Lossless compression

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Quick Questions

Shade in the A or B circle for each question.

1. Which statement describes lossless compression?

A Some detail is thrown away for good

B The original data can be rebuilt exactly

2. You unzip a ZIP file of your English essay. What do you get back?

A The exact essay, word for word

B An essay with a few words missing

3. Which file format uses lossless compression for images?

A PNG

B A format that discards detail to save space

4. A program (app) file is compressed before download. Which type of compression must be used?

A Lossy

B Lossless

5. True or false: A lossless file is smaller than the original, so some of its data is gone for good.

A False - the data is stored more efficiently, not deleted

B True - smaller files always lose data

Match the Pairs

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

Left column		Right column
1report.txt -> report.zip -> report.txt		AZIP is lossless, so unzipping must give back the exact original file, not a slightly different one.
2photo.bmp -> photo.png		BLosing words means this is not lossless. Text must be restored exactly, so lossless compression is needed.
3Lossless compression is needed for programs, because one missing or changed bit could break them		Cgame.exe -> game.zip -> game.exe
4After lossless compression, decompressing gives data identical to the original		DThe image file gets smaller but every original pixel can still be restored exactly
5essay.docx -> compress -> 'a few words missing' -> OK		Eoriginal == restored # True
6ZIP file -> unzip -> slightly different file		FThe file is shrunk into a ZIP and then unzipped to get back the exact original, with nothing lost

Definitions

lossless compression

Fill in the Blanks

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

compressed ← compress(original) restored ← __________(compressed)

restored ← decompress(compressed) IF restored _ original THEN OUTPUT "Lossless"

IF format = "ZIP" __ format = "PNG" THEN type ← "lossless"

needsExact ← TRUE IF __________ THEN method ← "lossless"

zipFile ← compress(original) copy ← decompress(zipFile) differences ← _

Spot the Error

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

Question 1

A type = "lossy"

B packed = compress(file)

C back = decompress(packed)

D print(back == file)

Question 2

A file = "essay.txt"

B method = "JPEG"

C lossless = True

D print(file, method)

Question 3

A file = "game.exe"

B needs_exact = False

C if needs_exact:

D use = "lossless"

Question 4

A format = "JPG"

B lossless = True

C print("No pixels lost")

Question 5

A original = "AAAAAB"

B packed = compress(original)

C restored = decompress(packed)

D same = restored != original

E print(same)

Answer Key

Lossless compression

Section A: Quick Questions

B — The original data can be rebuilt exactly

A — The exact essay, word for word

A — PNG

B — Lossless

A — False - the data is stored more efficiently, not deleted

Section B: Match the Pairs

report.txt -> report.zip -> report.txt → The file is shrunk into a ZIP and then unzipped to get back the exact original, with nothing lost

photo.bmp -> photo.png → The image file gets smaller but every original pixel can still be restored exactly

Lossless compression is needed for programs, because one missing or changed bit could break them → game.exe -> game.zip -> game.exe

After lossless compression, decompressing gives data identical to the original → original == restored # True

essay.docx -> compress -> 'a few words missing' -> OK → Losing words means this is not lossless. Text must be restored exactly, so lossless compression is needed.

ZIP file -> unzip -> slightly different file → ZIP is lossless, so unzipping must give back the exact original file, not a slightly different one.

Section C: Definitions

lossless compression — Shrinks a file so the exact original data can be rebuilt perfectly, with nothing lost. ZIP and PNG use it. Needed for text and programs.

Section D: Fill in the Blanks

compressed ← compress(original)
restored ← decompress(compressed)

restored ← decompress(compressed)
IF restored = original THEN
  OUTPUT "Lossless"

IF format = "ZIP" OR format = "PNG" THEN
  type ← "lossless"

needsExact ← TRUE
IF needsExact THEN
  method ← "lossless"

zipFile ← compress(original)
copy ← decompress(zipFile)
differences ← 0

Section E: Spot the Error

A — Lossy compression throws data away, so back would not equal file. The program tests for a perfect rebuild, so it should say type = "lossless".

B — JPEG is lossy and would damage the text. Text needs lossless compression like ZIP, so the line should be method = "ZIP".

B — A program must be rebuilt exactly or it may stop working. It should be needs_exact = True so that lossless compression is chosen.

A — JPG is a lossy format, so it does not match lossless = True or the message about no pixels lost. PNG is the lossless image format, so it should be format = "PNG".

D — With lossless compression the restored data is identical to the original, so the test should use == (equal), not != (not equal).