C++ isalpha()

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

int main() {

  // check if '7' is an alphabet
  int result = isalpha('7');

  cout << result;

  return 0;
}

// Output: 0

isalpha() Syntax

The syntax of isalpha() is:

isalpha(int ch);

Here, ch is the character that we want to check.

isalpha() Parameters

The isalpha() function takes the following parameter:

• ch - the character to check, casted to int or EOF

isalpha() Return Value

The isalpha() function returns:

• non-zero value if ch is an alphabet
• zero if ch is not an alphabet

isalpha() Prototype

The prototype of isalpha() as defined in the cctype header file is:

int isalpha(int ch);

Here, ch is checked for alphabets as classified by the currently installed C locale. By default, the following characters are alphabets:

• Uppercase Letters: 'A' to 'Z'
• Lowercase Letters: 'a' to 'z'

isalpha() Undefined Behavior

The behaviour of isalpha() is undefined if:

• the value of ch is not representable as unsigned char, or
• the value of ch is not equal to EOF

Example: C++ isalpha()

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

using namespace std;

int main() {
  char str[] = "ad138kw+~!$%?';]qjj";
  int count = 0, check;

  // loop to count the no. of alphabets in str
  for (int i = 0; i <= strlen(str); ++i) {

    // check if str[i] is an alphabet
    check = isalpha(str[i]);

  // increment count if str[i] is an alphabet
    if (check)
      ++count;
  }

  cout << "Number of alphabet characters: " << count << endl;
  cout << "Number of non-alphabet characters: " << strlen(str) - count;

  return 0;
}

Output

Number of alphabet characters: 7
Number of non alphabet characters: 12

In this program, we have used a for loop and the isalpha() function to count the number of alphabets in str.

The following variables and codes are used in this program:

• strlen(str) - gives the length of the str string
• check - checks if str[i] is an alphabet with the help of isalpha()
• count - stores the number of alphabets in str
• strlen(str) - count - gives the number of non-alphabets in str