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1996-12-04
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Week 2 Homework 1
Liberty BASIC programming course
Copyright 1996 Shoptalk Systems
All Rights Reserved
FOR/NEXT Loops
==============================================================================
Up to this point most of the loops that we've used in our code have had this
form:
variable = 0
[loopPoint]
'put some code in here
variable = variable + 1
if variable < 10 then [loopPoint]
This will loop 10 times. The first time through the loop the value of
variable will be 0, and the tenth time through the loop it will be 9. This
kind of coding does work, but it can look complicated and it takes some
care to make sure it is coded properly (it's easy to make a mistake). Now
let's take a look at an alternative:
for variable = 0 to 9
'put some code here
next variable
This is called a FOR/NEXT loop. The code that is inserted between the FOR
and NEXT statement will be executed 10 times. The value of variable will
be 0 the first time through and 9 the last time.
We can use this kind of code to do something a specifed number of times or
to do something using a range of values (for example 0 to 9).
FOR/NEXT loops require fewer lines of code. When programming, it is
usually best to use the simplest possible way to code any solution. This
kind of simplicity doesn't come without a price. There are things you can
do with the first kind of looping that you cannot do with a FOR/NEXT loop.
Look at this example:
variable = 0
[loopPoint]
'put your code here
if someCondition = 1 then [specialException]
variable = variable + 1
if variable < 10 then [loopPoint]
Notice the line of code that branches to [specialException] if someCondition
is equal to 1. This is perfectly acceptable coding practice. Now look at
this example:
for variable = 0 to 9
'put your code here
if someCondition = 1 then [specialException]
next variable
This is not acceptable. Liberty BASIC expects the loop to finish properly.
It is OK to use GOSUB to call a subroutine from inside a FOR/NEXT loop
because when the subroutine returns, execution will resume inside the loop
and it will be allowed to finish properly.
A good question to ask when deciding whether to use a FOR/NEXT loop is "Do
I know how many times this loop will be executed?" In our example above
we don't know because at any time someCondition could suddenly be equal to
1, and then our loop is finished.
You can cheat. If you set the value of variable to its finishing value (or
limit), the loop will think it's all done and it will quit. For example the
following code will only count to 5.
for x = 0 to 9
print x
if x = 5 then x = 9
next x
It's also worth noting that FOR/NEXT loops don't need to use integer values.
Here is an example that counts from 0 to 1 in steps of less than 1.
for i = 0 to 1 step 0.04
print i
next i
By using the word STEP and a value we specified what value to add to our
variable (i in this case). We can count backwards using a similar
technique.
for j = 10 to 1 step -1
print j
next j
Finally, if a starting value for the variable in our FOR/NEXT loop is
greater than its limit, the code between the FOR and NEXT statements will be
skipped over. The following code will not display any numbers at all.
for x = 1 to 0
print x 'this line will not be executed
next x
Similarly...
for x = 0 to 1 step -1
print x 'this line will not be executed
next x
Here is our ARRAYS.BAS program taken from our Week 2 Solutions to Challenge
Exercises. We've modified it by replacing one of the loops with a FOR/NEXT
loop. Study it carefully.
'ARRAYS.BAS
'List handling with arrays
'Here is a version that displays the names entered
'after [quit]
dim names$(10) 'set up our array to contain 10 items
[askForName] 'ask for a name
input "Please give me your name ?"; yourName$
if yourName$ = "" then print "No name entered." : goto [quit]
index = 0
[insertLoop]
'check to see if index points to an unused item in the array
if names$(index) = "" then names$(index) = yourName$ : goto [nameAdded]
index = index + 1 'add 1 to index
if index < 10 then [insertLoop] 'loop back until we have counted to 10
'There weren't any available slots, inform user
print "All ten name slots already used!"
goto [quit]
[nameAdded] 'Notify the name add was successful
print yourName$; " has been added to the list."
highestSlot = index 'this is a new line of code
goto [askForName]
[quit]
'display all the entered names
print
print "Here is a list of the names entered:"
print "------------------------------------"
for index = 0 to highestSlot
if names$(index) <> "" then print names$(index)
next index
end
Here's the assignment
-------------------------------------------------------------------------
1) In similar fashion to our modification of ARRAYS.BAS (above), modify
the AGES.BAS program below, replacing appropriate looping code with
FOR/NEXT loops.
'AGES.BAS
'Accept some names and ages from the user, then total and average them
dim numbers(20)
dim names$(20)
print "AGES.BAS"
print
'loop up to 20 times, getting numbers
print "Enter up to 20 non-zero values."
print "A zero or blank entry ends the series."
[entryLoop] 'loop around until a zero entry or until index = 20
'get the user's name and age
print "Entry "; index + 1;
input name$
if name$ = "" then [endSeries] 'quit if name$ is blank
print "Age ";
input age
index = index + 1 'add one to index
names$(index) = name$ 'set the specified array item to be name$
numbers(index) = age 'set the specified array item to be age
total = total + age 'add entry to the total
if index = 20 then [endSeries] 'if 20 values were entered, exit loop
goto [entryLoop] 'go back and get another entry
[endSeries] 'entries are finished
'Set entryCount to index
entryCount = index
if entryCount = 0 then print "No Entries." : goto [quit]
print "Entries completed."
print
print "Here are the "; entryCount; " entries:"
print "-----------------------------"
'This loop displays each entered value in turn.
'Notice that we re-use the index variable. It
'can be confusing to use a new variable for each
'new loop.
index = 0
[displayLoop]
index = index + 1
print "Entry "; index; " is "; names$(index); ", age "; numbers(index)
if index < entryCount then [displayLoop]
'Now display the total and average value
print
print "The total age is "; total
print "The average age is "; total / entryCount
[quit]
end
2) Add a routine onto the end of AGES.BAS that asks the user for a name.
Then using a FOR/NEXT loop searches for that name in the names$ array and
if the name is found displays the age for that name.