Subtracting Strings

[Josh]

I am having problems subtracting two strings.

Let's say string1 is: acbf and string2 is: abcde

How do I return the characters that are left? Do I have to use substr?

This should leave just character f. Even though they are not in order those characters where still present in that string.

This is c++ I am doing it in.

[Chris]

Do you need efficiency on a large scale (i.e. comparing things at a size of 10k characters)? If so, I'd look into use a hash table ("map" I think in C++. Minimal experience with it). More details on that here under Unit 3: Hashing. The recitation notes might prove more useful than the lecture notes.

Also, how do you want to consider duplicates?... That's important

Anyways, if you're comparing -very- short strings, it's simplest to just do something like this (Written in pseudo-code/python):

for char1 in string1: # Loop over characters of string 1
   foundchar = false
   for char2 in string2: # Same for string 2
      if char1 == char2:
          foundchar = true
          delete char2 from string2 # This way we don't run into the character again. This depends on how you
           want to handle duplicates
          break
   # Outside the for now...
   if foundchar == false:
      print char1 + " is not in string 2"

print "String 2 contains " + string2 + " which are not in string 1" # We deleted chars as we found them, so this
 will contain only chars not found in string 1.

[Josh]

Thanks, that gives me some sort of idea. I was thinking most of this I just having problems implementing it. Hopefully I will get it soon.