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FOR...NEXT

Description

A control structure that is a numerical iteration, used to execute one or more statements a specified number of times.

Syntax

FOR varname = start TO end {STEP increment}
   statementblock
NEXT

Parameter

Description

varname

The name of the iteration counter variable. It can be any numerical type (byte, integer, double, real, long, longlong, or decimal), but integers provide the fastest performance.

start

Starting value of varname.

end

Ending value of varname.

increment (optional)

The increment value. Increment must be a constant and the same datatype as varname. If you enter an increment, STEP is required. +1 is the default increment.

statementblock

The block of statements you want to repeat.


Ending statement

You can end the FOR loop with the keywords END FOR instead of NEXT.

Usage

Using the start and end parameters

For a positive increment, end must be greater than start. For a negative increment, end must be less than start.

When increment is positive and start is greater than end, statementblock does not execute. When increment is negative and start is less than end, statementblock does not execute.

When start and end are expressions, they are reevaluated on each pass through the loop. If the expression's value changes, it affects the number of loops. Consider this example -- the body of the loop changes the number of rows, which changes the result of the RowCount function: 

FOR n = 1 TO dw_1.RowCount( )
      dw_1.DeleteRow(1)
NEXT

A variable as the step increment

If you need to use a variable for the step increment, you can use one of the DO...LOOP constructions and increment the counter yourself within the loop.

Nesting

You can nest FOR...NEXT statements. You must have a NEXT or END FOR for each FOR.

Avoid overflow

If start or end is too large for the datatype of varname, varname will overflow, which might create an infinite loop. Consider this statement for the integer li_int: 

FOR li_int = 1 TO 50000

The end value 50000 is too large for an integer. When li_int is incremented, it overflows to a negative value before reaching 50000, creating an infinite loop.

Examples

Example 1

These statements add 10 to A as long as n is >=5 and <=25: 

FOR n = 5 to 25
      A = A+10
NEXT

Example 2

These statements add 10 to A and increment n by 5 as long as n is >= 5 and <=25: 

FOR N = 5 TO 25 STEP 5
      A = A+10
NEXT

Example 3

These statements contain two lines that will never execute because increment is negative and start is less than end: 

FOR Count = 1 TO 100 STEP -1
   IF Count < 1 THEN EXIT // These 2 lines
   Box[Count] = 10        // will never execute.
NEXT

Example 4

These are nested FOR...NEXT statements:

Int Matrix[100,50,200]
FOR i = 1 to 100
      FOR j = 1 to 50
      FOR k = 1 to 200
          Matrix[i,j,k]=1
      NEXT
      NEXT
NEXT
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