How to manipulate a string to lower case and store in same variable

Question

I have a piece of code that asks for user input, it is type "string" and its a really simple process, i want whatever the user inputs to be converted using the tolower() function. It does exactly as its supposed to do, but i can't seem to assign it to the same variable. Any help please?

#include <locale>
#include <string>
#include <iostream>
//maybe some other headers, but headers aren't the problem, so I am not going to list them all

while (nCommand == 0)
        {  
            locale loc;
            string sCommand;
            cin >> sCommand;

            for (int i = 0; i < sCommand.length(); ++i)
            {
            sCommand = tolower(sCommand[i],loc);
            cout << sCommand;
            }

For example if the user types in Help sCommand would be h

How I want it to look like it that if the user types in HELP or Help or HeLp

sCommand should be 'help' either way

This was closed as duplicate but the referred selection "solution", at http://stackoverflow.com/questions/313970/stl-string-to-lower-case, is UB code (in general), so I voted to reopen. Please before closing as dup make sure that the referred to answer works. It's difficult to make an OP of some other question change his or her selection of "solution". — Cheers and hth. - Alf, May 29 '14 at 17:36
@Cheersandhth.-Alf I noted the UB and notified the poster, and I intended to re-open this but forgot. I am tempted to add a correct answer to that post, but it would get lost amongst all the highly voted answer. It is annoying when buggy answers get so many up-votes. — juanchopanza, May 29 '14 at 18:02
@Cheersandhth.-Alf: *sigh* No it's not. What total nonsense. You wanna quit stomping around, baselessly accusing everybody else of being wrong? (`[C++11: 21.4.1.1.2/10]`/`[C99: 7.4.2.1/3]`) — Lightness Races in Orbit, May 29 '14 at 23:12

score 1 · Answer 1 · answered May 29 '14 at 18:16

You're assigning a string to a character when really what you want to do is assign the character stored at the position to the lower case version.

Therefore change this:

sCommand = tolower(sCommand[i], loc);

to this:

sCommand[i] = tolower(sCommand[i], loc);
//      ^^^

score 1 · Answer 2 · answered May 29 '14 at 18:57

1

This is another case where Boost String Algorithms would reduce the whole problem to one single expression:

boost::algorithm::to_lower(sCommand)

Try the Boost libraries. It will help you immensely in the long run and let you concentrate on real problems rather than silliness like being the one-millionth programmer to write their own "convert string to lower-case" function.

answered May 29 '14 at 18:57

Christian Hackl

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This can be done without boost by using std::transform and a lambda. – user2672165 May 29 '14 at 19:01
@user2672165: Indeed, but I think the String Algorithms approach is more readable, as it explicitly names the entire operation after what it really does. – Christian Hackl May 29 '14 at 19:05
Agreed, but introduces dependency on boost which is more of a strategic decision. – user2672165 May 29 '14 at 19:07
@user2672165: This is correct, too. OTOH, the OP seems to be learning the language, not working in a professional team where strategic decisions are made. – Christian Hackl May 29 '14 at 19:10

How to manipulate a string to lower case and store in same variable

2 Answers2