Character Classification Functions in C++

 

C++ provides several character classification functions in the cctype library to check whether a character is a digit, letter, uppercase, lowercase, etc. These functions are useful for input validation, parsing, and formatting strings.

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1️⃣ List of Character Classification Functions

The <cctype> header contains functions for checking character types.

Function	Description
isalpha(c)	Returns true if c is a letter (A-Z or a-z).
isdigit(c)	Returns true if c is a digit (0-9).
isalnum(c)	Returns true if c is a letter or digit (A-Z, a-z, 0-9).
islower(c)	Returns true if c is a lowercase letter (a-z).
isupper(c)	Returns true if c is an uppercase letter (A-Z).
tolower(c)	Converts uppercase letter to lowercase.
toupper(c)	Converts lowercase letter to uppercase.
isspace(c)	Returns true if c is a whitespace (' ', '\t', '\n', etc.).
ispunct(c)	Returns true if c is a punctuation character.
isprint(c)	Returns true if c is a printable character (not control character).

📌 All functions return true (non-zero) if the condition is met, otherwise false (0).

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2️⃣ Example: Using Character Classification Functions

#include <iostream>
#include <cctype>  // Required for character functions
using namespace std;

int main() {
    char ch;

    cout << "Enter a character: ";
    cin >> ch;

    if (isalpha(ch))
        cout << ch << " is a letter.\n";
    if (isdigit(ch))
        cout << ch << " is a digit.\n";
    if (isalnum(ch))
        cout << ch << " is alphanumeric.\n";
    if (islower(ch))
        cout << ch << " is a lowercase letter.\n";
    if (isupper(ch))
        cout << ch << " is an uppercase letter.\n";
    if (isspace(ch))
        cout << ch << " is a whitespace character.\n";
    if (ispunct(ch))
        cout << ch << " is a punctuation character.\n";

    return 0;
}

🔹 Example Output

Enter a character: A
A is a letter.
A is alphanumeric.
A is an uppercase letter.

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3️⃣ Example: Convert Case Using tolower() and toupper()

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

int main() {
    char ch;

    cout << "Enter a character: ";
    cin >> ch;

    cout << "Uppercase: " << (char)toupper(ch) << endl;
    cout << "Lowercase: " << (char)tolower(ch) << endl;

    return 0;
}

🔹 Example Output

Enter a character: b
Uppercase: B
Lowercase: b

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4️⃣ Example: Count Letters, Digits, and Spaces in a String

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

int main() {
    string str;
    int letters = 0, digits = 0, spaces = 0;

    cout << "Enter a string: ";
    getline(cin, str);

    for (char ch : str) {
        if (isalpha(ch)) letters++;
        if (isdigit(ch)) digits++;
        if (isspace(ch)) spaces++;
    }

    cout << "Letters: " << letters << endl;
    cout << "Digits: " << digits << endl;
    cout << "Spaces: " << spaces << endl;

    return 0;
}

🔹 Example Output

Enter a string: Hello 123!
Letters: 5
Digits: 3
Spaces: 1

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5️⃣ Summary

✔ isalpha() → Checks if a character is a letter.
✔ isdigit() → Checks if a character is a digit.
✔ isalnum() → Checks if a character is a letter or digit.
✔ isspace() → Checks if a character is a space.
✔ toupper(), tolower() → Convert letter case.

Would you like an example for password validation using these functions? 🚀