Selection: making decisions in programs

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Lesson notes

What is Selection?

Selection is a programming construct that makes decisions by choosing which code to run based on a condition.
A condition is a Boolean expression – it evaluates to either true or false.
Selection changes the flow of control in a program: it decides which statement or block of statements executes next.
It is one of the three basic building blocks of programming, alongside sequence and iteration.

A Boolean condition decides which one of two paths runs; the other is skipped.

The if-then-else Statement

The most common form of selection is the if-then-else statement.
General syntax: `if condition then consequent else alternative end if`.
If the condition is true, the consequent (the statements after `then`) runs.
If the condition is false, the alternative (the statements after `else`) runs.
The `else` part is optional – if omitted and the condition is false, nothing happens.
The consequent and alternative can be a single statement or a block of statements.

Writing Conditions with Comparison Operators

Conditions often use comparison operators to compare values.
Common operators: `==` (equal to), `≠` (not equal to), `<` (less than), `>` (greater than), `≤` (less than or equal to), `≥` (greater than or equal to).
Example: `if age ≥ 18 then print "Adult" else print "Minor"`.
Conditions can be combined using logical operators like `AND`, `OR`, and `NOT`.
Example: `if score > 50 AND score ≤ 100 then print "Pass"`.

Nested Selection

Selection statements can be placed inside other selection statements – this is called nesting.
Nesting allows more complex decisions, e.g., checking multiple conditions in a hierarchy.
Example: `if x > 0 then if x > 10 then print "large positive" else print "small positive" end if end if`.
Be careful with the dangling else problem: an `else` pairs with the nearest preceding `if` unless braces or `end if` are used.
Use indentation and clear block structure to make nested selection readable.

Chaining Conditions: else if

To check multiple conditions in sequence, use else if (or `elif` in some languages).
Only the statements after the first true condition run; all later branches are skipped.
Example: `if discount == 10 then print "10% off" else if discount == 20 then print "20% off" else print "no discount" end if`.
This is more concise than nested `if` statements and avoids deep indentation.
Different languages use different keywords: `else if` (C, Java), `elif` (Python), `ElseIf` (Visual Basic).

Tracing Selection

Tracing means manually following the flow of a program to determine which branch executes for given inputs.
To trace, evaluate the condition with the given input, then follow the appropriate branch.
Example: for `if x > 5 then print "big" else print "small"`, if `x = 7`, the condition is true, so "big" is printed.
Tracing helps debug and verify that selection logic works correctly.
Always consider edge cases like boundary values (e.g., `x == 5`) when tracing.

Switch Statements

A switch statement is an alternative for multiway branching when comparing one expression against many constant values.
It transfers control to the code of the first matching case; if none match, a default action runs.
Example: `switch day: case 1: print "Monday" case 2: print "Tuesday" default: print "Other"`.
Switch statements can be more efficient and readable than long chains of `else if`.
Some languages (like C) require `break` to exit a case; others (like Python) use dictionaries instead.

Common Pitfalls

Dangling else: an `else` may pair with the wrong `if` if blocks are not properly delimited.
Off-by-one errors: using `>` instead of `≥` can exclude boundary values.
Forgetting the else when a default action is needed.
Over-nesting makes code hard to read – consider `else if` or switch.
Using assignment `=` instead of equality `==` in conditions (common in C-like languages).

The same if-then-else written as pseudocode and drawn as a flowchart.

Nested selection tests a second condition only when the first is false, giving three possible outcomes.

Slides

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Practice questions

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1.What does an if statement do in a program?
Easy
- AMakes a decision based on a condition
- BRepeats a block of code
- CStores a value in a variable
- DPrints output to the screen
2.A Boolean expression can evaluate to either true or false.
Easy
True or false?
3.What is the purpose of the 'else' clause in an if-then-else statement?
Easy
- ATo specify what happens when the condition is false
- BTo repeat the condition
- CTo define a new variable
- DTo end the program
4.In the pseudocode: if discount == 10 then print "pay $30" else if discount == 20 then print "pay $20" else print "pay $10". If discount = 20, what is printed?
Medium
- Apay $30
- Bpay $20
- Cpay $10
- DNothing
5.In a chained if-else-if statement, only the statements after the first true condition are executed.
Easy
True or false?
6.Which keyword is used in Python to chain conditions without increasing indentation?
Easy
- Aelseif
- Belsif
- Celif
- Delse if
7.Which of the following is a valid way to avoid the dangling else problem in many languages?
Medium
- AUsing explicit block-ending syntax like 'end if'
- BUsing more nested if statements
- CRemoving all else clauses
- DUsing goto statements
8.Which of the following are examples of comparison operators commonly used in conditions? (Select all that apply)
Medium
- A==
- B=
- C<
- D≥
- E++

