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)

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)

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:

gEEKs

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