Value not multiplying by -1

I have a nested if statement which works as it is supposed to until the last iteration.

Does anyone know why it is not multiplying the last value by -1?

w = x_0     % zero vector 
k = 0       % zero value
i=1         % initial value for count
j=-1        % coefficient

    for i = 1:n

      if Raw_Data(i,2) == Raw_Data (i,3)
          fprintf ('ERROR!!!!!)
      else if Raw_Data(i,3) == val            
              w(i,1) = Raw_Data(i,4)          
          else if Raw_Data(i,2) == val        
                  w(i,1) =  j* Raw_Data(i,4)   

                  % Rationalisation Not Working

              else  w(i,1)= j* Raw_Data(i,4)                        

              end
          end
      end
      i=i+1
  end

[Adam]

Where does 'val' come from?

[Jay]

Val is specified in previous code used prior.

I have not added it in yet as I was using it with prior code.

I use val to = 1 for this subroutine.

[Adam]

So since you used breakpoints in the code where exactly is the problem? Is it not going into the relevant else-if statement or is the code in there not executing correctly?

[Jay]

The code is not executing correctly.

Namely the

elseif Raw_Data(i,2) == val
w(i,1)= j* Raw_Data(i,4)

The element is not multiplying by -1.

The value is already -ve so I want to multiply it by -1 again to = +ve value.

Answer 1

[Thorsten]

I would suggest to avoid the loop and use logical indexing:

ind = Raw_data(i,3) == val;
w(ind) = Raw_data(:,4);
w(~ind) = j*Raw_data(:,4);

if any(Raw_Data(i,2) == Raw_Data (i,3))
 error('values in column 2 and 3 are identical.')  
end

Note that in your code

else if Raw_Data(i,2) == val

is not needed, because

w(i,1) =  j* Raw_Data(i,4)

in any case.

And if w is a vector, w(i) is the same as w(1,i).

Answer 2

[Ingrid]

in a for loop there is no need to increment the i so just leave out

    i = i+1

as this is done automatically.

Also do not use the i as this can be confusing when using imaginary numbers, better practice is to use ii or something completly else (for example counter )

It is not clear why there is an

else if Raw_Data(i,2) == val

when you perform the same function regardless of true/false of this expression

[Jay]

I will leave out the incremental addition.

I will also change the variable from i to something more suitable.

I don't see how

else if Raw_Data(i,2) == val

is performed regardless, val is a dynamic variable.

else  w(i,1)= j* Raw_Data(i,4)

was used to try and rectify the issue of the final element not multiplying by -1.

Answer 3

[Ilham Hardy]

Learn on how to debug your code,

http://nl.mathworks.com/help/matlab/matlab_prog/debugging-process-and-features.html#brqxeeu-189

[Jay]

I use break points to run through the code.

This is how I know where the problem originates from.

[Ilham Hardy]

Ok, did you also notice the mlint warning on your code?

e.g.:

There should not be empty char (read: space) between else and if. So, elseif instead of else if .

[Jay]

Ok, I will do that from now on.

It does not throw an error if I use else if. Is there a difference in the function elseif and else if?