isupper() and islower() and their application in C++

Difficulty Level : Easy
Last Updated : 13 Sep, 2021

In C++, isupper() and islower() are predefined functions used for string and character handling. cstring.h is the header file required for string functions and cctype.h is the headerfile required for character functions.

isupper() Function
This function is used to check if the argument contains any uppercase letters such as A, B, C, D, …, Z.

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Syntax
int isupper(int x)

C++

// Program to check if a character is in
// uppercase using isupper()
#include <iostream>
#include <cctype>
using namespace std;
int main()
{
    char x;
    cin >> x;
    if (isupper(x))
        cout << "Uppercase";
    else
        cout << "Not uppercase.";   
    return 0;
}

Input : A
Output : Uppercase

Input : a
Output : Not uppercase

islower() Function
This function is used to check if the argument contains lowercase letters such as a, b, c, d, …, z.

Syntax
int islower(int x)

C++

// Program to check if a character is in
// lowercase using islower()
#include <iostream>
#include <cctype>
using namespace std;
int main()
{
    char x;
    cin >> x;
    if (islower(x))
        cout << "Lowercase";
    else
        cout << "Not Lowercase.";   
 
    return 0;
}

Input:A
Output : Not Lowercase

Input : a
Output : Lowercase

Application of islower(), isupper(), tolower(), toupper() function.
Given a string, task is to convert the characters in the string into opposite case, i.e. if a character is in lowercase, we need to convert it into uppercase and vice versa.

Syntax of tolower():

int tolower(int ch);

Syntax of toupper():

int toupper(int ch);

Examples:

Input : GeekS
Output :gEEKs

Input :Test Case
Output :tEST cASE

1. Traverse the given string character by character upto its length, check if character is in lowercase or uppercase using predefined function.
3. If lowercase, convert it to uppercase using toupper() function, if uppercase, convert it to lowercase using tolower() function.
4. Print the final string.

C++

// C++ program to toggle cases of a given
// string.
#include <iostream>
#include <cstring>
using namespace std;
 
// function to toggle cases of a string
void toggle(string& str)
{
    int length = str.length();
    for (int i = 0; i < length; i++) {
        int c = str[i];
        if (islower(c))
            str[i] = toupper(c);
        else if (isupper(c))
            str[i] = tolower(c);       
    }
}
 
// Driver Code
int main()
{
    string str = "GeekS";
    toggle(str);
    cout << str;
    return 0;
}

Output:

gEEKs

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