R2010a: eval -> output sometimes double, sometimes single

Hallo there,

do you have anyidea, when would eval output the same statement with double, and when as single ?

Because Im experiencing that

evalOut = eval(['round(myExpression)']);

is returning sometimes double, and sometimes single values. For me not deterministically.

Any idea why?

Best,

[Oleg Komarov]

Not enough info, what's myExpression?

Answer 1

[Jan]

If myExpression is a single and "round" is the function round, the output is a single also. But if "round" is a double array, the values of myExpression are interpreted as indices and the output has the type double.

So at first you have to check what "round" and "myExpression" is:

whos round
whos myExpression

In all possible cases this does not depend on eval! But you could use this to confuse Matlab - and its users:

round(3.14);
eval('round = 5');
eval('round(1)');

The behaviour can depend on the debug status of Matlab! Cruel. So I suggest to avoid this:

evalOut = eval(['round(myExpression)']);

under all circumstances and use this instead:

out = round(myExpression);

I do not know a reason why a program should be as confusing as possible. And as your question shows: With enough EVALs the behaviour of Matlab will look non-deterministic -- but of course it is not.