replace_if in string to replace

Question

I was solving one simple problem with the use of replace_if but every time it gives me error. Problem:
in a given string remove the adjacent character and make string as compact as possible.
Ex: aabcc will become b and aabbcc will become empty string.

int main()
{
    string s;
    cin>>s;    
    replace_if(s.begin(),s.end(),[](char l, char r){return l == r;},"");
    cout << string(s.begin(),s.end());   
    return 0;
}

Error:

/usr/include/c++/6/bits/stl_algo.h:4280:12: note:   candidate expects 3 arguments, 2 provided  
solution.cc:19:51: note: candidate:  
main()::<lambda(char, char)> 
replace_if(s.begin(),s.end(),[](char l, char r){

What error are you getting? Have you read any [documentation of `replace_if`](http://en.cppreference.com/w/cpp/algorithm/replace)? — Angew is no longer proud of SO, May 19 '17 at 17:02
@InvI `std::replace_if` can't change the size of the string, it can only change elements from one value to another. Look for another method. — Blastfurnace, May 19 '17 at 17:08
It would be helpful for everyone if you followed the guidelines here: https://stackoverflow.com/help/mcve — Rook, May 19 '17 at 17:08
@Blastfurnace Thanks now i know why this function is not working. — InvI, May 19 '17 at 17:12
Why would you try to use algo with `replace` in its name to remove elements? — Slava, May 19 '17 at 17:12
@Slava i got confuse to replace element with blank.but now clear..Thanks — InvI, May 19 '17 at 17:15
That you've read the documentation but didn't understand it should be mentioned in your question, ideally spcifying what exactly you didn't understand (if you're able to tell). — Angew is no longer proud of SO, May 19 '17 at 17:16
Unrelated to the problem at hand, but you might be interested in our list of [good C++ books](http://stackoverflow.com/q/388242/1782465). — Angew is no longer proud of SO, May 19 '17 at 17:16
I do not think `std::remove_if` is easy solvable for your task, it is possible but overhead would be bigger than writing loop — Slava, May 19 '17 at 17:25

score 4 · Answer 1 · answered May 19 '17 at 17:25

std::replace_if operates on a sequence of values of a certain type (in your case, of type char), and can be used to examine each value in the sequence and potentially replace it with another value of the same type. In other words, if allows you to replace all as with bs, but doesn't allow removing elements or basing the predicate on more than the inspected element itself.

What you need is a bit more complicated to compose, but could be done e.g. like this:

auto it = s.begin();
for (;;)
{
  it = std::adjacent_find(it, s.end()); // Find two adjacent same characters
  if (it == s.end()) // If there are none, we're done
    break;
  auto next = std::find_if(it, s.end(), [it](char c) { return c != *it; }); // Find next different character
  it = s.erase(it, next); // Erase all in range [it, next)
}

I think just loop without algos would be simpler, more readable and efficient — Slava, May 19 '17 at 17:26
@Slava I disagree, since keeping track of adjacent indices/iterators is prone to off-by-one errors. And as far as efficiency goes, I'd assume it to be identical: it's all just one iteration, after all. — Angew is no longer proud of SO, May 19 '17 at 17:37

replace_if in string to replace

1 Answers1