Comments on: A Logical Story

By: Ed Limbaugh

Ed Limbaugh — Wed, 01 Nov 2006 22:00:17 +0000

Ahhhh. Thanks for clarifying! I understand more clearly now.

By: Loren

Loren — Wed, 01 Nov 2006 21:48:22 +0000

Ed-

The reason the 2nd one doesn’t need to do scalar expansion is because it is true itself before doing the next evaluation
( xxx | [] ) since 4 is nonzero. The short-circuiting happens as soon as truth can be determined.

This was meant to demonstrate that you are MUCH safer using || and && and reducing your expressions to scalar values yourself. Your code will more likely be readable later and do as you expect. Using the arrays versions, | and &, lead to weirdness, especially when empty arrays might be around.

–Loren

By: Ed Limbaugh

Ed Limbaugh — Wed, 01 Nov 2006 21:43:23 +0000

If it is a scalar expansion, why does that not take place in the first example, ‘if 4 | []‘? Are the two cases treated differently with respect to operator priority? What are the rules for the priority of evaluating the operators in an ‘if’ or ‘while’ statement? Why would they differ in the two examples?

By: Loren

Loren — Wed, 01 Nov 2006 20:48:55 +0000

All-

I didn’t explain this well, I think. What happens is that

[] | 4 does a scalar expansion, ends up [] and therefore evaluates to false. So it’s not the short-circuit that causes the answer, but the scalar expansion. I will update the article to indicate this.

–Loren

By: Ed Limbaugh

Ed Limbaugh — Wed, 01 Nov 2006 20:35:55 +0000

Wouldn’t the defintion of ‘short-circuit’ (with respect to ‘if’ or ‘while’) be to stop evaluating arguments to an ‘or’ or ‘and’ operator as soon as the final result of the operation can be definitively determined? Therefore, an ‘or’ operator (|) should continue evaluating arguments until
1) it comes to the first TRUE argument, or
2) it has evaluated ALL of the arguments as false.

Similarly, I would expect an ‘and’ operator (&) to continue evaluating arguments until
1) it comes to the first FALSE argument, or
2) it has evaluated ALL of the arguments as true.

In the first example (‘if 4 | []‘), the ‘or’ operator short-circuits after evaluating 4 as a true argument. In the second example (‘if [] | 4′), why should ‘or’ stop evaluating? The first argument is false, as you’ve explained: “Empty arrays evaluate to false for the purposes of if and while.” The ‘or’ operator should then continue to evaluate as it has not yet determined whether the remaining arguments are true or false.

By: Bjorn Gustavsson

Bjorn Gustavsson — Wed, 01 Nov 2006 18:55:47 +0000

How can you have ‘if’ shortcut ‘|’ with something that is false?
In my (rapidly made up) opinion this is a severe bug. Lets compare:

if 0 | 4
disp(‘Must be true’)
else
disp(‘Must be false’)
end
Displays:
Must be true

As it should, since | is true since 4 is true. No shortcut in this case.

By: Oliver A. Chapman, PE

Oliver A. Chapman, PE — Wed, 01 Nov 2006 17:04:17 +0000

There must be some performance benefit to justify, “The nonscalar | and & operators short-circuit in if and while expressions, but not otherwise.” Can you give some examples of this benefit?

Before this week’s column, I would have thought that the purpose of the operation was return a true if either of the arguments existed. Thus, MatLab doesn’t operate as I expect, which is a flaw and should be corrected.

Because of the flaw, I have to add complexity to the code, something like this:

if ~isempty([]) | ~isempty(4)
disp(‘Must be true’)
else
disp(‘Must be false’)
end

if ~isempty(4) | ~isempty([])
disp(‘Must be true’)
else
disp(‘Must be false’)
end

Now I get the expected result.