Repetition In today’s lesson we will look at: why you would want to repeat things in a program different ways of repeating things creating loops in Just BASIC

Repetition Imagine you wanted to print the word “hello” ten times... You could do it like this: print “hello” print “hello” print “hello” print “hello” print “hello” Is that a sensible way to do it? What about if you change your mind about what you want it to say? What about if you want to change the number of times you say it?

Sometimes you might want to repeat a section of your program… a fixed number of times until something happens while a particular condition exists forever! In programming, these are called loops Repetition

How many times do you need to repeat to: choose lottery numbers for a “lucky dip”? print a list of all of the square numbers less than 100? ask the user for a number and make sure that it’s between 1 and 10? Examples

In Just BASIC there are three types of loop: FOR... NEXT WHILE... WEND DO... UNTIL/WHILE (most languages have similar loops, but often the bottom one is called REPEAT...) BASIC

A FOR... NEXT loop is probably the most common and most useful, e.g. FOR x = 0 to 100 STEP 10 PRINT x NEXT You can use any number variable in place of x You only need the STEP part if you are counting down, or counting up in steps other than 1 You can use the value of the variable inside the loop It is traditional to indent lines of code inside the loop FOR Example

A WHILE loop looks like this: x = 1 WHILE x*x < 100 print x; " squared is "; x*x x = x + 1 WEND Rather than repeating a fixed number of times, it repeats while a condition – in this case that the square is less than 100 – is true. The check is done at the start so the bit inside the loop may never run at all WHILE Example

A DO WHILE loop looks like this: DO input "Give me a number from 1 to 10: "; x LOOP WHILE x 10 print "Thank you" With a DO loop the test condition comes at the end, so it always loops at least once Could you do this same example with a WHILE loop? What would happen? DO WHILE Example

A DO UNTIL loop looks like this: DO UNTIL x >= 1 AND x <= 10 input "Give me a number from 1 to 10: "; x LOOP print "Thank you" This program does the same as the last one, but the condition is the opposite way round Because they are interchangeable, not all programming languages have both WHILE and REPEAT UNTIL loops DO UNTIL Example

An algorithm is a plan for a program Think about what the task requires in everyday terms before you rush to program it, e.g. –Ten green bottles –Times tables –The Twelve Days of Christmas Algorithm First!