comp sci aqa

• Algorithm


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


  • A fairly relaxed version of program code that should be understood by any programmer
  • AQA has its own version of pseudocode
• Flowchart symbols


  • Start/end
  • Process box
  • Input/output
  • Choice (for if statements and loops)
• Abstraction


  The process of removing unnecessary detail from a problem, simplifying it
• Decomposition


  Breaking a problem down into many sub-problems that each represent a specific task
• Trace table


  A table that shows how values change when an algorithm is carried out, going through every line of code
• Trace tables do not correspond one-to-one with lines of code, there may be multiple lines of code in one row of the table
• Linear search


  A searching algorithm that compares each item in the list one by one until the target item is found or not found
• Binary search


  A more efficient searching algorithm that compares the middle item in the list to the target, and discards half the list that cannot contain the target
• Binary search


  • Requires the list to be sorted
  • More time-efficient than linear search as the list grows larger
• Bubble sort


  A sorting algorithm that goes through the list in 'passes', swapping pairs of items to put the list in order
• Merge sort


  A more efficient sorting algorithm that works by repeatedly dividing the list in half and merging the sorted sub-lists
• Merge sort


  • More time-efficient than bubble sort, especially as the list grows larger
  • Uses more memory overall due to the sub-lists
• Data type


  Determines how data is stored on a computer and the operations it can do
• Arithmetic operators


  Division (/, div), modulus (mod)
• Relational and Boolean operators


  Used in conditions for if statements and loops (=, >, <, and, or)
• String handling operations


  Substring, concatenation
• Variable


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


  An identifier that holds a value that cannot be changed after it is set
• Constants


  • Help with readability
  • Make updating values easier in the future
  • Reduce risk of accidental changes
• Input/output


  How to get user input and display output
• Sequence


  Two or more lines of code that run one by one
• Selection (if statements)


  Deviate the path of execution depending on a condition
• Iteration (loops)


  Repeat a block of code multiple times
• If statements are always checked, whereas else-if statements are only checked if previous conditions were false</b>
• Common mistakes with if statements include adding conditions to else blocks and forgetting about else-if
• If it's always checked if you've got an if part this will always be checked no matter if you have several back to back
• If price is greater than 30 and if price is greater than or equal to 20


  Both premium and standard would be outputted because both are checked
• Else ifs or e-lifts sit below an if statement


  The else if is only checked if previous conditions were false
• You can have several else ifs if you want to, just make sure they come after the if
• Mistakes in exams are people adding conditions to else's and also forgetting about else if
• Definite iteration


  A set number of times the loop will repeat
• Indefinite iteration


  The loop will repeat until a condition is broken or met
• For loop


  1. Repeats a set number of times

  2. Counter variable starts at a set number and goes to another set number
• While loop


  1. Repeats until a condition is broken

  2. Asks user for password and keeps asking until they get it right
• Repeat until loop


  1. Repeats until a condition is met

  2. Condition comes after the loop, so it always runs at least once
• Array


  An organised collection of items of the same data type
• Indexing


  Specifying a particular item within an array, starting from 0
• Iterating through an array


  Using a for loop, starting at 0 and going to size of array - 1
• 2D array


  An array of arrays