Conversion of whole String to uppercase or lowercase using STL in C++ Last Updated : 10 Jan, 2023 Comments Improve Suggest changes Like Article Like Report Try it on GfG Practice Given a string, convert the whole string to uppercase or lowercase using STL in C++. Examples: For uppercase conversionInput: s = "String"Output: s = "STRING" For lowercase conversionInput: s = "String"Output: s = "string" Functions used : transform : Performs a transformation on given array/string. toupper(char c): Returns the upper case version of character c. If c is already in uppercase, return c itself. tolower(char c) : Returns lower case version of character c. If c is already in lowercase, return c itself. CPP // C++ program to convert whole string to // uppercase or lowercase using STL. #include<bits/stdc++.h> using namespace std; int main(){ // s1 is the string which is converted to uppercase string s1 = "abcde"; // using transform() function and ::toupper in STL transform(s1.begin(), s1.end(), s1.begin(), ::toupper); cout<<s1<<endl; // s2 is the string which is converted to lowercase string s2 = "WXYZ"; // using transform() function and ::tolower in STL transform(s2.begin(), s2.end(), s2.begin(), ::tolower); cout<<s2<<endl; return 0; } OutputABCDE wxyz Time complexity: O(N) where N is length of string ,as to transform string to Upper/Lower we have to traverse through all letter of string once.Auxiliary Space: O(1) Comment More infoAdvertise with us Next Article Conversion of whole String to uppercase or lowercase using STL in C++ J Jatin Goyal Improve Article Tags : Strings Computer Science Fundamentals DSA STL cpp-string C++ Basic Programs C++ String Programs +3 More Practice Tags : STLStrings Similar Reads Case conversion (Lower to Upper and Vice Versa) of a string using BitWise operators in C/C++ Given a string, write a function that converts it either from lower to upper case or from upper to lower case using the bitwise operators &(AND), |(OR), ~(NOT) in place and returns the string. 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