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

Syntax:
int isupper(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;
}

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)

Output

Not 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. 
2. If lowercase, convert it to uppercase using toupper() function, if uppercase, convert it to lowercase using tolower() function. 
3. Print the final string.

Implementation:

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

Time complexity : O(n) 
Auxiliary Space : O(1)