FOR...DO is a loop construct in Pascal. Of course, that may raise the question "What is a loop?".
Looping means repeating a statement or compound statement over and over until some condition is met.
There are three types of loops:
- fixed repetition - only repeats a fixed number of times
- pretest - tests a Boolean expression, then goes into the loop if TRUE
- posttest - executes the loop, then tests the Boolean expression
In Pascal, the fixed repetition loop is the for loop. The general form is:
for index := StartingLow to EndingHigh do statement;
The index variable must be of an ordinal data. The index can be used in calculations within the body of the loop, but its value cannot be changed (i.e. count:=5 will cause a program exception.)
In Pascal, the for loop can only count in increments (steps) of 1. A loop can be interrupted using the break statement. An example of using the index is:
sum := 0; for count := 1 to 100 do begin sum := sum + count; if sum = 38 then break; end;
The computer would do the sum the long way and still finish it in far less time than it took the mathematician Gauss to do the sum the short way (1+100 = 101. 2+99 = 101. See a pattern? There are 100 numbers, so the pattern repeats 50 times. 101*50 = 5050. This isn't advanced mathematics, its attribution to Gauss is probably apocryphal.).
In the for-to-do loop, the starting value MUST be lower than the ending value, or the loop will never execute! If you want to count down, you should use the for-downto-do loop:
for index := StartingHigh downto EndingLow do statement;