C++ isdigit() function explanation with example

C++ isdigit() function explanation with example:

The isdigit() function is used to check if a given character is decimal digit or not. This function is defined in the cctype header file. In this post, we will learn how to use this function with examples.

isdigit() function syntax:

The syntax of isdigit function is defined as like below:

int isdigit(int c)

It takes only one argument. It is the character to check. The provided value is casted to an integer or EOF.

isdigit() return value:

The isdigit() function returns if the provided value is a decimal digit or not. It returns an integer value. If it is zero, it is not a decimal digit. For decimal digits, it returns a nonzero value.

Example of isdigit() function:

Let’s try isdigit() function with an example:

#include <iostream>
#include <cstring>
#include <cctype>

using namespace std;

int main()
{
	char charArr[] = "1abc%$#-30";

	for (int i = 0; i < strlen(charArr); i++)
	{
		cout << "isdigit(" << charArr[i] << ") = " << isdigit(charArr[i]) << endl;
	}
	return 0;
}

Here,

charArr is an array of characters, or a string.
The for loop iterates through the characters of this array one by one and uses isdigit with each of the character.
- Inside the loop, it prints the result of isdigit for the character it is currently iterating.
The strlen method returns the length of charArr. So, i will start from 0 to length of charArr - 1, i.e. it will iterate through all characters of charArr.
For strlen, we have to use cstring header and for isdigit, we have to use cctype header.

If you run this program, it will print:

isdigit(1) = 1
isdigit(a) = 0
isdigit(b) = 0
isdigit(c) = 0
isdigit(%) = 0
isdigit($) = 0
isdigit(#) = 0
isdigit(-) = 0
isdigit(3) = 1
isdigit(0) = 1

Example of isdigit() with user input values:

Let’s use isdigit with user input values. This program will take a character as input from the user and print out if this is a decimal digit or not.

#include <iostream>
#include <cctype>

using namespace std;

int main()
{
	char c;

	cout << "Enter a character: " << endl;
	cin >> c;

	if (isdigit(c))
	{
		cout << "It is a decimal digit." << endl;
	}
	else
	{
		cout << "It is not a decimal digit." << endl;
	}

	return 0;
}

Here,

c is a character variable.
It is asking the user to enter a character. It reads that character and stores that in the variable c.
The if block is checking if the user entered character is a decimal digit or not.
If it is a decimal digit, it will enter the if block and print out a message that it is a decimal digit. Else it will enter the else block and print that it is not.

If you run this program, it will print output as like below:

Enter a character: 
1
It is a decimal digit.

Enter a character: 
c
It is not a decimal digit.

Enter a character: 
$
It is not a decimal digit.

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Updated on 25 Jan 2022

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