What is valid matlab syntax in if statement?

Question

Clarisha Nijman 2018-7-17

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评论： Clarisha Nijman 2018-7-17

If I use "==" in the 9th line of this code it says: inappropiate use of == operator. If I use "=" I am getting a parse error.

CODE:

for j=3:size(A,2)
   StartTijd=A(1,2);
   tijdIndex=1;
    for i=1:(size(A,1)-1)
        if (A(i,j)-A(i+1,j))==1
           Parkingtime(tijdIndex,j)=A(i+1,2)-tijd;%service time
           tijd=(A(i+1,2));
        else
           A(i,j)-A(i+1,j)==-1;
           StartTijd=A(i+1,2);
           tijdIndex=i+1;
             if Parkingtime(i+1,j)<0
                Parkingtime(i+1,j)=0;   
             end      
          end    
      end  
  end

[SL: applied code formatting. Clarisha Nijman, in the future please use the {}Code button to format your code so it's easier to read.]

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Pawel Jastrzebski 2018-7-17

编辑：Pawel Jastrzebski 2018-7-17

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In your 'if' you're checking for the condition, hence '=='

if (A(i,j)-A(i+1,j))==1

And also you've got whole left side encompassed with the bracket (you don't have that with the 'else' bit)

if (L-side expression) == 1

Now further down the code, if 'if' not met, the code executes 'else' bit. In my opinion there's no reason for the comparison - it's the consequence of 'if' not being met. I think the code should like like this:

(A(i,j)-A(i+1,j)) = -1;

Which is simply assigning -1 to the expression on the left hand side.

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Answer 1

Steven Lord 2018-7-17

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What do you expect line 9, which is A(i,j)-A(i+1,j)==-1;, to do? Do you want to run lines 10-15 only if that condition is satisfied? If so, what you should do depends on whether A(i, j)-A(i+1, j) can only take on the values 1 or -1 or if it can take on other values (like 0) as well.

If that difference can ONLY be 1 or -1, you don't need the check. If you reach the else you know the difference was not 1, so by process of elimination it must be -1.

If the difference can be something other than 1 or -1, you probably want to use an elseif statement. The first example on the documentation page for the if keyword shows how to use elseif to check for a series of conditions in turn.

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Clarisha Nijman 2018-7-17

Thank you for the feedback. It helped me. The problem was indeed the if. Instead of else it should be elseif. The difference can take value -1, 0, and 1. If the difference equals zero I do not want matlab to do anything.

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