Lesson Two Homework

Lesson Two Homework
Copyright 2007 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 also acceptable. You can jump out of a loop this way using Run BASIC. It is also okay 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 also use an EXIT FOR statement to break out of a FOR/NEXT loop. For example the following code will only count to 5.

for x = 0 to 9
    print x
    if x = 5 then exit for
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.