Revision notes

## Selection: Making Decisions in Programs Selection is how a program chooses which code to run based on a **condition** (a Boolean expression that is either **true** or **false**). ### The `if` statement - **Basic form:** `if condition then ... else ... end if` (pseudocode). - If the condition is **true**, the code after `then` runs; if **false**, the code after `else` runs (if present). - The `else` part is optional. - Example: ``` if discount == 10% then print "you have to pay $30" else print "pay full price" end if ``` ### `else if` (chaining) - Use `else if` (or `elif` in Python, `elsif` in Ruby/Perl, `ElseIf` in Visual Basic) to test multiple conditions in sequence. - Only the first condition that is **true** runs; all later branches are skipped. - Example: ``` if discount == 10% then print "you have to pay $30" else if discount == 20% then print "you have to pay $24" else print "no discount" end if ``` - In languages like C, Java, JavaScript, you can write `else if` without a special keyword because each `if` is a single statement. ### Nested selection - You can put an `if` inside another `if` to test more than one condition. - Watch out for the **dangling else** problem: an `else` always pairs with the **nearest** unmatched `if`. Use braces `{}` or `end if` to make it clear. ### Switch statement - A `switch` statement is an alternative to long chains of `else if` when you are comparing one expression against many constant values. - It jumps to the first matching case; a `default` case runs if no match. - Example (pseudocode): ``` switch (day) { case "Monday": print "Start of week"; break; case "Friday": print "End of week"; break; default: print "Midweek"; } ``` ### Tracing - To trace which branch runs, evaluate the condition step by step with the given input. - Remember: only one branch executes in an `if...else if...else` chain. ### Key points - Conditions use comparison operators like `==`, `!=`, `<`, `>`, `<=`, `>=`. - Selection is a core part of **structured programming** (replacing messy `goto` statements). - Always test your conditions with different inputs to make sure the right branch runs.

Conditional statement: A programming construct that directs control flow based on whether a condition (a Boolean expression) is true or false.
Boolean expression: An expression that evaluates to either true or false.
if-then-else: A conditional statement that executes a block of code if a condition is true, and optionally another block if false.
else if (elif): A construct used to chain multiple conditions; only the first true condition's block is executed.
Nested selection: Placing one conditional statement inside another to test multiple conditions.
Dangling else: An ambiguity in nested conditionals where an else clause may be paired with the wrong if; resolved by using braces or explicit end markers.
Switch statement: A multiway branching statement that compares an expression against constant values and jumps to the first matching case, with an optional default.
Structured programming: A programming paradigm that uses clear control flow structures like if-then-else and loops instead of goto statements.

Flashcards

What is a conditional statement in programming?

A conditional statement directs program control flow based on the value of a condition, which is a Boolean expression (true or false).

What is the general syntax of an if-then-else statement?

The general syntax is: if condition then consequent else alternative end if. The condition evaluates to true or false; if true, the consequent runs, otherwise the alternative runs. The else part is optional.

What is the 'dangling else' problem?

It occurs when an else clause is ambiguously paired with a preceding if in nested conditionals. It is resolved by using explicit block-ending syntax (like 'end if') or block enclosures (like curly braces).

What is an else-if (elif) construct?

An else-if construct allows chaining multiple conditions. Only the statements following the first true condition are executed; all others are skipped. It is often written as 'else if' or 'elif'.

In the pseudocode example, if discount is 10%, what is printed?

If the discount is 10%, the first if statement is true, so 'you have to pay $30' is printed, and all other statements are skipped.

Why does Python use the keyword 'elif'?

Because Python uses indentation for structure, a repeated 'else if' would require increased indentation after every condition. 'elif' avoids that.

Which languages use 'elsif'?

Perl and Ruby use 'elsif' to avoid large numbers of braces.

What is a switch statement?

A switch statement supports multiway branching by comparing the value of an expression with constant values and transferring control to the code of the first match. It often has a default action if no match is found.

What is the purpose of a default case in a switch statement?

The default case is executed when no other case matches the expression's value.

What is the Guarded Command Language (GCL)?

GCL, by Edsger Dijkstra, uses guarded commands: a list of Boolean guards with corresponding statements. Exactly one statement whose guard is true is executed, but which one is arbitrary.

What is the difference between a conditional statement and a conditional expression?

A conditional statement directs control flow, while a conditional expression evaluates to a value without changing control flow.

What is structured programming?

Structured programming supports control flow via code blocks, making code easier to read and maintain, as opposed to using goto statements.

What is the 'else if' construct in Ada?

In Ada, 'elseif' is syntactic sugar for the two words 'else if'.

What is the purpose of the 'end if' in pseudocode?

'end if' marks the end of an if-then-else statement, helping to avoid the dangling else problem.