if else in a loop

Hi, I have a variable extracted from a filename that I need to use to map the data in that file to a location in a big data set so that I can plot the whole dataset. The variable is a location variable. However it takes the range 1 to 4, and is also dependant on another value extracted from the filename for its final location in the dataset.

modules 1&3 are in a straight line, modules 2&4 are behind 1&3 and offset to lay between them

4_2.

_3_1

arrangement. Anyway, the problem should be easier to see in the code...

name={'Y1Z1','Y1Z3','Y2Z2','Y2Z4','Y3Z1','Y3Z3','Y4Z2','Y4Z4'}
Sizzy=size(name)
S=Sizzy(1,2)
rowCount=nan(S,1)

for k=1:S
    A=name{k};

      modn=str2double(A(1,2));
      rown=str2double(A(1,4));

      if modn==2|4
          rown==rown+4
      else modn==1|3
          rown==rown
      end

      rowCount(k)=rown
  end

which gives

rowCount =

          1.00          3.00          2.00          4.00          1.00          3.00          2.00          4.00

whereas I need

rowCount =

          1.00          3.00          6.00          8.00          1.00          3.00          6.00          8.00

I think the problem is with my if if else statement, maybe a misuse of the logical or, but I cannot see what I am doing wrong.

Best regards,

Steve

Answer 1

[Walter Roberson]

if modn==2|4

means the same as

if ((modn==2)|4)

The result of the modn==2 test is either 0 (false) or 1 (true), and you then or() that 0 or 1 with 4. 4 is a non-zero value so it is considered true, 1. You are therefore or() 0 or 1 with 1, and the results of that will always be true.

I am not aware of any computer language that has a comparison operation in the form you gave.

MATLAB's test to determine whether a value is any of a list of values is the ismember() function.

[Stephen Devlin]

Ah, so if I'd used dismember and got a logical 1 I could then have used that, sorry. For some reason 'if modn==2|4' looks like it made sense to me but I didn't appreciate the precedence rules etc. Thank you Walter.

Answer 2

[Stephen23]

This syntax does not do what you think it does:

modn==2|4

MATLAB will execute these from left to right, giving:

(modn==2)|4

equivalent to:

0|4   or   1|4

which will clearly always be true (any non-zero value is true in MATLAB, so 4 is clearly true).

Read more about the sequence of operations here:

https://www.mathworks.com/help/matlab/matlab_prog/operator-precedence.html

You might like to try something like this:

name = {'Y1Z1','Y1Z3','Y2Z2','Y2Z4','Y3Z1','Y3Z3','Y4Z2','Y4Z4'};
S = numel(name);
Z = nan(S,1);
for k = 1:S
    A = name{k};
    modn = str2double(A(2));
    rown = str2double(A(4));
    Z(k) = rown + 4*~mod(modn,2);
end

[Stephen Devlin]

Hi Stephen, I'll accept this, what does "4*~mod(modn,2)" mean.

I'm seeing 4 multiplied by NOT modulus of modn second column, is that correct?

[Stephen Devlin]

Thank you Stephen, this works perfectly.