Why does re-using an iterator variable in nested loops not cause issues in ColdFusion?

Question

I've been looking at some old code at work and I've found several instances of nested loops in which the variable used for iterating over an object is re-assigned within the inner loop but it does not cause issues. For instance given the following example.cfm:

<cfscript>

    // Get every second value in an array of arrays
    function contrivedExampleFunction(required array data) {
        var filtered = [];

        for (i = 1; i lte arrayLen(arguments.data); i++) {
            var inner = arguments.data[i];

            for (i = 1; i lte arrayLen(inner); i++) {
                if (i eq 2) {
                    arrayAppend(filtered, inner);
                }
            }
        }

        return filtered;
    }

    data = [
        [1,2,3],
        [4,5,6],
        [7,8,9]
    ];

    // Expected (working function): [2, 5, 8]
    // Expected (short-circuiting): [2]
    // Actual result: [1, 2, 3]
    writeDump( contrivedExampleFunction(data) );

</cfscript>

I would expect the inner i declaration to re-assign the outer i and cause the function to 'short-circuit', especially as i isn't even scoped. However the function will return an unpredictable result. Why?

Answer 1

You're not desk-checking the code properly. It's wrong, but it works as expected.

The outer loop will loop i from 1-3
    First iteration i=1
    inner = data[1] => [1,2,3]
    The inner loop will loop i from 1-3
        First iteration i=1
            nothing to do
        Second iteration i=2
            append inner to filtered => [1,2,3]
        Third iteration i=3
            nothing to do
    end inner loop
    i=3, which meets the exit condition of the outer loop
end of outer loop

filtered = [1,2,3]

I think you are misreading this line of code:

arrayAppend(filtered, inner);

You're reading it as:

arrayAppend(filtered, inner[i]);

But it doesn't say that.

Make sense?