MATLAB: What is valid matlab syntax in if statement

parse error "=" in if statement

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.]

Best Answer

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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