Selection: making decisions in programs

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レッスンノート

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.

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練習問題

無料プレビュー — 59問中8問。すべて見るには登録を。

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++

もっと問題

9.Match each language to the keyword it uses for else-if chaining.
Medium
- Python
- PHP
- Perl
- elif
- elseif
- elsif
10.In a switch statement, if no case matches, a default action is always executed.
Easy
True or false?
11.Consider the pseudocode: if x > 0 then if x > 10 then print "large" else print "small" else print "negative". If x = 5, what is printed?
Hard
- Alarge
- Bsmall
- Cnegative
- DNothing
12.Arrange the steps to correctly evaluate an if-else-if chain.
Medium
- Evaluate the first condition
- If false, evaluate the next condition
- If a condition is true, execute its block and skip the rest
- If all conditions are false, execute the else block (if present)
13.What is the general syntax of an if-then-else statement in pseudocode?
Easy
- Aif condition then consequent else alternative
- Bif condition then consequent
- Cif condition else consequent then alternative
- Dif consequent then condition else alternative
14.In the Guarded Command Language (GCL), exactly one of the statements whose guard is true is evaluated, but which one is arbitrary.
Easy
True or false?
15.Which of the following is an example of a conditional expression that evaluates to a value without changing control flow?
Medium
- Ax = (a > b) ? a : b
- Bif (a > b) then print a
- Cfor i = 1 to 10
- Dgoto label
16.The else clause is mandatory in an if-then-else statement.
Easy
True or false?
17.In the pseudocode: if discount == 10 then print "you have to pay $30" else if discount == 20 then print "you have to pay $20" else print "you have to pay $10". If discount = 10, which statements are executed?
Medium
- AOnly the first print
- BThe first and second prints
- CAll prints
- DOnly the else print
18.Which of the following is NOT a way to implement a switch statement if a language does not have one?
Medium
- AUse a chained if-then-else
- BUse a control table with an optimizing compiler
- CUse nested if statements
- DUse a for loop
19.In ALGOL 68, the keyword 'elif' was standardized in the 1973 Revised Report.
Medium
True or false?
20.What is the main advantage of using 'else if' instead of nested 'else' followed by 'if'?
Easy
- AIt avoids the dangling else problem
- BIt reduces the number of 'end if' statements needed
- CIt makes the code run faster
- DIt allows multiple conditions to be true
21.Match the term to its description.
Medium
- Conditional statement
- Conditional expression
- Boolean expression
- Directs control flow
- Evaluates to a value
- Evaluates to true or false
22.What is the purpose of an 'if' statement in a program?
Easy
- ATo repeat a block of code a fixed number of times
- BTo make a decision based on a Boolean condition
- CTo store a value in a variable
- DTo display output to the user
23.The 'else' clause is optional in an if-then-else statement.
Easy
True or false?
24.Which of the following are comparison operators commonly used in conditions? (Select all that apply)
Easy
- A==
- B=
- C≤
- D≥
- E<>
- F≠
25.In the pseudocode: if x > 5 then print "big" else if x > 0 then print "small" else print "negative". If x = 3, what is printed?
Medium
- Abig
- Bsmall
- Cnegative
- Dnothing
26.Which of the following is a recommended way to avoid the dangling else problem in many programming languages?
Medium
- AUsing indentation to show which if the else belongs to
- BUsing explicit block delimiters or 'end if'
- CAvoiding nested if statements entirely
- DUsing the 'goto' statement
27.A program checks the temperature of a room. If the temperature is above 25°C, it turns on the air conditioning. Which part of the if-then-else statement is the condition?
Easy
- Atemperature > 25
- Bturn on air conditioning
- Cthe program
- Dthe room
28.A student writes: if score ≥ 50 then print "Pass" else print "Fail". What is the output if score = 50?
Medium
- APass
- BFail
- CNo output
- DError
29.In an if-then-else statement, if the condition is false, the statements in the else block are executed.
Easy
True or false?
30.A program uses the following pseudocode to assign a grade based on a score: if score ≥ 90 then grade = 'A' else if score ≥ 80 then grade = 'B' else if score ≥ 70 then grade = 'C' else grade = 'D'. What is the grade if score = 85?
Medium
- AA
- BB
- CC
- DD
31.A program uses the following pseudocode: if x > 0 then if x > 10 then print "large" else print "small" else print "negative". What is printed if x = 5?
Medium
- Alarge
- Bsmall
- Cnegative
- Dnothing
32.Which of the following best describes the purpose of the 'else' clause in an if-then-else statement?
Easy
