Best idiom for a decrementing loop

Question

What's the best/preferred idiom for a decrementing for loop, and why?

for(int idx=(len-1); idx>=0; idx--) {...}

or

for(int idx=(len-1); idx>-1; idx--) {...}

or (based on sylvarking's answer, but using for to constrain the index scope)

for(int idx=len; idx-->0; ) {...}

Is one or the other more likely to trip up another developer?

Answer 1

I recommend the following because comparison against zero is optimized at the byte code level.

for(int idx=(len-1); idx>=0; idx--) {...}

This is a suggestion from Java Performance Tuning by Jack Shirazi

Answer 2

Another array-oriented variant you might see is this:

int n = foo.length;
while (n-- > 0) {
  /* Do something with foo[n] */
}

Of the options you show, I prefer the first.

• 0 is (arguably) more readable than -1.
• Comparison to zero is usually faster than comparison to other constants. It is a single instruction that examines a single value on the stack. Other comparisons use one instruction to push a constant onto the stack and another instruction to compare the top two values.