Thread: For-looping, if-else statements, charAt(), etc. Beginner programming problem

This is what I was assigned:

Programming Problem: Wild Equality

For this assignment you are to write a single class, with main included. The job of the class is as follows: you are to read in two strings from the keyboard, string1 and string2, and then you are to determine if string1 and string2 are equal, character for character. Your program should report its answer as true or false. Note that if the strings are different lengths, then they are definitely not equal.

There are two "catches" to the problem. First, capitalization in your solution should not matter. Thus if the strings are "dog" and "DoG", then your program should report true - they are equal. The second catch concerns a wild card symbol, here the character '*'. If * appears in one of the strings, then it can match any other character. So "Dog" and D*g" should be judged as equal, since the *, as a wild card, matches any other character, in this case the 'o'.

More examples:

Dog and d** -> true
Dog and *** -> true
Dog and **** -> false (different lengths)
bla*k and B***k -> true


One further suggestion: use theScanner method nextLine() rather than the method next() for reading input.

Your solution should be in a single class containing main, and that class MUST BE called WildEquality. Enter (paste) your code in the box below. Note that the import statement for Scanner is provided. DO NOT include any import statements in the code you submit.

import java.util.Scanner;

Okay. So I imported the scanner easily, and I also converted both strings to lowercase already. With the code I have now, I can type in two of the same word, with DIFFERENT capitalization, and the program recognizes that they are the same word. I am STILL having problems with the wildcard method however.

import java.util.Scanner;
public class WildEquality{
  public static void main(String[] args){
    Scanner scan=new Scanner(System.in); //creates a Scanner object
    String string1=scan.nextLine();
    String string2=scan.nextLine();
    string1=string1.toLowerCase();
    string2=string2.toLowerCase();
    string1.toCharArray();
    string2.toCharArray();
    char wildcard='*';
    for (int j=0; j<string1.length();j++){
      if (string1.charAt(j)==wildcard) wildcard=string2.charAt(j);
    }
    for (int k=0; k<string2.length();k++){
      if (string2.charAt(k)==wildcard) wildcard=string1.charAt(k);
    }
    if ((string1.length()==string2.length())&&(string1.equals(string2))) {
      System.out.println("true");
    } else {
      System.out.println("false");
    }
  }
}

This is what I have so far. This wildcard stuff is confusing me so much. I would literally appreciate ANY feedback! Thanks in advance!

The easiest way to tackle this might be to check first if *either* char is a wildcard, and if it is go on to the next char.

Originally Posted by Sean4u

The easiest way to tackle this might be to check first if *either* char is a wildcard, and if it is go on to the next char.

So wait, how would I check for something like that? A for loop that can check both strings at once and then ONLY continue if there is a wildcard? Maybe I misunderstood what you were saying, but I've literally been in this course for three weeks and we just started for looping this week, so it's still a little fuzzy! Thank you though!

You should only be using one for loop for this like (pseudo code)

check strings are same length
for (all characters or positions in one of the strings)
  if (character in first string is *)
    great!
  else if (character in second string is *)
    great!
  else if (character in first string is same as character in second string)
    great!
  else
    strings are different - stop comparing

Originally Posted by Sean4u

You should only be using one for loop for this like (pseudo code)

check strings are same length
for (all characters or positions in one of the strings)
  if (character in first string is *)
    great!
  else if (character in second string is *)
    great!
  else if (character in first string is same as character in second string)
    great!
  else
    strings are different - stop comparing

Okay, so I already have part of the code that will check to see if the strings are the same length, that was the easy part. I've done this from the pseudo code you provided, but I'm still confused as to what I should be putting in place of the "great!"s. If those statements are true, then great! but I'm not sure how to translate something like that into code!

    for (int j=0; j<string1.length();j++){
      if (string1.charAt(j)==wildcard) 
        wildcard=string2.charAt(j); 
      else 
        if (string2.charAt(j)==wildcard)_________
      else
        if (string1.charAt(j)==string2.charAt(j))__________
      else 
        string1!=string2; __________
    }

The string1!=string2; at the end does not work, I'm aware. I'm sure there's a very simple reason why, but I figured it'd be fine just as a placeholder for now until I figure out the wildcard part. Blahh, thank you for being to patient!

