Why doesn't my for loop work?

I am trying to do an assignment and I cannot seem to figure out why the code below does not work. I know how to do it with just indexing arrays but they want me to use a for loop. I used almost an identical code for a different assignment except the if statement's relational operator was < instead of == so I have no idea why this isn't working! Please help.

Assignment: Assign numMatches with the number of elements in userValues that equal matchValue. Ex: If matchValue = 2 and userVals = [2, 2, 1, 2], then numMatches = 3.

    % function numMatches = FindValues(userValues, matchValue)
% userValues: User defined array of values  
% matchValue: Desired match value    
    arraySize = 4;     % Number of elements in userValues array
    numMatches = 0;    % Number of elements that equal desired match value
    % Array solution (rather than loop):
    for (i = 1:arraySize)
        if (userValues(i) == matchValue)
            userValues(i) = userValues(i);
        else
            userValues(i) = [];
        end
    end
       numMatches = numel(userValues);
end

[Charles Teasley]

OK, the line that says "userValues(i) = userValues(i);" is incorrect. It should be something like "numMatches = numMatches+1;". Also, get rid of the "else" statement altogether. At the end of the for loop, numMatches is automatically returned with your answer. The code I used is as follows:

********Code

function numMatches = FindValues(userValues,matchValue)

arraySize = 4;
numMatches = 0;

    for ( i = 1:arraySize)
        if (userValues(i) == matchValue) % Test for a match
          numMatches = numMatches +1; % If values match increment the 
                                      % counter
        end % End If
    end % End for
  end % End function

[Mae]

Thank you!

As per your suggestion I came up with the following

function numMatches = FindValues(userValues, matchValue)
  % userValues: User defined array of values  
  % matchValue: Desired match value    
      arraySize = 4;     % Number of elements in userValues array
      numMatches = numel(userValues);    % Number of elements that equal desired match value
  % Array solution (rather than loop):
  for (i = 1:arraySize)
      if (userValues(i) ~= matchValue)
          numMatches = numMatches -1;
      end
  end
end

[Charles Teasley]

Hello Mae,

It looks very close. I will make some notes using your code.

arraySize = 4;     % Number of elements in userValues array

"arraySize" declares the number of elements in the array. Why set this here when you get the same number on the next line?

numMatches = numel(userValues);

Use the "numel(userValues)" function to get the size of the array, and set "arraySize" to the "numMatches" value. This way the size of the "userValues" can be dynamic if/when calling this function. You did the count the opposite way I did, but it should work. Did you get everything working?

Answer 1

[Stefan Raab]

Hello,

I think the error is in this assignment:

userValues(i) = [];

This will not just delete the value from the array but also the element. If in your example the for loop runs 4 times and for example the 3rd value does not match your matchValue, in the 4th loop run there won't be a userValues(4) as the vector got shortened before. This would cause an error. You could use

userValues(i) = NaN;

instead and in the end you could find the number of matching elements with

numMatches = sum(double(~isnan(userValues)));

But in general I think this could be made much easier by using indexing as you already said. The following code should be equal to the whole for loop structure:

numel(userValues(userValues==matchValues))

Hope this helps.

Kind regards, Stefan

Answer 2

[J. Webster]

The method you are using...remove all elements in the array that aren't matchValue, then when done, counting the number of elements in the array, is not a good way to do this. Instead of the numel method, try looking at your code.

This line...

userValues(i) = userValues(i);

does nothing, right? it just sets something equal to itself. so that can go too.

After that, you're actually really close. When the match value equals a uservalue, you want to add one to the variable numMatches...

numMatches = numMatches+1;

You just need to figure out where to put that.