Programming
Loops
There are 3 questions to ask before deciding which type of loop to use :
1) Do you know how
many times you want to repeat code?
(This kind of loop is top tested before code is executed)
Some
examples :
a) for
each of twelve months
b) for
each student in a class
c) for
each question on a test
If
the number of times the code will be executed is not known :
2) Do you want to execute the code at least one time?
(This kind of loop is bottom tested after code is executed at least once)
Some
examples :
a) To
let the user enter something, and possibly quit
i)
Such
as used in a menu :
(1) the user will do this at least once,
or as many times as necessary ?
3) Is it possible that the code will not execute even once?
(This kind of loop is top tested before code is executed even one time)
Some
examples :
a) entering a list of data, one at a
time, until finished
i)
user may choose to enter many items, or
none at all !
b)
Reading
data from a file, where there not be any!
i)
There
may be many items, or none at all, in the file !
Summary:
There
are several ways to repeat instructions in nearly any programming language.
1. The
For loop
2. The
Do
Until
3. The
Do
While loop (confusing variation of
the Do
Until loop)
4. The
While Loop
(Executes code a
preset number of times)
If you know how many
times you want to execute the code, the for loop is the best choice.
Primarily, the for
loop lets the programmer specify what value to start, and when to stop.
By default, the loop
counter increments by 1, but optionally this can be any value even fractions!
Loop
start from start value to end value, and counts by ones (unless specified)
// Checks <Terminating
Condition> to decide whether to it again ?
<program code>
End of loop
The For loop is top tested before code is
executed.
The body of code in a For loop may execute any number of times,
or may not
execute at all, because it is tested before the code executes the first
time.
Both of the following types of loops let the user (or data from a
file) control how many times the code is executed :
The Do
Until or Do
While loop :
(Code may be executed at least once, or more times)
This loop starts by executing the code before it
gets to the terminating condition that checks to see if the code should be
done again?
If the terminating condition is false, the
code is executed again, otherwise the loop ends.
As you can see, the code is executed before checking
to see if it should be done again.
The format looks like this :
Loop
start
<program code>
<Data is entered that may tell loop to quit >
End of
loop < Terminating Condition value is checked>
NOTE : Some languages have both a
Do
While and a Do
Until loop structure.
The
difference between the Do
While and the Do
Until is subtle:
These are both essentially the same thing though.
The Do
While
performs the code while the condition is true,
then exits when it is false.
During the Do
While loop, the
condition is true.
The body of code in a Do
While loop will execute at least once,
because it is tested after the code executes
the first time.
The Do
Until
performs the code until the condition is true,
then exits when it is true.
During the Do
Until loop, the
condition is false.
The body of code in a Do
While loop will execute at least once,
because it is tested after the code executes
the first time.
(This kind of loop is bottom tested
after code is executed)
The program lets the user
(or data from a file) control how many times the code is executed
:
Loop
start <While Terminating Condition is True :
value is checked before code is executed>
<program code>
<Data is entered that may tell loop to quit >
End of
loop
Since the terminating
condition is checked before the code is executed, it is possible that the body
of code never gets done at all!
The body of code in a While loop may
execute any number of times,
or may not
execute at all, because it is tested before the code executes the first
time.