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I have always wondered if, in general, declaring a throw-away variable before a loop, as opposed to repeatedly inside the loop, makes any (performance) difference? A (quite pointless) example in Java:
a) declaration before loop:
double intermediateResult; for(int i=0; i < 1000; i++){ intermediateResult = i; System.out.println(intermediateResult); } b) declaration (repeatedly) inside loop:
for(int i=0; i < 1000; i++){ double intermediateResult = i; System.out.println(intermediateResult); } Which one is better, a or b?
I suspect that repeated variable declaration (example b) creates more overhead in theory, but that compilers are smart enough so that it doesn't matter. Example b has the advantage of being more compact and limiting the scope of the variable to where it is used. Still, I tend to code according example a.
Edit: I am especially interested in the Java case.
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Declaring variables inside or outside of a loop, It's the result of JVM specifications But in the name of best coding practice it is recommended to declare the variable in the smallest possible scope (in this example it is inside the loop, as this is the only place where the variable is used).
As a general rule, I declare my variables in the inner-most possible scope. So, if you're not using intermediateResult outside of the loop, then I'd go with B. Show activity on this post. A co-worker prefers the first form, telling it is an optimization, preferring to re-use a declaration.
Often the variable that controls a for loop is needed only for the purposes of the loop and is not used elsewhere. When this is the case, it is possible to declare the variable inside the initialization portion of the for.
Rationale: It's best to declare variables when you first use them to ensure that they are always initialized to some valid value and that their intended use is always apparent.
Which is better, a or b?
From a performance perspective, you'd have to measure it. (And in my opinion, if you can measure a difference, the compiler isn't very good).
From a maintenance perspective, b is better. Declare and initialize variables in the same place, in the narrowest scope possible. Don't leave a gaping hole between the declaration and the initialization, and don't pollute namespaces you don't need to.
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