10.2.1 FOR...NEXT Loops
  In a FOR...NEXT loop, you specify a loop control variable
  (the loop index) that determines the number of loop itera-
  tions. This number must be a scalar (unsubscripted) variable.
  When BASIC begins execution of a FOR...NEXT loop, the
  starting and ending values of the loop control variable are
  known.

  The FOR statement assigns the control variable a starting
  value and an ending value. You can use the optional STEP
  clause to specify the amount to be added to the loop control
  variable after each loop iteration.

  When a FOR loop block executes, the BASIC compiler per-
  forms the following steps:

  1. Evaluates the starting value and assigns it to the control
        variable.
  2. Evaluates the ending value and the step value and assigns
        these results to temporary storage locations.
  3. Tests whether the ending value has been exceeded. If
        the ending value has already been exceeded, BASIC exe-
        cutes the statement following the NEXT statement. If the
        ending value has not been exceeded, BASIC executes the
        statements in the loop.
  4. Adds the step value to the control variable and trans-
        fers control to the FOR statement, which tests whether
        the ending value has been exceeded. Steps 3 and 4 are
        repeated until the ending value is exceeded.

  Note that BASIC performs the test before the loop executes.
  When the control variable exceeds the ending value, BASIC
  exits the loop, and then subtracts the step value from the con-
  trol variable. This means that after loop execution, the value
  of the control variable is the value last used in the loop, not
  the value that caused loop termination.

  Example 10-1 assigns the values 1 to 10 to consecutive array
  elements 1 to 10 of New_array , and Example 10-2 assigns
  consecutive multiples of 2 to the odd-numbered elements of
  New_array .

  Note that the starting, ending, and step values can be run-
  time expressions. You can have BASIC calculate these values
  when the program runs, as opposed to using a constant value.
  For instance, the following example assigns sales information
  to array Sales_data . The number of iterations depends on
  the value of the variable Days_in_month , which represents
  the number of days in that particular month.
  FOR I% = 1% TO Days_in_month
        Sales_data(I%) = Quantity_sold
  NEXT I%

  Because the starting, ending, and step values can be numeric
  expressions, they are not evaluated until the program runs.
  This means that you can have a FOR...NEXT loop that does
  not execute. The following example prompts the user for the
  starting, ending, and step values for a loop, and then tries to
  execute that loop. The loop executes zero times because it is
  impossible to go from 0 to 5 using a step value of -1.
  counter% = 0%
  INPUT "Start"; start%
  INPUT "Finish"; finish%
  INPUT "Step value"; step_val%
  FOR I% = start% TO finish% STEP step_val%
        counter% = counter% + 1%
  NEXT I%
  PRINT "This loop executed"; counter%; "times."

  Output
  Start? 0
  Finish? 5
  Step value? -1
  This loop executed 0 times.
  Whenever possible, you should use integer variables to con-
  trol the execution of FOR...NEXT loops because some decimal
  fractions cannot be represented exactly in a binary com-
  puter, and the calculation of floating-point control variables is
  subject to this inherent imprecision.

  In the following example, the first loop uses an integer con-
  trol variable while the second uses a floating-point control
  variable. The first loop executes 100 times and the second 99
  times. After the ninety-ninth iteration of the second loop, the
  internal representation of the value of Floating_point_variable
  exceeds 10 and BASIC exits the loop. Because the first loop
  uses integer values to control execution, BASIC does not exit
  the loop until Integer_variable equals 100.
  EXAMPLE: Click to display example.

  Output
  Integer loop count: 100
  Integer loop end: 100
  Real loop count: 99
  Real loop end: 9.9
  Although it is not recommended programming practice, you
  can assign a value to a FOR...NEXT loop's control vari-
  able while in the loop. This affects the number of times a
  loop executes. For example, assigning a value that exceeds
  the ending value of a loop will cause the loop's execution to
  end as soon as BASIC performs the termination test in the
  FOR statement. Assigning values to ending or step variables,
  however, has no effect at all on the loop's execution.