'Replace' from <algorithm> not functioning correctly

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For some reason, the replace template from <algorithm> does not want to work for me. See example code below:

C++

#include <iostream>
#include <string>
#include <algorithm>
using namespace std;

int main() {
	string s = "Hello, world!";
	replace(s.begin(), s.end(), "l", "i");
	cout << s << endl;
	return 0;
}

There are no syntax errors with the code, but the compiler is still throwing me the following errors:

In file included from /usr/include/c++/4.9/algorithm:62:0,
                 from prog.cpp:3:
/usr/include/c++/4.9/bits/stl_algo.h: In instantiation of 'void std::replace(_FIter, _FIter, const _Tp&, const _Tp&) [with _FIter = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >; _Tp = char [2]]':
prog.cpp:8:38:   required from here
/usr/include/c++/4.9/bits/stl_algo.h:4234:15: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
  if (*__first == __old_value)
               ^
/usr/include/c++/4.9/bits/stl_algo.h:4235:13: error: invalid conversion from 'const char*' to 'char' [-fpermissive]
    *__first = __new_value;
             ^

Thanks, all!

What I have tried:

I have tried multiple versions of this, but nothing seems to work. Google just gives me the template, which I already know.

Posted 18-Aug-16 17:52pm

Anti-Antidote

Updated 18-Aug-16 21:32pm

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Jochen Arndt · Answer 1 · 2016-08-18T21:32:00

Solution 1

Your call replace(iterator, iterator, const char*, const char *) does not match the std::replace template:

C++

template<typename _FIter, typename _Tp>
  void
  replace(_FIter, _FIter, const _Tp&, const _Tp&);

_Tp is char here because the std::string iterators are pointing to char.

So you must use

replace(s.begin(), s.end(), 'l', 'i');

Posted 18-Aug-16 21:32pm

Jochen Arndt

Comments

Anti-Antidote 19-Aug-16 8:28am

...would I need to use char s rather than string s?

Jochen Arndt 19-Aug-16 8:48am

No.

You still did not get it. I will try to explain it in a different way:

The replace() template works with an iterator and the corresponding type.
The first thing is to understand what an iterator is. From http://www.cplusplus.com/reference/iterator/:

"An iterator is any object that, pointing to some element in a range of elements (such as an array or a container), has the ability to iterate through the elements of that range using a set of operators (with at least the increment (++) and dereference (*) operators)."

In the case of std::string you have an array of characters. So the type (_Tp) is char.

Or just think about what you get when using the iterator to access the element:

std::string s = "Hello, world!";
cout << "The first char is " << *s.begin() << endl;

Anti-Antidote 19-Aug-16 10:29am

Using *s.begin() and *s.end() aren't working for me... am I still doing it wrong?