switch within if statement vs if/elseif efficiency

Hi!

I'm doing an assignment where I have to code Conway's Game of Life.

Currently I'm attempting to create a GUI for ease of entering initial conditions.

prev=false(m,n)
[rows,cols]=size(prev);
clf
plotroutine(prev)
title({'Instructions'})
gate=true;
while gate==true
    [x,y,key]=ginput(1);
    if key~=[]  %This is line 9!
    switch key
        case 1
            if 0<x&&x<cols&&0<y&&y<rows
                col=ceil(x);
                row=rows-floor(y);
                prev(row,col)=~prev(row,col);
                clf
                plotroutine(prev)
                title({'Instructions'})

              end
          case 114
              prev=logical(randi([0,1],rows,cols));
              clf
              plotroutine(prev)
              title({'Instructions'})

          case 32
              gate=false;
              title([])
      end
      end

While I was testing edge case scenarios, more specifically the [Enter] and [Windows] key, ginput returned []. However, this is not a valid argument for switch. Thus I had to write line 9 instead.

My question is whether this implementation, or merging the if and switch statements to a single if/elseif statment is more efficient/preferrable from a best practice point of view?

[Stephen23]

The comparison key~=[] does not do what you seem to think it does: it will return an empty output whenever key is scalar or the same size as []. There does not seem to be any point to that. Much better would be to test if key is empty using standard MATLAB commands:

if ~isempty(key)

[Shuai Shao]

Oh, the function managed to behave as I'd expected despite the error so didn't notice, thanks!

Still, would you recommend using the switch statment (many online resources seem to advocate for it), or get rid of the nesting and use a single if, elseif, elseif, elseif?

[Stephen23]

@Shuai Shao: I don't see any problem with using if and nested switch, if it makes the code intent clear. Both of these are going to be fast, so I don't think you should be prematurely optimizing this code.

[Shuai Shao]

Okay, thanks for your input!

Answer 1

[Stephen23]

Use whichever one is clearest for understanding the program. You will spend more time reading and fixing the code than one if statement will spend running.

https://shreevatsa.wordpress.com/2008/05/16/premature-optimization-is-the-root-of-all-evil/

Answer 2

[Steven Lord]

Your code doesn't handle the case where the user does something to finish the ginput call other than click the left mouse button, press the 'r' key, or press the space key. Therefore the while loop simply continues. If you want a "catch all" that lets you handle those other cases differently than simply continuing on, use the otherwise keyword.

Inside the otherwise block you could:

• check if key isempty as Stephen suggested
• display some text telling the user what they need to do
• issue a warning
• throw an error
• let the while loop simply continue while the otherwise block makes it explicitly clear that that's how you want those other cases to be handled
• or some combination of any or all of the above.

[Shuai Shao]

I think you might've misunderstood the question slightly.

In my implementation the key variable is the argument for the switch statement. Since a 0x0 empty array isn't a valid argument for a switch, I wouldn't be able to check key isempty in an otherwise-keyword in this switch.

As far as I've understood I'm pretty much forced to use an if statement with isempty(key) as argument and redirect to the current switch statement should it not be the case in order to prevent the switch from crashing the program if the user presses an key which returns [], e.g. [Enter].

My question was simply how to handle the flow control;

Is it preferrable to have this nested structure with a switch statement, whose use many seem to advocate.

or

Compile the structure into a single if statement, as such.

    if isempty(key)

      elseif key==1&&0<x&&x<cols&&0<y&&y<rows
   *do something*
      elseif key==114
   *do something*
      elseif key==32
   *do something*

As for the need to "catch all", my intention was to let the while loop continue indefinitely until the user pressed spacebar as the full 'instructions' reads:

title({'[LMB] - edits the state of induvidual cells','[R] - generates random values for all cells','[Space] - finalises the initial configuration'})

Sending an error message every time the user presses an insignificant key seems redundant, I simply don't want the program to crash in case the user does (Solved by Stephen C.).

He also provided an answer to the main question, however, I don't know how to mark a topic as solved if the answer was in a comment.

(Edit: The user is selecting cells which are to be alive for the initial state of Conway's Game of Life in case that wasn't clear.)

[Steven Lord]

Ah yes, I had forgotten about switch requiring the expression to be a scalar or a char vector.

You could combine the two.

if isempty(key)
  % handle the empty case
else
  switch key
  % add case and/or otherwise blocks here
  end
end

or

if ~isempty(key)
  switch key
    case 1
    % etc
  end
end