programming-for-dummies

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Looping at Least Once with the DO Loop
The first line defines the Answer variable as a string data type. The
second line asks the user for a password and stores that answer in the
Answer variable.
The WHILE loop first checks if the Answer variable is SECRET. If not, the
loop runs the two commands that print Invalid password and then asks
Enter password: on-screen once more.
Whatever reply the user types gets stored in the Answer variable. Then the
WHILE loop checks this Answer variable again before running.
You can make a WHILE loop count like a FOR-NEXT loop. Suppose you had
the following FOR-NEXT loop:
FOR I = 10 DOWNTO 1
PRINT I
NEXT I
PRINT “BLASTOFF!”
The equivalent WHILE loop might look like this:
I = 10
WHILE (I >= 1)
PRINT I
I = I - 1
WEND
PRINT “BLASTOFF!”
Although the WHILE loop can count, notice that it takes more lines of code
to do so, and the WHILE loop isn’t as easy to understand as the FOR-NEXT
loop. If you need a loop to run a fixed number of times, use the FOR-NEXT
loop. If you aren’t sure how many times you need a loop to run, use the
WHILE loop.
Looping at Least Once with the DO Loop
Before a WHILE loop runs, it checks a Boolean expression to see if it’s True
or False. If this Boolean expression is False, the WHILE loop never runs at
all. What if you want to insure that the loop runs at least once? In that case,
you must use a DO loop.
A DO loop acts like an upside-down WHILE loop. First the DO loop runs once
and then it checks a Boolean expression. A typical DO loop looks like this:
DO
Command
Command to change the Boolean expression
LOOP WHILE (True or False Boolean expression)