- AIt specifies what to do if the condition is false.
- BIt specifies what to do if the condition is true.
- CIt always executes regardless of the condition.
- DIt checks a second condition if the first is false.
33.In an if-then-else statement, the 'else' clause is mandatory.
Easy
True or false?
34.What is the output of the following pseudocode if x = 7? if x > 10 then print "large" else if x > 5 then print "medium" else print "small" end if
Easy
- Amedium
- Blarge
- Csmall
- DNo output
35.A program uses the following nested if statement: if x > 0 then if x > 10 then print "big" else print "small" end if else print "negative" end if What is printed if x = 5?
Medium
- Asmall
- Bbig
- Cnegative
- Dnothing
36.Match each programming language to the keyword it uses for else-if chaining.
Medium
- Python
- Visual Basic
- ALGOL 68
- elif
- ElseIf
37.Arrange the steps to correctly evaluate an if-else-if chain in pseudocode.
Medium
- Evaluate the first condition.
- If true, execute the first block and skip the rest.
- If false, evaluate the next condition.
- Repeat until a condition is true or all are false.
- If none are true, execute the else block (if present).
38.In the Guarded Command Language (GCL), if multiple guards are true, which statement is executed?
Medium
- AExactly one, chosen arbitrarily
- BAll of them, in order
- CThe first one listed
- DNone of them
39.A conditional statement uses a Boolean expression to decide which part of the program to run.
Easy
True or false?
40.In pseudocode, which keyword is used to chain conditions without increasing indentation in Python?
Easy
- Aelseif
- Belsif
- Celif
- Delse if
41.Complete the sentence.
Easy
In an if-then-else statement, if the condition is ____ the program executes the consequent; if it is ____ it executes the alternative.
42.What is the name of the construct that supports multiway branching by comparing an expression with constant values?
Medium
43.In a program, a variable 'score' is set to 75. The code is: if score ≥ 70 then grade = 'A' else if score ≥ 60 then grade = 'B' else grade = 'C'. What is the value of grade? (Assume grade is a string, answer as the letter)
Hard
44.What is the keyword used in Visual Basic for else-if chaining?
Easy
45.A program uses the following code: if x mod 2 == 0 then print "even" else print "odd". If x = 7, what is printed? (Answer as 1 for even, 0 for odd)
Hard
46.Complete the sentence: In many programming languages, the ___ statement is used when you need to compare a single expression against multiple constant values.
Easy
In many programming languages, the ____ statement is used when you need to compare a single expression against multiple constant values.
47.Name the two possible values of a Boolean expression.
Easy
Name the two possible values of a Boolean expression: ____ and ____.
48.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 amount is printed? (Give the numeric value in dollars)
Medium
49.What is the term for the problem where an 'else' clause can be ambiguously paired with one of several nested 'if' statements?
Medium
50.A shop offers discounts based on the price of an item. The pseudocode is: if price > 100 then discount = 20 else if price > 50 then discount = 10 else discount = 5. What is the discount (in pounds) for an item priced at £75?
Medium
51.Fill in the blank:
Easy
In an if-then-else statement, the condition is a ____ expression that evaluates to either true or false.
52.What are the two possible values of a Boolean expression?
Easy
53.Explain why using 'goto' statements in early if-then-else constructs led to 'spaghetti code', and how structured programming solves this.
Challenge
54.Describe the difference between a conditional statement and a conditional expression, giving an example of each.
Hard
55.Describe the dangling else problem and explain one way that programming languages resolve it.
Hard
56.In a program, a variable 'score' is set to 75. The code is: if score ≥ 70 then grade = 'A' else if score ≥ 60 then grade = 'B' else grade = 'C'. What is the value of grade? (Assume grade is a string, answer as the letter)
Hard
- A0
- B-1
- C1
- D2
57.A program uses the following code: if x mod 2 == 0 then print "even" else print "odd". If x = 7, what is printed? (Answer as 1 for even, 0 for odd)
Hard
- A-1
- B0
- C2
- D1
58.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 amount is printed? (Give the numeric value in dollars)
Medium
- A22 $
- B24 $
- C20 $
- D18 $
59.A shop offers discounts based on the price of an item. The pseudocode is: if price > 100 then discount = 20 else if price > 50 then discount = 10 else discount = 5. What is the discount (in pounds) for an item priced at £75?
Medium
- A12 £
- B9 £
- C11 £
- D10 £

復習ノート

## 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.

フラッシュカード

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.