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Code Crew G6+ / Week 05 / Tuesday
2/6
Week 05 · Lists and dictionaries

Tuesday

Changing lists
// Keeping many values together
⏱ about 30 min

Tuesday: Changing lists

The supply cart for the next mission keeps changing. Someone adds a radio, someone takes the last item off to pack it, and someone else wants the list in alphabetical order.

Rocket has been retyping the whole list every time. "There must be a quicker way," he says, flexing his tired fingers.

Raven points to the word supplies at the start of each line. "What do you notice?" she asks. "Every change is something you do to the list. Add to it. Take from it. Sort it."

Nova draws a dot after the word supplies and a menu of glowing actions. "I found a pattern," she says. "Lists come with their own tools, called methods. You reach them with a dot."

Rocket types supplies and a dot, and wonders what he will find.

List methods

MethodWhat it does
supplies.append(x)adds x to the end
supplies.pop()removes the last item and gives it back
supplies.pop(0)removes the first item and gives it back
supplies.sort()puts the items in order (A to Z, or smallest first)
x in suppliesTrue if x is in the list (not a method, but very handy)
# supplies.py
supplies = ["water", "tools", "maps"]
supplies.append("radio")
print(supplies)
last = supplies.pop()
print("Packed:", last)
print(supplies)
supplies.sort()
print(supplies)
print("maps" in supplies)
print(len(supplies))
['water', 'tools', 'maps', 'radio']
Packed: radio
['water', 'tools', 'maps']
['maps', 'tools', 'water']
True
3

Notice that append, pop and sort change the list itself. You do not write supplies = supplies.sort(); just supplies.sort() on its own line.

TRACE THE METHODS
  • Read the question.
  • Tap your answer.
crew = ["Lee", "Ana"]. Then crew.append("Sam"), then crew.pop(0). What is crew now?
nums = [5, 2, 9]. After nums.sort(), what is nums[0]?
tools = ["drill", "tape"]. What does "rope" in tools give?
At your computer
1. Type supplies.py in a new file, save it and run it.
2. Add a line that appends "snacks" and prints the list again.
3. Change supplies.pop() to supplies.pop(0). Predict which item is packed, then run it.
4. In the shell, make a list of five numbers and sort it.

Practice: insert and remove

Two more methods are useful. The insert(i, x) method puts x at index i and moves the later items along. The remove(x) method takes away the first item equal to x.

pop uses a position, but remove uses the value itself.

# launch_steps.py
steps = ["fuel check", "weather check", "launch"]
steps.insert(2, "crew check")
print(steps)
steps.remove("weather check")
print(steps)
done = steps.pop(0)
print("Done:", done)
print(steps)
['fuel check', 'weather check', 'crew check', 'launch']
['fuel check', 'crew check', 'launch']
Done: fuel check
['crew check', 'launch']
TRACE LAUNCH_STEPS.PY
  • Read the question.
  • Tap your answer.
Where does crew check go?
Which method uses the value, not the position?
How many items are left at the end?
colors = ["red", "blue"]. After colors.insert(0, "green"), what is colors[0]?
nums = [3, 7, 3]. After nums.remove(3), what is nums[0]?
stack = ["a", "b", "c"]. What does stack.pop() give back?
StatementTrue or false?
append always adds to the end of a list.?
remove takes away every copy of a value.?
pop(0) removes the first item.?
sort puts words in A to Z order.?
WHY THIS EXERCISEappend adds at the end, pop(0) takes the front, sort orders A to Z, and remove takes one copy.
PUT THESE LIST CHANGES IN THE ORDER THE PROGRAM DOES THEM
  • ?Insert crew check at index 2
  • ?Remove weather check
  • ?Pop the first step
  • ?Print what is left
WHY THIS EXERCISEA program changes a list one line at a time, in the order the lines appear.

You can now grow, shrink and sort lists. Tomorrow you will loop through a list and work with every item.

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