SIMSCRIPT II.5 Programming Language

SIMSCRIPT II.5 Programming Language
those values in the sequence, however, for which the logical expression in the with phrase is true
(i.e., all values but zero) are passed on to the statements in the do loop.
Sequences of with, unless, while, and until phrases can be attached to for phrases in any
combination. More than one of each type of phrase is permitted. While and until control phrases
may also be modified by with and unless phrases, and may be nested with other independent
while and until phrases to control statement repetition. Again, the use of such combinations of
control phrases will become more apparent in later examples.
1.16 Altering the Flow of Control Within a Loop
There are occasions when it may be necessary to exercise more explicit direction on the flow of control
than may readily be specified using the control structures discussed so far. Consider the previous
example:
for COUNT = 1 to NUMBER.OF.SEATS,
while TOTAL.WEIGHT < LIMIT
do
read PASSENGER.NO, FARE, and BAGGAGE.WEIGHT
add BAGGAGE.WEIGHT to TOTAL.WEIGHT
loop
Examination of the statements within the loop shows that if the loop is terminated by the while
phrase, the weight of the last passenger's baggage must have overflowed the LIMIT value. This
may not be quite what was intended. The problem arises because the test of the condition is made
only at the beginning of the loop, and all the statements in the loop are executed at each iteration.
There are occasions when the test should logically be made within the loop. The following example
uses a leave statement to transfer control out of the loop when the LIMIT is about to be exceeded:
for COUNT = 1 to NUMBER.OF.SEATS
do
read PASSENGER.NO, FARE, and BAGGAGE.WEIGHT
if TOTAL.WEIGHT + BAGGAGE.WEIGHT > LIMIT
leave
otherwise
add BAGGAGE.WEIGHT to TOTAL.WEIGHT
loop
let LOAD.FACTOR = (COUNT-1)/NUMBER.OF.SEATS
.
.
The leave statement, which may only be used within a do loop, causes the flow of control to pass
to the statement immediately following the loop statement that delimits the do loop. This addition
to the construction of a do loop proves useful in those cases where the terminating condition is dependent
on some of the actions performed within the loop. Note the use of otherwise in this example.
This usage is discussed in the next paragraph. Note also that the value of the control phrase
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