Question

This particular assignment has to do with removing substrings from strings; I am trying some of the Stanford SEE courses online to learn some new languages.

What I’ve got so far is below, but if text = "hello hello" and remove ="el", it gets stuck in a loop, but if i change text to text = "hello hllo", it works, making me think I’m doing something obviously stupid.

There is a stipulation in the assignment not to modify the incoming strings, and instead to return a new string.

string CensorString1(string text, string remove){
    string returned;
    size_t found=0, lastfound=0;
    found = (text.substr(lastfound,text.size())).find(remove);
    while (string::npos != found ){
        returned += text.substr(lastfound,found);
        lastfound = found + remove.size();
        found = (text.substr(lastfound,text.size())).find(remove);
    }
    returned += text.substr(lastfound,found);
    return returned;
}

Guidance would be appreciated 🙂 Thanks

UPDATE

Took the very kind advice given and modified my code to this :

string CensorString1(string text, string remove){
string returned;
size_t found=0, lastfound=0;
found = text.find(remove);
while (string::npos != found ){
    returned += text.substr(lastfound,found);
    lastfound = found + remove.length();
    found = text.find(remove,lastfound);
}
returned += text.substr(lastfound);
return returned;
}

But still behaves the same

Any more ideas folks?

Answer 1

found = (text.substr(lastfound,text.size())).find(remove); is incorrect. It returns the index of the searched string in text.substr(lastfound,text.size()), but not in text.

You should perhaps change this to found = text.find(text, lastfound);.

Besides of being incorrect, taking a substring (this means, allocating a new string) and calculating an index in it is quite inefficient, unless the optimizer is super-smart.

Moreover, the final returned += text.substr(lastfound,found); is incorrect too: you need to add the last chunk of the text, not the one till found index (which is most likely empty, as lastfound can be smaller than found. Better would be returned += text.substr(lastfound);

Edit:
In the second example, you need to replace returned += text.substr(lastfound,found);
with returned += text.substr(lastfound,found-lastfound);. The second argument to substr is length, not position.

With this change, the test example runs fine in my test program.

(Addition by J.F. Sebastian:)

string CensorString1(string const& text, string const& remove){
  string returned;
  size_t found = string::npos, lastfound = 0;
  do {
    found = text.find(remove, lastfound);
    returned += text.substr(lastfound, found-lastfound);
    lastfound = found + remove.size();
  } while(found != string::npos);
  return returned;
}