Encode and decode

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Quick Questions

Shade in the A or B circle for each question.

1. Which word means turning information into a code?

A Decode

B Encode

2. Which word means turning a code back into the information?

A Encode

B Decode

3. Mia swaps each letter in her message for a number, so A becomes 1 and B becomes 2. What is she doing?

A Encoding the message

B Decoding the message

4. Leo gets the secret message 8 9. He uses the key A=1, B=2 and so on to read it as HI. What is he doing?

A Decoding the message

B Encoding the message

5. Which statement is true about encode and decode?

A They are opposites

B They mean the same thing

Match the Pairs

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

Left column		Right column
1encode("CAT") gives 3 1 20		ADecoding needs the key. Without it you cannot tell which number stands for which letter.
2decode("3 1 20") gives CAT		Bencode(picture) gives 0 1 1 0
3Encoding a black-and-white picture: black pixels become 1 and white pixels become 0		CThis turns the word into a code, so it is encoding. Decoding should turn the code back into the word.
4Decoding with a key: the key says 1 means black and 0 means white, so the digits become a picture again		DUses the key to turn the number codes back into the word
5decode("CAT") gives 3 1 20		ETurns the word CAT into number codes, so the information is now in code
6decode("3 1 20") without a key		Fdecode("0 1 1 0", key) gives picture

Definitions

encode / decode

Fill in the Blanks

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

message ← "CAB" code ← ______(message, key)

code ← "3 1 2" message ← ______(code, key)

code ← "3 1 2" message ← decode(code, ___)

picture ← [0, 1, 1, 0] code ← encode(_______, key)

code ← encode(message, key) original ← ______(code, key) output original

Spot the Error

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

Question 1

A key ← A=1, B=2, C=3

B message ← "CAB"

C code ← encode(message, key)

D answer ← encode(code, key)

E show answer

Question 2

A key ← A=1, B=2, C=3

B message ← BAC

C code ← encode(message, key)

D show code

Question 3

A row ← black, white, black

B key ← black=1, white=0

C code ← decode(row, key)

D show code

Question 4

A code ← 1, 0, 1

B key ← 1=black, 0=white

C row ← decode(code)

D show row

Question 5

A key ← A=1, B=1, C=3

B word ← "ABC"

C code ← encode(word, key)

D show code

Answer Key

Encode and decode

Section A: Quick Questions

B — Encode

B — Decode

A — Encoding the message

A — Decoding the message

A — They are opposites

Section B: Match the Pairs

encode("CAT") gives 3 1 20 → Turns the word CAT into number codes, so the information is now in code

decode("3 1 20") gives CAT → Uses the key to turn the number codes back into the word

Encoding a black-and-white picture: black pixels become 1 and white pixels become 0 → encode(picture) gives 0 1 1 0

Decoding with a key: the key says 1 means black and 0 means white, so the digits become a picture again → decode("0 1 1 0", key) gives picture

decode("CAT") gives 3 1 20 → This turns the word into a code, so it is encoding. Decoding should turn the code back into the word.

decode("3 1 20") without a key → Decoding needs the key. Without it you cannot tell which number stands for which letter.

Section C: Definitions

encode / decode — To encode is to turn information into a code. To decode is to turn the code back into the information, using a key. They are opposites.

Section D: Fill in the Blanks

message ← "CAB"
code ← encode(message, key)

code ← "3 1 2"
message ← decode(code, key)

code ← "3 1 2"
message ← decode(code, key)

picture ← [0, 1, 1, 0]
code ← encode(picture, key)

code ← encode(message, key)
original ← decode(code, key)
output original

Section E: Spot the Error

D — To get the message back from the code you must decode, not encode. Line 4 should be answer ← decode(code, key).

B — The secret message is text, so it needs quote marks. Line 2 should be message ← "BAC".

C — Turning a picture row into 0s and 1s is encoding, not decoding. Line 3 should be code ← encode(row, key).

C — Decoding needs the key to know what each number means. Line 3 should be row ← decode(code, key).

A — A and B share the number 1, so you could not decode it back. Each letter needs its own number. Line 1 should be key ← A=1, B=2, C=3.