Thread: Comparing strings problem.

Hello!

So I have a problem with finishing this task from C++ Primer 5th edi, Chapter 9:

Write a function that takes three strings, s, oldVal, and newVal. Using iterators, and the insert and erase functions replace all instances of oldVal that appear in s by newVal. Test your function by using it to replace common abbreviations, such as “tho” by “though” and “thru” by “through”.

if I know the string s, it would be rather easy to do the task, because then I would know the positions of where to insert and erase right? Problem is, the task isnt specified, and I had a lot of trouble figuring out a way of comparing two ranges of iterators or comparing two words and etc etc.

I have made a function that works, but it uses stringstreams and no iterators or erase and insert functions... Does anyone know a good way of doing the task?

Code:

string abbrev(string s, string oldVal, string newVal)
{
    string word, newString;
    istringstream val(s);
    while (val >> word)
    {
        if (word == oldVal)
            word = newVal;
        newString += word + " ";
    }
    return newString;
}

This is the function I have written, just to show some work.

Your function won't work because if I specify abbrev("Hello World", "Hel", "Bel"), it won't change anything.
Unfortunately, to my knowledge, there are no algorithms in the standard library that allows you to search a range where both the input what to search and the token to find are both given as iterators, so you may have to do it the old-fashioned way.
An iterator represents a character in a string, so given that you can iterate through each character in a string, given some input i and something to find f, how would you go about finding this f in i?

Originally Posted by Iceboxes

if I know the string s, it would be rather easy to do the task, because then I would know the positions of where to insert and erase right?

Yes, and you do indeed "know" the string s since it is an argument to the function.

Originally Posted by Iceboxes

Problem is, the task isnt specified, and I had a lot of trouble figuring out a way of comparing two ranges of iterators or comparing two words and etc etc.

The task appears to be specified: "replace all instances of oldVal that appear in s by newVal".

Originally Posted by Iceboxes

I have made a function that works, but it uses stringstreams and no iterators or erase and insert functions... Does anyone know a good way of doing the task?

Start by observing what is wrong with your function. Your instructions recommend: 'Test your function by using it to replace common abbreviations, such as "tho" by "though" and "thru" by "through".' This should be easy, e.g.,

Code:

int main()
{
    string text = "tho I did not study,           I pulled thru.";
    text = abbrev(text, "tho", "though");
    text = abbrev(text, "thru", "through");
    cout << text << endl;
}

Observe that the result using your implementationm of abbrev is:

Code:

though I did not study, I pulled thru.

Obviously, "thru" was not replaced by "through" because it was parsed as "thru.". Furthermore, the spaces after the comma were not retained, which might not be a problem, but technically violates the requirements.

You can find the positions of the search string by using the find member function of std::string, though that is index-based rather than iterator based.