Three levels of nesting

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Three levels of nesting

Name:
Date:
Class:
A

Quick Questions

Shade in the A or B circle for each question.

1. A FOR loop contains an IF, and that IF contains another IF. Which closing order is correct?
A ENDIF, ENDIF, then NEXT
B NEXT, ENDIF, then ENDIF
2. Which statement about indentation in a three-level nested algorithm is true?
A All three levels are written at the same indentation
B Each level is indented one step more than the structure around it
3. A WHILE loop contains a FOR loop, and the FOR loop contains an IF. Which structure is closed first?
A ENDIF
B ENDWHILE
4. Which shows the correct closing order for WHILE containing FOR containing IF?
A ENDWHILE, NEXT, ENDIF
B ENDIF, NEXT, ENDWHILE
5. How should you read a three-level nested algorithm to work out what it does?
A From the inside out, starting with the innermost structure
B Only the outermost structure matters, so ignore the rest
B

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left columnRight column
1ENDIF ENDIF NEXT i AOpens three levels of nesting: a WHILE containing a FOR containing an IF, each indented one level further than the one before.
2WHILE n > 0 DO FOR i <- 1 TO 3 IF i = n THEN BFOR i <- 1 TO 3 IF i > 1 THEN IF i < 3 THEN OUTPUT i
3A WHILE containing a FOR containing an IF must be closed in reverse order: ENDIF, then NEXT, then ENDWHILE. C ENDIF NEXT j ENDWHILE
4Indent each level of nesting one step further, so the innermost statement sits furthest to the right. DNEXT i closes the FOR before the outer IF has been closed. The structures overlap instead of nesting. The outer ENDIF must come before NEXT i.
5FOR i <- 1 TO 5 IF a > 0 THEN IF b > 0 THEN OUTPUT i ENDIF NEXT i ENDIF EThe outermost WHILE loop is never closed. ENDWHILE is missing after NEXT i.
6WHILE n > 0 DO FOR i <- 1 TO 3 IF i = n THEN OUTPUT i ENDIF NEXT i FCloses the inner IF first, then the outer IF, then the FOR loop. This is the reverse of the order they were opened.
C

Definitions

nesting
D

Fill in the Blanks

Complete each line of code by filling in the missing parts.

1.
FOR Day <- 1 TO 7 IF Temp > 20 THEN IF Rain = 0 ____
2.
ENDIF NEXT Count ________
3.
WHILE Total < 100 __ FOR Count <- 1 TO 5 IF Mark > 50 THEN
4.
WHILE Total < 100 DO ___ Count <- 1 TO 5 IF Mark > 50 THEN
5.
ENDIF ENDIF ____ Day
E

Spot the Error

Shade in the letter of the line that contains an error.

Question 1
A FOR Day <- 1 TO 7
B IF Rain > 10
C IF Temp < 15 THEN
D ENDIF
E ENDIF
F NEXT Day
Question 2
A WHILE Total < 100 THEN
B FOR N <- 1 TO 5
C IF N MOD 2 = 0 THEN
D ENDIF
E NEXT N
F ENDWHILE
Question 3
A FOR S <- 1 TO 30
B IF Mark[S] >= 50 THEN
C IF Mark[S] >= 80 THEN
D ENDWHILE
E ENDIF
F NEXT S
Question 4
A WHILE Lives > 0 DO
B FOR Shot <- 1 UNTIL 3
C IF Hit = TRUE THEN
D ENDIF
E NEXT Shot
F ENDWHILE
Question 5
A FOR Seat <- 1 TO 40
B IF Seat < 21 THEN
C IF Seat MOD 2 == 0 THEN
D ENDIF
E ENDIF
F NEXT Seat

Answer Key

Three levels of nesting
Section A: Quick Questions
Q1
A — ENDIF, ENDIF, then NEXT
Q2
B — Each level is indented one step more than the structure around it
Q3
A — ENDIF
Q4
B — ENDIF, NEXT, ENDWHILE
Q5
A — From the inside out, starting with the innermost structure
Section B: Match the Pairs
Q1
ENDIF ENDIF NEXT i → Closes the inner IF first, then the outer IF, then the FOR loop. This is the reverse of the order they were opened.
Q2
WHILE n > 0 DO FOR i <- 1 TO 3 IF i = n THEN → Opens three levels of nesting: a WHILE containing a FOR containing an IF, each indented one level further than the one before.
Q3
A WHILE containing a FOR containing an IF must be closed in reverse order: ENDIF, then NEXT, then ENDWHILE. → ENDIF NEXT j ENDWHILE
Q4
Indent each level of nesting one step further, so the innermost statement sits furthest to the right. → FOR i <- 1 TO 3 IF i > 1 THEN IF i < 3 THEN OUTPUT i
Q5
FOR i <- 1 TO 5 IF a > 0 THEN IF b > 0 THEN OUTPUT i ENDIF NEXT i ENDIF → NEXT i closes the FOR before the outer IF has been closed. The structures overlap instead of nesting. The outer ENDIF must come before NEXT i.
Q6
WHILE n > 0 DO FOR i <- 1 TO 3 IF i = n THEN OUTPUT i ENDIF NEXT i → The outermost WHILE loop is never closed. ENDWHILE is missing after NEXT i.
Section C: Definitions
Q1
nesting — Putting one structure inside another, three levels deep. Indent each level and close them in reverse order: innermost first, e.g. ENDIF, ENDIF, then NEXT.
Section D: Fill in the Blanks
Q1
FOR Day <- 1 TO 7 IF Temp > 20 THEN IF Rain = 0 THEN
Q2
ENDIF NEXT Count ENDWHILE
Q3
WHILE Total < 100 DO FOR Count <- 1 TO 5 IF Mark > 50 THEN
Q4
WHILE Total < 100 DO FOR Count <- 1 TO 5 IF Mark > 50 THEN
Q5
ENDIF ENDIF NEXT Day
Section E: Spot the Error
Q1
B — The outer IF is missing THEN. Every IF condition must end with THEN, so the line should be IF Rain > 10 THEN.
Q2
A — A WHILE loop uses DO, not THEN. THEN belongs to IF. The line should be WHILE Total < 100 DO.
Q3
D — The innermost structure is an IF, so it must be closed first with ENDIF. ENDWHILE belongs to a WHILE loop, and there is none here.
Q4
B — A FOR loop gives its end value with TO, not UNTIL. UNTIL belongs to REPEAT. The line should be FOR Shot <- 1 TO 3.
Q5
C — Cambridge pseudocode tests equality with a single =, not ==. The line should be IF Seat MOD 2 = 0 THEN.