Appian enumerate()

The shape

enumerate() takes a count and hands back that many whole numbers, starting at 0.

enumerate(5)   →   {0, 1, 2, 3, 4}

Five numbers, and the last one is 4. It is a list of positions, not a list of things.

Why it exists

Every other way of looping in Appian starts from a list you already have. enumerate() is for when you do not have one — you know only how many times something should happen.

Ten blank rows in a grid, a retry counter, the first N of a series: there is no list to loop over, so you make one.

The trap: it counts from 0, and nothing else does

This is the whole reason the function is worth a lesson. Appian counts from 1 nearly everywhere else:

• index(list, 1, "") gives you the first item
• fv!index is 1 on the first pass of a loop
• enumerate(3) gives you 0, 1, 2

So the numbers from enumerate() are almost always one less than the number you actually want. Adding one fixes it for the whole list at once, with no loop:

enumerate(5) + 1   →   {1, 2, 3, 4, 5}

That works because arithmetic already applies itself to every item of a list. It is the same element-wise behaviour that makes most a!forEach() calls unnecessary.

Edges worth knowing

• enumerate(0) is an empty list. Useful rather than annoying: loop over it and the body never runs.
• A negative count is an error, not an empty list. Guard the count if it comes from data.
• A decimal count is cut down to a whole number, so enumerate(2.7) gives two numbers.

Pairing it with a real list

Because the numbers are positions, they pair with index() to walk a list by position:

a!forEach(items: enumerate(3), expression: index(names, fv!item + 1, ""))

Note the + 1 again. When you already have the list, looping over the list itself and using fv!item is shorter and harder to get wrong — reach for this pairing only when the position is genuinely what you are working with.