Computer Science

• ELIF

  It is simple, efficient, and easier to read when there are fewer conditions
• Indentation in programming


  Indentation refers to the spaces at the beginning of a code line
• Algorithm to ask user to enter a number between 1 and 10 and output if it is low or high


  OUTPUT "enter a number"

  nom = USERINPUT

  IF nom <= 5

  OUTPUT (nom) "is a low number"

  ELSE

  OUTPUT (nom) "is a high number"
• Boolean operators


  Named after George Boole, a 19th century English mathematician who formulated an algebraic system of logic, can be represented as true or false / 1 or 0
• Boolean operators


  • AND - Logical AND operator, if all operands are true then condition is true

  OR - Logical OR operator, if any operands are true then condition is true

  NOT - Logical NOT operator
• Indentation


  Spaces at the beginning of a code line
• Algorithm to ask user to enter a number between 1 and 10

  1. OUTPUT "enter a number"

  2. nom = USERINPUT

  3. IF nom <= 5

  4. OUTPUT (nom) "is a low number"

  5. ELSE

  6. OUTPUT (nom) "is a high number"
• Boolean operators


  • AND
  • OR
  • NOT
• AND operator


  Logical AND operator, if all operands are true then the condition becomes true
• OR operator


  Logical OR operator, if any of the operands are true then the condition becomes true
• NOT operator


  Logical NOT operator, used to reverse the logical state of the operand
• Algorithm to input student year and tutor group

  1. year ← USERINPUT

  2. OUTPUT "Please enter the tutor group."

  3. tutorGroup ← USERINPUT

  4. IF year < 7 AND > 13 THEN

  5. OUTPUT "The year group is out of range"

  6. ENDIF

  7. IF tutorGroup ≠ "red"OR tutorGroup ≠ "green" THEN

  8. OUTPUT "There is no tutor group with this colour"

  9. ENDIF
• Nested IF statement


  An 'IF' statement inside another 'IF' statement
• Algorithm to find the largest of 3 numbers

  1. OUTPUT "enter a number"

  2. A = USERINPUT

  3. OUTPUT "enter a number"

  4. B = USERINPUT

  5. OUTPUT "enter a number"

  6. C = USERINPUT

  7. IF A > (B OR C)

  8. OUTPUT A "is the biggest number"

  9. ENDIF

  10. IF B > (A OR C)

  11. OUTPUT B "is the biggest number"

  12. ENDIF

  13. IF C > (B OR A)

  14. OUTPUT C "is the biggest number"

  15. ENDIF
• Algorithm to output month name based on number input

  1. OUTPUT "enter a number"

  2. A = USERINPUT

  3. CASE A

  4. 1: OUTPUT "January"

  5. 2: OUTPUT "February"

  6. ...

  7. 12: OUTPUT "December"

  8. ENDCASE
• Top-down problem solving


  Breaking down a difficult problem into sub-tasks to make it easier
• Computational thinking involves decomposition, abstraction, pattern recognition and algorithm design
• Bottom-up problem solving


  Starts with a collection of small problems and tries to combine them together to form a larger system
• Benefit of structured programming


  Subroutines can be called and used in the program as many times as needed without having to rewrite the code each time, reused in different programs
• Sub-routines in 'User login' process

  • Enter a username
  • Check if username exists
  • Enter password
  • Check password
• Procedure


  A process to do an activity till completion
• Function


  A command
• SUBROUTINE count()

  FOR x ← 5 TO 10

  OUTPUT "x"

  ENDFOR

  ENDSUBROUTINE
• In Python, functions do not have to return values, but in languages with both functions and procedures, functions must return values
• Python game to throw 3 dice and keep highest score

  import random

  import time

  HS = 0

  while True:

  A = input("Will you roll the dice? (yes/no) ")

  if A == "yes":

  print("rolling....")

  time.sleep(1)

  print("still rolling...")

  time.sleep(1)

  input ("you still there? ")

  N1 = random.randint(1, 6)

  time.sleep(1)

  print("dice 1 =", N1)

  N2 = random.randint(1, 6)
• Iteration


  Repeating steps or instructions over and over again until a particular condition is met (LOOP)
• Why use iteration


  • It simplifies an algorithm, it is quicker and does not include lots of unnecessary steps (Abstraction)
• Condition


  A situation that is checked every time. If the condition is false the iteration repeats and if it is true the condition stops
• Counter


  They can keep count of how many iterations have been completed
• Iteration is the act of repeating a process until there is a desired outcome
• Real-world examples of humans iterating


  • Practising sports
  • Boiling water
  • Practising an instrument
• WHILE loop


  1. Index = 0

  2. WHILE index <= 10

  3. OUTPUT("The number is " + index)

  4. Index = index + 1

  5. ENDWHILE
• Python WHILE loop


  1. Print("Please insert £8 for a ticket")

  2. While total < 8

  3. coins = int(input("Enter pound coins"))

  4. total = total + coins

  5. print("Amount entered", total)

  6. print("Thank you!")
• FOR loop


  • Pseudocode uses a variable name and has to show the start and end e.g 0-10
  • Python uses a string and you only have to show the end but it doesn't go up to the end number e.g (14) it only goes from 0-13
• WHILE loop


  • Pseudocode similar to FOR loop but you use < or .
  • Python once you do what is asked e.g put money in the ticket machine, the loop is dropped and it proceeds with the code
• Definite iteration


  The number of iterations that are going to happen before the iteration of the loop e.g 5 lessons in a day, definitely going to happen
• WHILE loop algorithm


  1. number = int(input('Please enter your number.'))

  2. Index = 1

  3. While index < = 12:

  4. print(index + 'x' + number + '=' + number + index

  5. INDEX = INDEX + 1
• Python algorithm for entering numbers

  1. a = int(input("Enter a low number: ")

  2. b = int(input("Enter a higher number: ")

  3. while a <= b

  4. print(a)

  5. a = a+1
• Indefinite iteration


  The number of iterations is not known before the loop is started
• Definite iteration


  The number of iterations is known before the execution of the loop is started