CS Notes

Decisions in Computational Thinking

4 min read

Identify the Points in a Solution where a Decision has to be Taken

  • Most languages use structured programming techniques including:
    • Sequence: one statement after another
    • Selection: Decision making, if/then/else, switch/case
    • Iteration: Loops, for/while/do while/do until
  • The purpose of these techniques is to aid the readability, **understanding **and maintainability of code
  • **Python **is an example of a block-structured language, which uses only these three constructs to control the flow of execution and data in a program. Each block of code should have a single entry and exit point and ideally minimise breaking out of iterative blocks. This is to prevent unintended consequences relating to the flow of control of a program such as entering or leaving a subroutine early or branching unintentionally
  • When designing algorithms, it is always best to **plan the algorithm **using flowcharts, pseudocode or structured english before coding in a language
  • **Languages **have specific syntax, constructs and idiosyncrasies that differ between other languages. This makes it challenging to create a solution that can immediately be implemented in another language
  • When determining the points in a solution where decisions are made, **flowcharts are useful **as they **visually **show the flow of control in a program. Decisions are clear and easy to follow, however flowcharts are time consuming to create
  • **Pseudocode **more **accurately **mimics the constructs of programming languages without the problem of syntax. As pseudocode has no syntax, any way of expressing an algorithm is acceptable as long as the pseudocode meaning is clear
  • Structured English can be an alternative to pseudocode but is usually more **verbose **and imprecise

How do you Identify the Points in a Solution where a Decision has to be Taken?

  • Most errors in a program occur when evaluating a Boolean condition, whether in a **sequence **as part of a statement, iteration or selection. It is therefore important to be careful when creating Boolean conditions, especially long, complex conditions involving multiple clauses
  • Decisions in programs usually occur in two situations, an if statement/select case or a loop, usually a while loop
  • Selection, also known as branching, involves directing the flow of control of a program, dependent on a **Boolean **condition or set of conditions
  • **Iteration **involves **repeating **a sequence of instructions based on a stopping **Boolean **condition

Determine how Decisions Affect Flow through a Program

  • Decisions **affect **the flow of control of a program
  • Decisions are Boolean conditions encountered in selection structures such as if/then/else statements and **iteration **structures such as while loops
  • **If **statements and switch/case statements can consist of many decision points as illustrated below. Each decision point directs the program through different statements

if/then/else example

if today == "Monday" then
    print("Eugh! Monday again!")
elseif today == "Tuesday" then
    print("Tuesday, one day closer the weekend!")
elseif today == "Wednesday" then
    print("Half way there!")
elseif today == "Thursday" then
    print("One more day to go!")
elseif today == "Friday" then
    print("I can't believe its Friday!")
elseif today == "Saturday" then
    print("Woo! Its Saturday!")
elseif today == "Sunday" then
    print("Aww, its Monday tomorrow!")
else
    print("That's not a day!")
endif

switch/case statement example

switch entry:
    case "Monday":
        print("Eugh! Monday again!")
    case "Tuesday"
        print("Tuesday, one day closer the weekend!")
    case "Wednesday":
        print("Half way there!")
    case "Thursday":
        print("One more day to go!")
    case "Friday":
        print("I can't believe its Friday!")
    case "Saturday":
        print("Woo! Its Saturday!")
    case "Sunday":
        print("Aww, its Monday tomorrow!")
    default:
        print("That's not a day!")
endswitch
  • **Iteration **and **selection **statements can be nested, leading to a structure as shown below
Count ← 0
REPEAT 
    INPUT Score[Count] 
    IF Score[Count] >= 70 THEN 
        Grade[Count] ← "A" 
    ELSE 
        IF Score[Count] >= 60 THEN 
            Grade[Count] ← "B" 
        ELSE 
            IF Score[Count] >= 50 THEN 
                Grade[Count] ← "C" 
            ELSE 
                IF Score[Count] >= 40 THEN 
                    Grade[Count] ← "D" 
                ELSE 
                    IF Score[Count] >= 30 THEN 
                        Grade[Count] ← "E" 
                    ELSE 
                        Grade[Count] ← "F" 
                    ENDIF 
                ENDIF 
            ELSE
        ENDIF 
    ENDIF 
Count ← Count + 1 
UNTIL Count = 30
  • Each **if statement **contains another **if statement **which affects the flow of the program. This is however functionally identical to the days of the week program shown above

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