Decoding run-length encoded image data

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Quick Questions

Shade in the A or B circle for each question.

1. Decode this run-length encoded row: 3R2B

A RRRBB

B RRBBB

2. Decode this run-length encoded row (W = white, K = black): 2W1K3W

A WWKWWW

B WWWKWW

3. In the encoded data 5G, what does the number 5 tell you?

A Repeat the colour code G five times

B Add 5 to the colour code G

4. An image is 8 pixels wide. A row is encoded as 4B3W2B. Is this a valid row for the image?

A Yes - the counts add up to 8

B No - the counts add up to 9, not 8

5. The image is 6 pixels wide. Which is the correct decoding of the row 1R4B1R?

A RBBBR

B RBBBBR

Match the Pairs

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

Left column		Right column
13R2B		ADecodes to RRRBB: three red pixels followed by two blue pixels
21W4K1W		BThe number and the colour code have been copied as they are instead of being repeated. The count must be used to repeat the colour code, so 3R2B should give RRRBB.
3To rebuild a row, repeat each colour code as many times as its count says, working left to right		C4B2W3B -> 4+2+3 = 9
4To check a decoded row has the right width, add up all the counts and compare the total with the image width		D2G3Y -> GGYYY
53R2B decoded as R3B2		EThe decoded row has only 7 pixels (2+3+2 = 7), but the image width is 8. The check should show the row is one pixel short, so it is wrong.
6Image width 8, row 2R3B2R decoded as RRBBBRR and accepted as correct		FDecodes to WKKKKW: one white, four black, then one white pixel, a row 6 pixels wide

Definitions

RLE decode

Fill in the Blanks

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

3R2B decodes to _____

_W1K2W decodes to WWWWKWW

3R2B1G: 3 + 2 + 1 = _ pixels wide

FOR i ← 1 TO _____ row ← row + colour NEXT i

IF LENGTH(row) _ width THEN OUTPUT "Row is correct" ENDIF

Spot the Error

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

Question 1

A data = [(3, "R"), (2, "B")]

B row = ""

C for count, colour in data:

D row = colour * count

E print(row)

Question 2

A runs = [(2, "W"), (3, "K")]

B row = ""

C for n, c in runs[0]:

D row += c * n

E print(row)

Question 3

A row = "RRBBB"

B width = len(row)

C ok = len(row) == width

D print(ok)

Question 4

A row = "RRRBB"

B width = 5

C if row == width:

D print("Row OK")

Question 5

A grid = []

B for runs in image:

C row = ""

D for n, c in image:

E row += c * n

F grid.append(row)

Answer Key

Decoding run-length encoded image data

Section A: Quick Questions

A — RRRBB

A — WWKWWW

A — Repeat the colour code G five times

B — No - the counts add up to 9, not 8

B — RBBBBR

Section B: Match the Pairs

3R2B → Decodes to RRRBB: three red pixels followed by two blue pixels

1W4K1W → Decodes to WKKKKW: one white, four black, then one white pixel, a row 6 pixels wide

To rebuild a row, repeat each colour code as many times as its count says, working left to right → 2G3Y -> GGYYY

To check a decoded row has the right width, add up all the counts and compare the total with the image width → 4B2W3B -> 4+2+3 = 9

3R2B decoded as R3B2 → The number and the colour code have been copied as they are instead of being repeated. The count must be used to repeat the colour code, so 3R2B should give RRRBB.

Image width 8, row 2R3B2R decoded as RRBBBRR and accepted as correct → The decoded row has only 7 pixels (2+3+2 = 7), but the image width is 8. The check should show the row is one pixel short, so it is wrong.

Section C: Definitions

RLE decode — Repeat each colour code count times along the row to rebuild the pixels, e.g. 3R2B gives RRRBB. Then check the row adds up to the image width.

Section D: Fill in the Blanks

3R2B decodes to RRRBB

4W1K2W decodes to WWWWKWW

3R2B1G: 3 + 2 + 1 = 6 pixels wide

FOR i ← 1 TO count
  row ← row + colour
NEXT i

IF LENGTH(row) = width THEN
  OUTPUT "Row is correct"
ENDIF

Section E: Spot the Error

D — Using = replaces the row on every pass, so only the last run survives. Use += to add each repeated colour code to the end of the row, giving RRRBB.

C — runs[0] is only the first (count, colour) pair, so the loop does not go through every run. Loop over runs so all runs are decoded.

B — The image width is a known value (5 pixels), not something worked out from the row. Setting width = len(row) makes the check always pass, so it can never spot a row of the wrong length. It should be width = 5.

C — This compares the row text with a number, so it is never true. Use len(row) to count the decoded pixels and compare that with the width.

D — The inner loop must go through the runs of the current row, not the whole image. It should be for n, c in runs: so each row is decoded from its own runs.