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Code Crew G5 / Week 10 / Thursday
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Week 10 · Trees

Thursday

Print the tree
// The fleet command tree
⏱ about 15 min

Thursday: Print the tree

New crew need a printed chart of the command tree. Rocket prints all the names in one long column.

"Hmm. It is all there, but you cannot tell who leads who," Rocket says.

Raven tilts the chart sideways. "What do you notice about how deep each person is in the tree?"

"Ada is at depth 0. Bo and Eve are 1 step down. Their crew are 2 steps down," Rocket says.

Nova hums. "Here is a hint. Push each name to the right by its depth."

Printing with depth

The show function takes a node and a depth. It prints two spaces for each level of depth, then the name. Each child is shown one level deeper.

fleet = {
    "name": "Ada",
    "crew": [
        {"name": "Bo", "crew": [
            {"name": "Cy", "crew": []},
            {"name": "Dee", "crew": []},
        ]},
        {"name": "Eve", "crew": [
            {"name": "Fen", "crew": []},
            {"name": "Gil", "crew": []},
            {"name": "Hal", "crew": []},
        ]},
    ],
}

def show(node, depth):
    print("  " * depth + node["name"])
    for child in node["crew"]:
        show(child, depth + 1)

show(fleet, 0)
Output:
Ada
  Bo
    Cy
    Dee
  Eve
    Fen
    Gil
    Hal

Notice the order. show finishes all of Bo's tree before it starts Eve. Each call goes as deep as it can before moving on.

TAP THE FIRST FIVE NAMES INTO THE ORDER SHOW PRINTS THEM.
  • Tap a card.
  • Then tap its spot.
11st
22nd
33rd
44th
55th
How many spaces are printed before Gil?
WHY THIS EXERCISEThe depth goes up by 1 at each level, and " " * depth makes the indent.
CHANGE THE PROGRAM.
  • Read the question.
  • Tap your answer.
If we call show(fleet["crew"][1], 0), which name prints first?
With that same call, how many names print?

Practice lab: show the ship

ship = {
    "name": "Ship",
    "parts": [
        {"name": "Deck A", "parts": [
            {"name": "Lab", "parts": []},
            {"name": "Dock", "parts": []},
        ]},
        {"name": "Deck B", "parts": [
            {"name": "Bridge", "parts": []},
            {"name": "Hold", "parts": []},
        ]},
    ],
}

def show(node, depth):
    print("  " * depth + node["name"])
    for part in node["parts"]:
        show(part, depth + 1)

show(ship, 0)
How many lines does show(ship, 0) print?
How many spaces are printed before Lab?
How many spaces are printed before Deck B?
Which name prints third?
SHOW CHECK
  • Read the question.
  • Tap your answer.
What prints right after Dock?
With show(ship, 1), how many spaces come before Ship?
If " " became "--", what would come before Lab?
StatementTrue or false?
Ship prints with no spaces.?
Bridge prints before Dock.?
Each child is shown one level deeper.?
show prints every node once.?
WHY THIS EXERCISEThe depth grows by 1 at each level of the tree.
How many names does show(ship["parts"][1], 0) print?
WHY THIS EXERCISEshow only visits the smaller tree it starts at.

Great job! You printed a tree the way computers show folders inside folders. Tomorrow we search the tree.

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