.
@ is not an alphabet character.

3. iscntrl(): This function returns true if the character is a control character like \t, \n, \b, \r, etc else returns false.

Example code:
// C++ program to check iscntrl()
#include <cctype>
#include <iostream>
using namespace std;

int main(){
	char ch[5] = "\nA\t\b";

	//working of iscntrl() function
	for (int i = 0; i < 4; i++) {
	   if (iscntrl(ch[i]))
		cout<<"Character is a control Character"<<endl;
	   else
		cout<<"Character is a not control Character"<<endl;		
	}
        return 0;
}
Output: 
Character is a control Character
Character is a not control Character
Character is a control Character
Character is a control Character

4. isdigit(): This function returns true if the character is a digit else it returns false. It includes all digits from 0 to 9. 

Example code:
//C++ program to check isdigit() function
#include<iostream>
#include<cctype>
using namespace std;

int main(){
  char ch[5] = "A123";

  //working of isdigit()
  for(int i = 0; i < 4; i++){
    if(isdigit(ch[i]))
      cout<<ch[i]<<" is a digit."<<endl;
    else
      cout<<ch[i]<<" is not a digit."<<endl;  
  }
  return 0;
}
Output: 
A is not a digit.
1 is a digit.
2 is a digit.
3 is a digit.

5. isgraph(): This function returns true if the character is a graphical character else it returns false. Graphical characters include digits (0-9), alphabets (a-z and A-Z), and punctuation characters (!"#$%&'()*+,-./:;<=>?@[\]^_`{|}~).

Example code:
//C++ program to check isgraph() function
#include<iostream>
#include<cctype>
using namespace std;

int main(){
  char ch[5] = "$\tA2";

  //working of isgraph()
  for(int i = 0; i < 4; i++){
    if(isgraph(ch[i]))
      cout<<ch[i]<<" is a graphical character."<<endl;
    else
      cout<<ch[i]<<" is not a graphical character."<<endl;  
  }
  return 0;
}
Output: 
$ is a graphical character.
  is not a graphical character.
A is a graphical character.
2 is a graphical character.

6. islower(): This function returns true if the character is a lowercase letter (a-z) else it returns false.

7. isupper(): This function returns true if the character is an uppercase letter (A-Z) else it returns false.

Example code:
//C++ program to check islower() and isupper() function
#include<iostream>
#include<cctype>
using namespace std;

int main(){
  char ch[5] = "ABcd";

  //working of isupper() and islower()
  for(int i = 0; i < 4; i++){
    if(islower(ch[i]))
      cout<<ch[i]<<" is a lowercase character"<<endl;
    if(isupper(ch[i]))
      cout<<ch[i]<<" is an uppercase character"<<endl;  
  }
  return 0;
}
Output:
A is an uppercase character
B is an uppercase character
c is a lowercase character
d is a lowercase character

8. isprint(): This function returns true if the character is a printable character else it returns false. All characters are printable character except the control characters. 

Example code:
//C++ program to check isprint() function
#include<iostream>
#include<cctype>
using namespace std;

int main(){
  char ch[6] = "A\tBc1";

  //working of isprint()
  for(int i = 0; i < 5; i++){
    if(isprint(ch[i]))
      cout<<ch[i]<<" is a printable character"<<endl;
    else
      cout<<ch[i]<<" is not a printable character"<<endl;  
  }
  return 0;
}
Output: 
A is a printable character
  is not a printable character
B is a printable character
c is a printable character
1 is a printable character

9. ispunct(): This function returns true if the character is a punctuation character else it returns false.

Example code:
//C++ program to check ispunct() function
#include<iostream>
#include<cctype>
using namespace std;

int main(){
  char ch[7] = ")#!\t2'";

  //working of ispunct()
  for(int i = 0; i < 6; i++){
    if(ispunct(ch[i]))
      cout<<ch[i]<<" is a punctuation character"<<endl;
    else
      cout<<ch[i]<<" is not a punctuation character"<<endl;  
  }
  return 0;
}
Output: 
) is a punctuation character
# is a punctuation character
! is a punctuation character
  is not a punctuation character
2 is not a punctuation character

10. isspace(): This function returns true if the character is a tab or a whitespace else it returns false.

Example code: 
//C++ program to check isspace() function
#include<iostream>
#include<cctype>
using namespace std;

int main(){
  char ch[7] = "A\n\t#2";

  //working of isspace()
  for(int i = 0; i < 5; i++){
    if(isspace(ch[i]))
      cout<<"Character is a space character"<<endl;
    else
      cout<<"Character is not a space character"<<endl;  
  }
  return 0;
}
Output: 
Character is not a space character
Character is a space character
Character is a space character
Character is not a space character
Character is not a space character

11. isblank(): This function returns true if the character is a blank character like a space or a tab else it returns false. 

Example code:
//C++ program to check isblank() function
#include<iostream>
#include<cctype>
using namespace std;

int main(){
  char ch[7] = " \n\t2";

  //working of isblank()
  for(int i = 0; i < 4; i++){
    if(isblank(ch[i]))
      cout<<"Character is a blank character"<<endl;
    else
      cout<<"Character is not a blank character"<<endl;  
  }
  return 0;
}
Output: 
Character is a blank character
Character is not a blank character
Character is a blank character
Character is not a blank character

12. isxdigit(): This function returns true if the character is a hexadecimal character else it returns false. It includes characters from 0-9 and (A-F) in both uppercase and lowercase.

Example code:
//C++ program to check isxdigit() function
#include<iostream>
#include<cctype>
using namespace std;

int main(){
  char ch[7] = "AB120X";

  //working of isxdigit()
  for(int i = 0; i < 6; i++){
    if(isxdigit(ch[i]))
      cout<<ch[i]<<" is a Hexadecimal character"<<endl;
    else
      cout<<ch[i]<<" is not a Hexacecima character"<<endl;  
  }
  return 0;
}
Output:
A is a Hexadecimal character
B is a Hexadecimal character
1 is a Hexadecimal character
2 is a Hexadecimal character
0 is a Hexadecimal character
X is not a Hexacecima character

13. toupper(): This is a transformation function that change the character to uppercase letter if a lowercase character is pass to this function and if the character is already an uppercase character then no change take place.

14. tolower(): This is a transformation function that change the character to lowercase letter if an uppercase character is pass to this function and if the character is already a lowercase character then no change take place.

Example code:
//C++ program to check tolower() and toupper() function
#include<iostream>
#include<cctype>
using namespace std;

int main(){
  char ch1[11] = "welcome";
  char ch2[11] = "ALGOLESSON";
  //converting to uppercase
  for(int i = 0; i < 7; i++){
    cout<<(char)toupper(ch1[i]);
  }
  cout<<"\t";
  //converting to lowercase
  for(int i = 0; i < 10; i++){
    cout<<(char)tolower(ch2[i]);
  }
  return 0;
}
Output: 
WELCOME algolesson

So here ends the complete list of built-in character functions in C++ programming language with example code to make you understand working of each function. These functions are very helpful for solving several string related coding problems.

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