Module basic::__Chapter_2::FOR

FOR <variable>=x TO y [STEP z]

Where x, y, and z are expressions.

Purpose

Used with NEXT to repeat execution of statements while iterating over a sequence of numbers.

Remarks

If we wanted the numbers 1,3,5,7 we would write FOR I=1 TO 7 STEP 2. On the first iteration, 1 will be assigned to variable I. Statements execute until a NEXT statement. On subsequent iterations, the variable I gets 2 added to it. If the result exceeds 7 the loop breaks. Otherwise the statements get executed again.

The first iteration will evaluate z, then y, then set variable to x. Note that some versions of BASIC evaluate x then y then z; this is the older style. The evaluated x and y along with the variable name is stored on the stack. Because FOR loops are stack-based, you can nest them for iterating over multiple dimensions.

The expressions are skipped if the variable is already past the end on the first execution. Some versions of basic always execute the statements because the logic is done at the NEXT statement.

Example 1

10 I=9
20 FOR I=1 TO 10 STEP I
30 PRINT "HELLO WORLD";i
40 NEXT I
RUN
HELLO WORLD 1
HELLO WORLD 10

Example 2

10 FOR X=1 TO 2
20 FOR Y=5 TO 6
30 PRINT x,y
40 NEXT Y,X
RUN
 1  5
 1  6
 2  5
 2  6