Selection and iteration

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Notes de leçon

Selection

Selection lets a program choose between paths using conditions. IF ... THEN ... ELSE ... ENDIF; ELSE IF for multiple options; nested IF statements put one decision inside another.
Example: IF mark ≥ 70 THEN OUTPUT "Distinction" ELSE IF mark ≥ 40 THEN OUTPUT "Pass" ELSE OUTPUT "Fail" ENDIF.

Selection chooses a path

Iteration

Count-controlled loops (FOR) repeat a set number of times: FOR i = 1 TO 5 ... NEXT i.
Condition-controlled loops (WHILE, REPEAT UNTIL) repeat until a condition is met. WHILE checks before; REPEAT UNTIL checks after (always runs at least once).
Nested loops: a loop inside a loop; useful for tables and 2D arrays.

Repeating instructions in a loop

Choosing

Use FOR when you know how many repeats; WHILE/REPEAT when you don't (e.g. keep asking for a password until correct).

Avoiding infinite loops

For WHILE, update the values used by the condition so it can become false. For REPEAT UNTIL, update them so the stopping condition can become true. Trace normal and boundary cases.

Trace a changing loop variable

Boolean conditions

Combine with AND, OR, NOT; use brackets for clarity.

Tracing

Use trace tables to follow loop values.

Diapos

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Questions d'entraînement

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1.What does IF do?
Easy
- Aruns code only if a condition is true
- Brepeats code
- Cends a program
- Dstores data
2.What does a loop do?
Easy
- Arepeats instructions
- Bmakes a decision
- Cstores a value
- Dends a program
3.What does ELSE do?
Easy
- Aruns when the IF condition is false
- Brepeats code
- Cends the loop
- Dstores a value
4.Which loop runs a fixed number of times?
Easy
- AFOR
- BWHILE
- CREPEAT UNTIL
- DIF
5.Which condition-controlled loop tests before executing its body?
Easy
- AWHILE
- BREPEAT UNTIL
- CFOR only
- Dnone
6.What is the output? x = 5 IF x > 3 THEN OUTPUT "A" ELSE OUTPUT "B" ENDIF
Easy
- AA
- BB
- C5
- Dnothing
7.What is the output? n = 1 WHILE n < 4 OUTPUT n n = n + 1 ENDWHILE
Easy
- A1 2 3
- B1 2 3 4
- C4
- D0 1 2 3
8.What is the output? FOR i = 2 TO 8 STEP 2 OUTPUT i NEXT i
Easy
- A2 4 6 8
- B2 3 4 5 6 7 8
- C2 8
- D8 6 4 2

Plus de questions

9.What does REPEAT UNTIL do?
Easy
- Arepeats until a condition is true
- Brepeats while a condition is true
- Cruns once
- Dnever runs
10.Why use a loop?
Easy
- Ato avoid repeating the same code many times
- Bto make code longer
- Cto stop errors
- Dto hide logic
11.What is a nested IF?
Easy
- Aan IF inside another IF
- Btwo IFs in a row
- Ca loop in an IF
- Dan IF with no ELSE
12.Which loop is best for asking for a password until it is correct?
Medium
- AWHILE or REPEAT UNTIL
- BFOR
- CIF
- Dnone
13.Which loop is best for printing a times table 1 to 12?
Medium
- AFOR
- BWHILE with unknown
- CIF
- Dnone
14.A WHILE loop has no separate exit or exception. What happens if its condition remains true?
Medium
- Aan infinite loop
- Bthe loop ends
- Can error
- Dthe loop skips
15.What does ELSE IF allow?
Medium
- Achecking another condition
- Bending the program
- Crepeating
- Dstoring
16.Which operator joins two conditions requiring both to be true?
Medium
- AAND
- BOR
- CNOT
- DMOD
17.What is the output? total = 0 FOR i = 1 TO 4 total = total + i NEXT i OUTPUT total
Medium
- A10
- B4
- C6
- D24
18.What is the output? x = 10 WHILE x > 0 x = x - 4 ENDWHILE OUTPUT x
Medium
- A-2
- B0
- C2
- D10
19.What is the output? FOR i = 1 TO 3 FOR j = 1 TO 2 OUTPUT i×j NEXT j NEXT i
Medium
- A1 2 2 4 3 6
- B1 2 3
- C2 4 6
- D1 2
20.How many times does the inner loop body run in FOR i = 1 TO 4, FOR j = 1 TO 5?
Medium
- A20
- B9
- C4
- D5
21.What is wrong with: x = 1 WHILE x > 0 OUTPUT x ENDWHILE
Medium
- Ax never changes so it loops forever
- Bit has no output
- Cit uses FOR
- Dit ends at once
22.What does STEP do in a FOR loop?
Easy
- Asets the increment
- Bends the loop
- Cadds an IF
- Dstores data
23.What is a condition?
Easy
- Aan expression that is true or false
- Ba loop
- Ca variable
- Dan output
24.What does NOT (x > 5) mean?
Medium
- Ax is less than or equal to 5
- Bx is greater than 5
- Cx must equal 5
- Dx is less than 5 only
25.What is a counter in a loop?
Medium
- Aa variable that counts repetitions
- Ba loop type
- Can error
- Da function
26.A FOR loop repeats a fixed number of times.
Easy
True or false?
27.A WHILE loop checks its condition before running.
Easy
True or false?
28.A REPEAT UNTIL loop may never run.
Easy
True or false?
29.Nested loops can be used to process 2D arrays.
Easy
True or false?
30.An IF statement repeats code.
Medium
True or false?
31.Which are loop types?
Medium
- AFOR
- BWHILE
- CREPEAT UNTIL
- Dnested loops
- EIF
32.In a WHILE loop with no other exit, which can cause infinite repetition?
Medium
- Aa condition that never becomes false
- Bforgetting to update the counter
- CWHILE TRUE with no exit
- Da correct FOR loop
- Ea finished loop
33.Write pseudocode to output the numbers 1 to 10 using a loop.
Medium

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