Home > empirical > To if-else-if or if-if, that is the question

## To if-else-if or if-if, that is the question

August 21st, 2009

I am currently measuring if-statements, occurring in visible source, that might be mapped to an equivalent switch-statement. The most obvious usage to look for is a sequence of if-else-if statements that all involve the same expression being tested against an integer constant, as in

```if (x == 1) stmt_1; else if (x == 2) stmt_2; else if (x == 3) stmt_3;```

Another possible sequence is:

```if (x == 1) stmt_1; if (x == 2) stmt_2; if (x == 3) stmt_3;```

provided all but the last conditionally executed arms do not change the value of the common control variable (e.g., `x`).

I started to wonder about what would cause a developer to chose one of these forms over the other. Perhaps the if-if form would be used when it was obvious that the common conditional variable was not modified in the conditionally executed arm. This would imply that there would be more statements in the arms of if-else-if sequences than if-if sequences. The following plot of percentage occurrence (over all detected if-else-if/if-if forms) of line number difference between pars of associated if-statements (e.g., when the controlling expression occurs on line x and the following if-statement controlling expression occurs on line x+2 the distance is 2) shows that this is not the case:

Just over a quarter of the arms contain a single statement (or to be exact the code is contained on a single line); this suggests that when using the if-else-if form most developers put the `else` and `if` on the same line. At the next distance along the percentage of if-else-if forms is twice as great as the if-if, probably because of `else` and `if` appearing on separate lines (as in the introductory example) in one case and less frequently a comment/blank line in the other. Next along, why the big increase in if-if forms? A comment + blank line, or perhaps no comment or blank line but the use of curly brackets (this is too off the track of where I am supposed to be going to investigate).

This morning I realized why the original plot did not look right, one of the data sets was a way off adding to 100%. An updated version has been uploaded.

It turns out that a single statement (or at least a single line) is more common in the if-else-if form, the opposite of what I had expected. At slightly larger distances there are still differences that can be attributed to `else` and `if` appearing on separate lines, curly brackets and a comment/blank line, but the effect is not as large as seen in the original, less accurate, plot.

I have a feeling that I ought to say something about the if-else-if form being preferred to the if-if form. One of the forms will have its behavior changed if the common control variable is modified in one of its arms. But is this an intended or unintended behavior? What is the typical characteristic usage of a common control variable, e.g., do they tend to be accessed but not modified in a given function definition? At the moment I see no obvious cost or benefit strongly favoring one usage over the other, so I will remain silent on the issue.

1. August 24th, 2009 at 05:08 | #1

Hey Derek
I have a doubt when you say that there is no obvious cost or benefit…If the control variable is being changed then obviously we would have to go for the 2nd form because the first wouldn’t cover all cases and if we assume that the variable is not being changed then I agree there might not be any difference in worst case scenario (last condition holds true) but wouldn’t the 2nd form cause penalties in best case scenario (first condition holds true) as many more tests would be done in 2nd case even though we don’t need to check anymore..

2. August 24th, 2009 at 15:16 | #2

More tests appear to be needed at runtime for the if-if usage. I say ‘appear’ because after gcc’s impressive performance on 3n+1 I thought it might spot when if-if could be implemented as if-else-if; a quick check shows that with -O2 optimization gcc does indeed implement this transformation.

1. August 31st, 2009 at 16:06 | #1
2. October 8th, 2009 at 19:25 | #2