switch within if statement vs if/elseif efficiency

2017 11 10
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更新时间：2017 11 10
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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?

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Stephen23
Stephen23 2017-11-10
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
Shuai Shao 2017-11-10
编辑：Shuai Shao 2017-11-10
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
Stephen23 2017-11-10
编辑：Stephen23 2017-11-10
@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
Shuai Shao 2017-11-10
Okay, thanks for your input!

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Stephen23
Stephen23 2017-11-10

1 个投票

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/

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Steven Lord
Steven Lord 2017-11-10

1 个投票

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.

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Shuai Shao
Shuai Shao 2017-11-10
编辑：Shuai Shao 2017-11-10
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
Steven Lord 2017-11-10
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

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