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

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 arguement contains any uppercase letters such as A, B, C, D……….Z.

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

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Input : A
Output : Uppercase
Input : a
Output : Not uppercase

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

Syntax
int islower(int x)
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// 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;
}

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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 uppercade and vice versa.

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.

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// 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;
}

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Output:

gEEKs

This article is contributed by Ayush Saxena. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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