13.1 General Description

The for-statement and next-statement provide for the construction of loops. The general syntactic form of the for-statement and next-statement is FOR v = initial-value TO limit STEP increment NEXT v where "v" is a simple numeric variable and the "initial-value", "limit" and "increment" are numeric expressions; the clause "STEP increment" is optional.

Examples

        FOR I = 1 TO 10 NEXT I
        

13.2 Syntax

            1. for-block                = for-line for-body
            2. for-body                 = block next-line
            3. for-line                 = line-number for-statement end-of-1ine
            4. next-line                = line-number next-statement end- of-line
            5. for-statement            = FOR control-variable equals-sign initial-value TO limit (STEP increment)?
            6. control-variable         = simple-numeric-variable
            7. initial-value            = numeric-expression
            8. limit                    = numeric-expression
            9. increment                = numeric-expression
            10.next-statement           = NEXT control-variable
        

13.3 Examples

            FOR I = 1 TO 10
            NEXT I
            FOR I = A TO B STEP -1
            NEXT I
        

13.4 Semantics

The for-statement and the next-statement are defined in con junction with each other. The physical sequence of statements beginning with a for-statement and continuing up to and including the first next-statement with the same control variable is termed a "for-block". For-blocks can be physically nested, i.e. one can contain another, but they shall not be interleaved, i.e. a for-block which contains a for-statement or a next- statement shall contain the entire for-block begun or ended by that statement. Furthermore, physically nested for-blocks shall not use the same control variable. In the absence of a STEP clause in a for-statement, the increment is assumed to be +1. The action of the for-statement and the next-statement is de fined in terms of other statements, as follows.

        FOR v = initial-value TO limit STEP increment (block) NEXT v
        

is equivalent to:

            LET ownl = limit
            LET own2 = increment
            LET v = initial-value
    line 1  IF (v-ownl) * SGN (own2) > 0 THEN line2
            (block)
            LET v = v + own2
            GOTO line 1
    line2   REM continued in sequence
        

Here v is any simple-numeric-variable, ownl and own2 are variables associated with the particular for-block and not accessible to the programmer, and linel and line2 are line-numbers associated with the particular for-block and not accessible to the programmer. The variables ownl and own2 are distinct fiom similar variables associated with other for-blocks. A program shall not transfer control into a for-body by any statement other than a return statement (see 12).

13.5 Exceptions

None.

13.6 Remarks

Where arithmetic is approximate (as with decimal fractions in a binary machine), the loop will be executed within the limits of machine arithmetic. No presumptions about approximate achievement of the end test are made. It is noted that in most ordinary situations where machine arithmetic is truncated (rather than rounded), such constructions as FOR X = 0 TO 1 STEP 0.1 will work as the user expects, even though 0.1 is not representable exactly in a binary machine. If this is indeed the case, then the construction FOR X = 1 TO 0 STEP -0.1 will probably not work as expected. As specified above, the value of the control-variable upon exit" from a for-block via its next-statement is the first value not usedj if exit is via a control-statement, the control variable retains the value it has when the control-statement is executed. The variables "ownl" and "own2" associated with a for-block are assigned values only upon entry to the for-block through its for-s tatement.