Using an expression as an array index
Name:
Date:
Class:
A
Quick Questions
Shade in the A or B circle for each question.
1. n ← 3. Which element does Scores[n+1] refer to?
2. When is the expression n+1 in StudentNames[n+1] worked out?
3. Which line copies the value in element n into the next element?
4. n ← 5. Which element does StudentNames[n-1] refer to?
5. An array is declared as ARRAY[1:10] OF INTEGER and n ← 10. What happens with Marks[n+1]?
B
Match the Pairs
Draw a line connecting each item on the left to its match on the right.
| Left column | Right column | |
|---|---|---|
| 1Scores[n + 1] | AWorks out n + 1 first, then uses the element at that index, the one after position n | |
| 2Scores[n - 1] | BWorks out n - 1 first, then uses the element at that index, the one before position n | |
| 3Copy the value in element n into the next element along | Cn - 1 works out to 0, which is below the lower bound of 1, so the index is out of range | |
| 4Check that n + 1 stays within the bounds of an array declared ARRAY[1:10] before using it | DStudentNames[n + 1] ← StudentNames[n] | |
| 5StudentNames[n + 1] ← StudentNames[n] when n = 10 and the array is ARRAY[1:10] | En + 1 works out to 11, which is above the upper bound of 10, so the index is out of range | |
| 6StudentNames[n - 1] ← StudentNames[n] when n = 1 and the array is ARRAY[1:10] | FIF n + 1 <= 10 THEN StudentNames[n + 1] ← StudentNames[n] ENDIF |
C
Definitions
Array[n+1]
D
Fill in the Blanks
Complete each line of code by filling in the missing parts.
1.
StudentNames[___] ← StudentNames[n]
2.
Diff ← Marks[n] - Marks[___]
3.
n ← 3
OUTPUT Scores[n+1]
// Shows element Scores[_]
4.
IF n + 1 __ 10 THEN
Names[n + 1] ← Names[n]
ENDIF
5.
// Names is ARRAY[1:10]
FOR n ← 1 TO _
Names[n + 1] ← Names[n]
E
Spot the Error
Shade in the letter of the line that contains an error.
Question 1
A
n ← 3
B
Names[n-1] ← Names[n]
C
OUTPUT Names[4]
Question 2
A
n ← 5
B
previous ← Score[n+1]
C
OUTPUT previous
Question 3
A
n ← 4
B
IF n+1 <= 5 THEN
C
Names[n]+1 ← "Zoe"
D
ENDIF
Question 4
A
n ← 3
B
Names(n+1) ← Names[n]
C
OUTPUT Names[n+1]
Question 5
A
n ← 2.5
B
Total ← Marks[n] + Marks[n+1]
C
OUTPUT Total
Answer Key
Using an expression as an array index
Section A: Quick Questions
Q1
A — Scores[4]
Q2
A — Before the element is chosen
Q3
A — StudentNames[n+1] ← StudentNames[n]
Q4
B — Element 4, the previous one
Q5
B — The index is 11, which is outside the bounds, so it is an error
Section B: Match the Pairs
Q1
Scores[n + 1] → Works out n + 1 first, then uses the element at that index, the one after position n
Q2
Scores[n - 1] → Works out n - 1 first, then uses the element at that index, the one before position n
Q3
Copy the value in element n into the next element along → StudentNames[n + 1] ← StudentNames[n]
Q4
Check that n + 1 stays within the bounds of an array declared ARRAY[1:10] before using it → IF n + 1 <= 10 THEN StudentNames[n + 1] ← StudentNames[n] ENDIF
Q5
StudentNames[n + 1] ← StudentNames[n] when n = 10 and the array is ARRAY[1:10] → n + 1 works out to 11, which is above the upper bound of 10, so the index is out of range
Q6
StudentNames[n - 1] ← StudentNames[n] when n = 1 and the array is ARRAY[1:10] → n - 1 works out to 0, which is below the lower bound of 1, so the index is out of range
Section C: Definitions
Q1
Array[n+1] — An index can be an expression like n+1. The expression is worked out first, then that element is used. It must stay within the array's bounds. Example: n=3 gives Array[4].Section D: Fill in the Blanks
Q1
StudentNames[n+1] ← StudentNames[n]Q2
Diff ← Marks[n] - Marks[n-1]Q3
n ← 3
OUTPUT Scores[n+1]
// Shows element Scores[4]Q4
IF n + 1 <= 10 THEN
Names[n + 1] ← Names[n]
ENDIFQ5
// Names is ARRAY[1:10]
FOR n ← 1 TO 9
Names[n + 1] ← Names[n]Section E: Spot the Error
Q1
B — To copy a value to the NEXT element the index must be n+1. n-1 is the previous element (2), so the value goes to the wrong place and Names[4] is not set. It should be Names[n+1] ← Names[n].
Q2
B — The previous element is found with n-1. n+1 gives the next element (6), not the previous one. It should be previous ← Score[n-1].
Q3
C — The +1 must be inside the square brackets so that the index expression n+1 is worked out before the element is chosen. It should be Names[n+1] ← "Zoe".
Q4
B — Array indexes are written in square brackets, not round brackets. It should be Names[n+1] ← Names[n].
Q5
A — An array index must be a whole number. With n as 2.5, n+1 is 3.5, which is not a valid element. It should be n ← 2.