Chapter 1: Elements of a Program (continued)¶

Variables vs. Names vs. Identifiers vs. References¶

In [1]:

variable = 20.0

In [2]:

variable

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20.0

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variable = 20

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variable

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variable *= 4  # same as variable = variable * 4

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variable

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variable //= 2  # same as variable = variable // 2; "//" to retain the integer type

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variable

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variable += 2  # same as variable = variable + 2

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variable

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variable

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del variable

In [13]:

variable

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
Cell In[13], line 1
----> 1 variable

NameError: name 'variable' is not defined

Naming Conventions¶

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pi = 3.14

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answer_to_everything = 42

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my_name = "Alexander"

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work_address = "WHU, Burgplatz 2, Vallendar"

Who am I? And how many?¶

In [26]:

a = 42

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b = a

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a = 87

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x = [1, 2, 3]

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y = x

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[1, 2, 3]

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x[0]

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x[0] = 99

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Out[38]:

[99, 2, 3]

In [39]:

Out[39]:

[99, 2, 3]

How Python reads "Commands"¶

Expressions¶

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a - 42

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(a - 42) // 9

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Statements¶

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a = 21 * 2

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del a

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print("Don't I change the state of the computer's display?")

Don't I change the state of the computer's display?

Logical vs. Physical Lines¶

In [49]:

result = 21 * 2; print("The answer is:", result)

The answer is: 42

In [50]:

result = 21 * 2
print("The answer is:", result)

The answer is: 42

In [51]:

result = 21 * 2
print(
    "The answer is:",
    result
)

The answer is: 42

In [52]:

for number in [1, 2, 3]:
    if number % 2 == 0:
        double = 2 * number 
        print(number, "->", double)

2 -> 4

Comments¶

In [53]:

distance = 891  # in meters
elapsed_time = 93  # in seconds

# Calculate the speed in km/h.
speed = 3.6 * distance / elapsed_time

In [54]:

seconds = 365 * 24 * 60 * 60  # = seconds in the year

In [55]:

seconds_per_year = 365 * 24 * 60 * 60