Programming in Lua : 4.3.4

This first edition was written for Lua 5.0. While still largely relevant for later versions, there are some differences.
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Programming in Lua

Part I. The Language Chapter 4. Statements

4.3.4 – Numeric for

The for statement has two variants: the numeric for and the generic for.

A numeric for has the following syntax:

    for var=exp1,exp2,exp3 do
      something
    end

That loop will execute something for each value of var from exp1 to exp2, using exp3 as the step to increment var. This third expression is optional; when absent, Lua assumes one as the step value. As typical examples of such loops, we have

    for i=1,f(x) do print(i) end
    
    for i=10,1,-1 do print(i) end

The for loop has some subtleties that you should learn in order to make good use of it. First, all three expressions are evaluated once, before the loop starts. For instance, in the first example, f(x) is called only once. Second, the control variable is a local variable automatically declared by the for statement and is visible only inside the loop. A typical mistake is to assume that the variable still exists after the loop ends:

    for i=1,10 do print(i) end
    max = i      -- probably wrong! `i' here is global

If you need the value of the control variable after the loop (usually when you break the loop), you must save this value into another variable:

    -- find a value in a list
    local found = nil
    for i=1,a.n do
      if a[i] == value then
        found = i      -- save value of `i'
        break
      end
    end
    print(found)

Third, you should never change the value of the control variable: The effect of such changes is unpredictable. If you want to break a for loop before its normal termination, use break.