The **`isdigit()`** function in C++ is used to check if a given character is a number or not. 

# Library
To use the `isdigit()` function, include the following library:
```C
#include <cctype>
```

# Declaration
The `isdigit()` function is declared as follows:
```C
int isdigit(int c);
```
* `c`: The ASCII value of the character that is to be checked whether it is a digit.

>Note: If `c` is not an unsigned char or EOF, the behavior of the `isdigit()` function is undefined.

# Return value
* The `isdigit()` function returns a non-zero value if `c` is a digit.
* The `isdigit()` function returns 0 if `c` is not a digit.

# Examples
## Example 1
Consider the code snippet below, which demonstrates the use of the `isdigit()` function:




#include <cctype>
#include <iostream>
using namespace std;

int main()
{
  char c1 = 'a';
  char c2 = '8';  

  cout<<c1<<" is digit: "<<isdigit(c1)<<endl;
  cout<<c2<<" is digit: "<<isdigit(c2)<<endl;
  
  return 0;
}

## Explanation
Two characters `c1` and `c2` are declared in *line 7-8*. 
`c1` is not a digit and `c2` is a digit.
* *line 10*: The `isdigit()` function is used in *line 10* to check whether `c1` is a digit. The `isdigit()` function returns 0 which means that `c1` is not a digit.
* *line 11*: The `isdigit()` function is used in *line 11* to check whether `c2` is a digit. The `isdigit()` function returns a non-zero value which means that `c2` is a digit.

## Example 2
Consider the code snippet below, which uses the `isdigit()` function to remove the digits from other characters in a string:

#include <cctype>
#include <cstring>
#include <iostream>
using namespace std;

int main()
{
  char str[] = "H5el6lo7 8w7o0rld!!123";
  int len = strlen(str);
  char withoutDigits[len];
  int digitCount = 0;

  int ind = 0 ;
  for(int i = 0; i < len; i++){
    if( isdigit(str[i])==0 ){
      withoutDigits[ind] = str[i];
      ind++;
    }else{
      digitCount++;
    }
  }
  
  cout<<"string = "<<str<<endl;
  cout<<"string without digits = "<<withoutDigits<<endl;
  cout<<"number of digits in string = "<<digitCount<<endl;
  return 0;
}

## Explanation
* *line 8*: A string `str` is declared that contains both digits and non-digits characters.
* *line 10*: A string `withoutDigits` is declared that will contain the non-digit characters of `str`.
* *line 11*: A variable
`digitCount` is declared that will contain the count of the number of digit characters of `str`.
* *line 14-17*: Loop over the characters of `str` such that if a character is a non-digit then copy the character in `withoutDigits` and if a character is a digit then add one to `digitCount`.


What is isdigit() in C++?

The isdigit() function in C++ checks if a character is a digit, returning non-zero if true and 0 if false. Include <cctype>.