فهرست عناوین

موضوع قبلی

9. Return

موضوع بعدی

11. A character in a string

10. Lists

Run the following program, first to select the correct world, and a second time to run the rest of it.

World("/worlds/many.json", "Many")
move()
print(object_here())

The result likely looks as follows:

['token', 'triangle', 'banana', 'carrot']

توجه

There is also a Python function named list which can be used to create lists! You have seen that we can use the = sign to assign a name to an object; you should avoid using list as the name of a variable since this would prevent you from using the list() function in Python.

This is an example of a Python list. A Python list is represented by square brackets, [ ], that contains zero, one, or many items. In the example above, the list contains 4 items, each of which is a Python string. Using the "bracket notation", I could create a list containing different types of objects:

a_list = [1, "Reeborg", move]

10.1. Accessing list elements

When you saw the first newspaper delivery task, you read the following:

In computer programming, we generally start counting at zero instead of 1. Since these worlds refer to three famous people in computer science, I thought it would be appropriate to number these worlds starting at zero.

Given a list, we can retrieve a single element using a bracket notation.

Try this!

Try the following program:

odd_numbers = [1, 3, 5, 7]
print( odd_numbers[0] )
print( odd_numbers[1] )
print( odd_numbers[3] )
print( "length = ", len(odd_numbers))

The Python function len() gives us the length of the list, which is equal to the total number of items of that list.

The number in brackets which is used to get a single item from the list is called the index of that item.

Sometimes, it is useful to get the last element of a list, or the one before that. One could use the len() function to figure out the position of the last item, but Python has a convenient shortcut.

Try this!

Try the following program:

odd_numbers = [1, 3, 5, 7]
print( odd_numbers[-1] )  # last item
print( odd_numbers[-2] )  # second last item

Notice how including a minus sign can be used as a convenient way to get item starting at the end of a list.

10.2. Using lists as conditions

When used as a condition, in an if or a while statement, lists behave differently depending on whether or not they have items in them.

Figure it out by yourself!

Run the following program:

# Only one of the following is correct.

if []:
    print("An empty list is considered to be equivalent to True,")
elif ['Reeborg']:
    print("An empty list is considered to be equivalent to False.")
    print("But a non-empty list is considered to be equivalent to True.")
elif [1, 2, 3, 4, 5]:
    print("A list is considered to be equivalent to True",
          "ONLY if it contains many items")
else:
    print("Perhaps lists are always considered to be False ...")

As you know, branches that contain a condition considered to be False are skipped; the first branch that contain a True condition is the one that is executed.

10.3. More harvesting challenges

First, a quick test

Run the following program, that will print the first type of objects found by Reeborg. Then change the program to print the second type of objects found.

World("/worlds/many.json", "Many")
move()
print(object_here()[0])

Reeborg's aunt is a fruit farmer. In her fields, many types of fruits can be found. On a given day, only a certain kind of fruit needs to be harvested. Have a look at worlds Harvest 4a, Harvest 4b, Harvest 4c and Harvest 4d. As he enters the field, Reeborg sees the type of fruit that needs to be harvested as his aunt put a sample there. He picks it up and proceed to harvest all fruits of the similar type.

Reeborg uses the function object_here() which, as we saw, returns a list containing the names of the objects found at that location; for the Harvest 4 worlds, the possible objects are "apple", "banana", "orange" or "strawberry".

Below is an incomplete program that would make Reeborg accomplish the required task in any of the four worlds mentioned. We use the variable FRUIT which we wrote using uppercase letters since it is the same variable used inside and outside some functions; it is essentially a global variable. However, since we do not assign it a value inside a function using the = sign, we do not need to use the global keyword.

def harvest_one_row ():
    while front_is_clear():
        if object_here()[0] == FRUIT:
            take(FRUIT)
        move()

def go_back_to_beginning_of_row():
    pass

def move_to_next_row():
    pass

def go_to_first_row():
    pass

def complete_one_row():
    harvest_one_row()
    go_back_to_beginning_of_row()
    move_to_next_row()

move()
FRUIT = object_here()[0]
take(FRUIT)
go_to_first_row()
repeat 6:
    complete_one_row()

Your turn!

Complete the above program so that it works in all four worlds: Harvest 4a, Harvest 4b, Harvest 4c and Harvest 4d.

10.4. One last experiment

Sometimes, when a program becomes too big, it makes sense to put it into many files. Here, your programs are not in files: they are in the Code Editor but parts of them can also be in the library. Imagine that your program above is so big that it has to be broken up in two pieces. Put the definition of the function harvest_one_row() in the library, and import it into the main program using

from library import harvest_one_row

Does your program still work? Does using the global keyword makes it work? The result of these experiments might point out some problems when using global variables, that is variable used inside a function but defined elsewhere.