Iteration in Computer Science: WHILE, REPEAT, and FOR Loops, Slides of Computer science

Download Iteration in Computer Science: WHILE, REPEAT, and FOR Loops and more Slides Computer science in PDF only on Docsity!

AQA

AS Level

Computer

Science

Paper 1

Unit 1 Fundamentals of programming

Objectives

• (^) Understand and use three different types of iterative

statement WHILE, REPEAT and FOR

• (^) Be familiar with, and be able to use, random number

generation

Unit 1 Fundamentals of programming WHILE … ENDWHILE loop

• (^) Using a WHILE .. ENDWHILE loop, the condition

is tested upon entry to the loop

• (^) It is possible that the instructions inside the loop

might not be executed at all if the entry condition

is not met

WHILE Condition x = True Execute statement a Execute statement b etc.... END WHILE

Unit 1 Fundamentals of programming Iteration entry condition

• (^) Trace through the pseudocode loop
  • (^) What values will x take?
  • (^) What will be the output?
• (^) What would happen if the first statement was

changed to x  2?

x  0 WHILE x < 2 x  x+ OUTPUT x ENDWHILE OUTPUT “The end” x Output 0

Unit 1 Fundamentals of programming Worksheet 3

• (^) Complete Questions 1 and 2 in Task 1

Unit 1 Fundamentals of programming REPEAT .. UNTIL loop

• (^) In a REPEAT. .UNTIL loop the statements in the

loop are executed before the condition is evaluated

• (^) The statement will always be executed at least once
• (^) Complete the values for x and Output
• (^) What would happen if the first statement was

changed to x  2?

x  0 Repeat x x+ OUTPUT x UNTIL x >= 2 OUTPUT “The end” x Output 0

Unit 1 Fundamentals of programming Performing a range check

• (^) A loop can be used to implement a range check
• (^) The loop will continually prompt for input until a valid

age between 12 and 18 is entered

• (^) Work through the algorithm below with the data 10,

11,18 and 19. What will be the output?

REPEAT

OUTPUT “Enter age” age USERINPUT UNTIL age > 11 AND age <= 18 OUTPUT “Age is”, age

Unit 1 Fundamentals of programming An equivalent WHILE loop

• (^) In Python the REPEAT..UNTIL loop is not supported
• (^) An equivalent loop can be created using a WHILE

loop

• (^) Work through the algorithm below with the data 10,

11,18 and 19. What will be the output?

age = 0 WHILE age < 12 OR age > 18 OUTPUT “Enter age” age USERINPUT ENDWHILE OUTPUT “Age is”, age

Unit 1 Fundamentals of programming Infinite loop

• (^) Infinite loops are often used in 2D games
  • (^) An “outer” WHILE True… ENDWHILE loop runs the game
  • (^) A brief pause can be included to slow the loop down, so that a sprite x,y position can be updated about 25 times a second for smooth animation WHILE True updatePlayerPosition() checkForCollision() redraw() ENDWHILE

Unit 1 Fundamentals of programming Infinite loops in control and data sensing applications

• (^) Computer control and data sensing applications use

infinite loops to gather data from sensors

• (^) A variety of sensors can control a number of output devices such as lights, buzzers and motors
• (^) After the setup code is run, the device enters an infinite loop to repeatedly check the value of the sensors

Unit 1 Fundamentals of programming FOR .. NEXT loop

• (^) The FOR .. NEXT loop is termed “definite iteration”,

and is used to repeat a block of instructions a

specified number of times

• (^) The FOR loop uses a counter variable which is

automatically incremented each time through the

loop

• (^) Optionally, a STEP value can be specified to make

the counter increase or decrease by any integer

Unit 1 Fundamentals of programming FOR .. NEXT loop

• (^) What values will index and x take? FOR index  1 TO 8 x = (index ** 2) % 3 OUTPUT x NEXT index index x 1

Unit 1 Fundamentals of programming Stepping backwards

• (^) The increment in a FOR loop can be negative
• (^) What values will index and x take? FOR index  15 to 1 Step
  • x = int (index /2) OUTPUT x NEXT index index x 15

Unit 1 Fundamentals of programming Using a WHILE .. ENDWHILE loop as an alternative

• (^) Are these loops logically equivalent?
• (^) What number would you use in the FOR loop to get

the same results as the WHILE loop?

• (^) Outside the FOR loop, the variable index is

undefined

index  1 WHILE index < 4 OUTPUT index index  index + 1 ENDWHILE FOR index  1 to? OUTPUT index NEXT index