Week 06 · Friday: Split and join (Code Crew G4)

Friday: The hidden message

A strange line appears in the comms log: lights are now dim. "That is odd," Rocket says. "The lights are fine."

Raven writes the four words in a column. "What do you notice about the first letter of each word?" she asks.

Rocket reads them down the side. "L, A, N, D. Land! It is a secret message from the shuttle!" Nova does a happy loop. "Great discovery! Would you like a hint? A loop could find the first letters for us."

Split, loop, join

This program splits the log line into words.

The loop appends the first letter of each word to a list.

Then join glues the letters together, and upper makes them capitals.

log = "lights are now dim"
letters = []
for w in log.split():
    letters.append(w[0])
print("".join(letters).upper())

The screen shows
LAND

Round	w	letters after the round
1	lights	['l']
2	are	['l', 'a']
3	now	['l', 'a', 'n']
4	dim	['l', 'a', 'n', 'd']

Change the log to "watch all its tanks". What does the program show?

WHY THIS EXERCISEThe program works for any log line. That is the power of a loop.

Debug the decoder

Rocket made one small change. Now the decoder shows only L.

log = "lights are now dim"
letters = []
for w in log.split(","):
    letters.append(w[0])
print("".join(letters).upper())

FIND THE BUG

Read the question.
Tap your answer.

Why does the decoder show only L?

Which fix makes it show LAND again?

Change to the working decoder	Would the screen still show LAND?
Change "".join to "-".join	?
Take out upper()	?
Rename letters to marks on every line	?

WHY THIS EXERCISEPredicting how a change affects output is a big debugging skill.

At the computer

This is never required.
If a grown-up has set up Python from python.org, type the decoder into IDLE.
Write your own secret log line and decode it.

Practice lab: the last-letter code

log = "bus hat sofa car"
letters = []
for w in log.split():
    letters.append(w[-1])
print("".join(letters).upper())

What word does the program show?

How many times does the loop run?

How many items are in letters at the end?

DECODER CHECK

Read the question.
Tap your answer.

What does w[-1] grab?

In round 3, which letter is appended?

Suppose the log is "rim two saw". What would the program show?

Change to the last-letter decoder	Would the screen still show STAR?
Change w[-1] to w[0]	?
Change "".join to " ".join	?
Change the log to "plus melt tuna door"	?
Take out .upper()	?

WHY THIS EXERCISEEach part of the decoder has a job. Change one and the output changes.

Use first letters this time. What word hides in "sun kites yellow"?

WHY THIS EXERCISEs, k and y spell sky.

Write a secret log line where the last letter of each word spells a word. Show how the decoder finds it.

Week six is done, Crew Engineer! You can split, join and decode. Next week, a new kind of container will make the cargo inventory easy.