How do i find character (variable) from a string?

Question

cin >> letter >> position; //This is inside the main(). void moveLetter(char letter, int position) {` if (upperRowPiecesToPlay.find(letter)) { upperRowPiecesToPlay.replace(letter,1 , " "); switch(position) { case 1: if(p1 == '.' ) { p1 = letter; } break;

So, this is my code.

I'd like to find a character(input from a user) from the given string. And replace it with empty space.

However, obviously this is not how I should use the find and replace.

Please teach me the right way...

Welcome to Stack Overflow. Please read the About page soon and also visit the links describing How to Ask a Question and How to create a Minimal, Complete, and Verifiable example. Right now all we can do is guess at what upperRowPiecesToPlay is (presumably std::string) and what p1 is (presumably char). What isn't working? — David C. Rankin
– David C. Rankin, Commented Feb 6, 2020 at 1:14

Remy Lebeau · Accepted Answer · 2020-02-06 01:22:23Z

You are not using the return value of std::string::find() correctly.

std::string::find() returns the index of the specified character, or std::string::npos (-1) if not found. It does not return a bool, like your code seems to think it does.

When an if statement is evaluated, a non-zero integer value is treated as true. Which means your code will try to execute upperRowPiecesToPlay.replace() if the letter is found at any index other than 0, or is not found at all.

But there is no overload of std::string::replace() that will take the letter as input. You need to instead give it the index that find() returns, if it is not npos.

Try this instead:

void moveLetter(char letter, int position) { std::string::size_type index = upperRowPiecesToPlay.find(letter); if (index != std::string::npos) { upperRowPiecesToPlay.replace(index, 1 , " "); // alternatively: // upperRowPiecesToPlay[index] = ' '; ... } ... }

@DavidC.Rankin std::string::at() performs bounds checking, std::string::operator[] does not. The index in this example is already guaranteed to be in-bounds by the if (assuming upperRowPiecesToPlay is not being modified by other thread), so at() would just be wasted overhead. It would make sense to use at() only if the if is omitted.
Yes, true, that's why upperRowPiecesToPlay.at(upperRowPiecesToPlay.find(letter)) = ' ' would work in one-shot.

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