'If' conditions evaluate to a boolean - one of two values. If you're studying computing, you should probably read up on DeMorgan's Laws for the background to why this code works out this way, but you can either try to invalidate the inputs and nest those ifs:

if (/* char in string1 is not '*' */)
  if (/* char in string2 is not '*' */)
    if (/* chars don't match */)
      fail

or AND them:

if (/* char in string1 is not '*' */ AND /* char in string2 is not '*' */ AND /* chars don't match */)
  fail

or you can look from the point of attempting to validate the inputs, in which case any valid condition is enough. In these kinds of cases I often - maybe it's not the most stylish thing to do - use else if and 'do nothing' as long as a valid condition is met:

if (/* string1's char is * */)
{ /* do nothing */ }
else if (/* string2's char is * */)
{ /* that's cool! */ }
else if (/* the two chars match */)
{ /* excellent! */ }
else
  fail

the comments in curly brackets are valid Java, not much else is in that code. The idea is that the loop should keep checking the remaining characters as long as the ones tested so far are good. As soon as the 'else' option is hit then you need to (usually) somehow remember or report that the input is invalid, and terminate the loop at that point (there's no point in continuing). Forum rules say I'm not allowed to spoonfeed, but you can exit a loop with the 'break' statement. Don't tell anybody I told you - but do put the link to this thread in your homework assignment if you get anything of value from it. If you surprise your tutor with excellent code, she will also want to check that someone didn't just give it to you like pureed tasty vegetables.

You would normally use that 'break' - or put the validity condition in your loop expression (for, while, do etc) - so that your loop stops as soon as the input is invalid because there's no more point in checking.

output has way too many "true"s.

It looks like the indentation in your code was meant to associate the two lines of code together:

      else
        result=false;
        System.out.println(result);
      }

Java doesn't work like that - it needs { and } to create blocks of code. (I think Python may work like that and perhaps a few other languages). That last } in your code snippet above is the closing curly bracket for the 'for' loop. Since you set 'result' to flag that the input is invalid, you could wait until the for loop terminates and then do System.out.prin... - just switch the last two lines above with each other so that the System.out.prin... is after the for loop's closing curly bracket.

Okay, that makes sense. So now I have:

import java.util.Scanner;
public class WildEquality{
  public static void main(String[] args){
    Scanner scan=new Scanner(System.in); //creates a Scanner object
    String string1=scan.nextLine();
    String string2=scan.nextLine();
    string1=string1.toLowerCase();
    string2=string2.toLowerCase();
    boolean result=false;
    if (string1.length()!=string2.length()){
      result=false;
    }
    else
      for (int j=0; j<string1.length();j++){
      if (string1.charAt(j)==string2.charAt(j)){
        result=true;
      }
      else if (string1.charAt(j)=='*')
      {}
      else if (string2.charAt(j)=='*')
      {}
      else
        result=false;
      }
      System.out.println(result);
    }
  }

The code works for every example the assignment requires EXCEPT this one:

Dog and *** -> true

And I can't figure out why. The loop should check and ignore every "*" starting with the character at position 0... so I'm not sure why it's doing this. Any idea?

I'm not sure why you picked that example - it seems like it should result in 'true'. I notice your logic is not quite right though. Your loop tests each character in turn to see if it is invalid for two-strings-matching. If *any* character is bad, then the whole match is bad. Otherwise, the strings are good. 'Otherwise' means you should default your flag to good. Your code should be:

flag = valid
for (all chars)
  if (char fails test)
    flag = invalid
if (flag is valid)
  "The strings match!"
else
  "That input is no good"

Nowhere should there be an instruction which could reset the flag back to valid if a previous validity test had set it to invalid

Ahhh, thank you so much! Seriously, you have been a HUGE help, and I think all the examples work now! Thanks again! I reallllly appreciate it!

The boolean result value should be originally set to true rather than false.

Where you have;

boolean result = false;

You should have;

boolean result = true;

If you do this the original value is set to true so "dog" and "***" will result in true rather than false. I struggled with this program for a while too. I'm in Moll's class also.

Kevin