whole paper 2 cs

• Algorithm


  A sequence of steps that can be followed to complete a task
• Algorithmic thinking


  Solving problems by defining steps and the sequence needed
• Program code is just implementing an algorithm
• Algorithms

  • Can be represented in pseudocode
  • Can be represented in flowcharts
• Pseudocode


  Resembling code but more relaxed
• Flowchart


  • Has a set group of symbols including start/end, process, sub-program, input/output
  • Uses a diamond for selection decisions
• Abstraction


  Removing unnecessary detail from a problem
• Decomposition


  Breaking down a problem into sub-problems
• Structure diagram


  Represents decomposition of a larger problem into sub-problems
• Trace table


  Shows how values change when an algorithm is carried out
• Trace tables


  • Go through every line of code
  • Only add to a column when that value changes
  • Generally move to a new row when moving to a new block of code
• Linear search


  Looks for a target by comparing each item in a list one by one
• Binary search


  Compares the middle item to the target, discards half the list, and repeats until the target is found or the list is exhausted
• Searching algorithms


  • Linear search doesn't require the list to be ordered
  • Binary search requires the list to be ordered
• Bubble sort


  Does a series of passes, swapping pairs if needed, until a full pass happens with no swaps
• Merge sort


  1. Divide the list continuously by two until each list has one item

  2. Merge the lists back together keeping them in order
• Insertion sort


  Starts with one item in a sorted part, moves items one by one from the unsorted part to the sorted part
• In section A of the exam, you can answer algorithm questions in your own way
• In section B of the exam, you may need to answer in a programming language, either a real one or the OCR exam reference language
• Data types


  • Integer
  • Real (float)
  • Boolean
  • String
  • Character
• Casting


  Converting between data types
• Arithmetic operations


  • Addition
  • Subtraction
  • Multiplication
  • Division (real and integer)
  • Exponentiation
• Modulus


  Gives the remainder of a division
• Comparison operators


  • Equal to
  • Not equal to
  • Greater than
  • Greater than or equal to
  • Less than
  • Less than or equal to
• Boolean


  Logical operations of AND and OR
• Substring


  Extracts part of a string
• Concatenation


  Joining two strings together
• Variable


  An identifier that holds a value that can be changed
• Constant


  An identifier that holds a value that cannot be changed
• Getting user input


  1. Assign the user input to a variable

  2. May need to cast the input to a different data type
• Random function


  Generates a random number
• Programming constructs


  • Sequence (lines executed in order)
  • Selection (if/elif/else statements, switch statements)
  • Iteration (for loops, while loops, do-until loops)
• While loop


  1. Repeat until a condition is broken

  2. Useful when you don't know how many times the loop needs to repeat
• Do until loop (do while loop)

  1. Repeat until a condition is met

  2. Runs at least once even if condition is false
• Subprogram (subroutine)


  Named out-of-line block of code
• Procedure


  Type of subprogram
• Function


  Type of subprogram that has a return value
• Parameter


  Variable used as an input to a subprogram
• Return value


  Output from a subprogram
• Local variable


  Variable that only exists when the subprogram is executing and is only accessible inside the subprogram