Any topic

Liberty BASIC - Help Online

Week Two Homework

Week 2 Homework

Liberty BASIC programming course

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

We can jump out of the loop and commence program execution at the first line of code after the NEXT statement if we use the EXIT FOR command, like this:

for x = 1 to 10

print x

if x > 5 then EXIT FOR

next x

'execution continues here

print "Final x value ";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 "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"

'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 "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 "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.

Week Two Homework Solution