10/13/2011 CS Variables and Expressions in Python

8/3/2019 10/13/2011 CS Variables and Expressions in Python

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Do Now: 10 minutes

Figure out what the % operator does in python.

1. Start by working in the interpreter window. Type random

things like 4%4, 9%2, 10%12, etc. Try to figure out why you

get the each answer.

2. Open a new window to write your own mini-program. The

program should be:

for i in range(10):

print("i is ", i, " and i%4 is ", i%4)


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Homework

Hand write lines of Python code that will swap the value of two

variables, no matter what the values of the two variableshappen to be. If you get the chance, test it out.


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Expressions

Examples:

2+2+2+2+2

8*6

10-5+6


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Expressions

Examples:

2+2+2+2+2

8*6

10-5+6

Evaluating an expression: taking a line like 2+2+2+2+2

and reducing it to a single value, i.e. 10


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Expressions

Examples:

2+2+2+2+2

8*6

10-5+6

Evaluating an expression: taking a line like 2+2+2+2+2

and reducing it to a single value, i.e. 8

Order: How does the computer evaluate 3*4 + 2*(8 + 2) ?PEMDAS applies


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Variables

Containers that hold the results of our

expressions for later use.

Example: spam = 15


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Variables

Containers that hold the results of our

expressions for later use.

Example: spam = 15

The = sign is actually called the assignment

operator


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In the interpreter window:

Explanation:

1. Assigning the spam variable to be 152. spam + 5 is the same as 15 + 5 = 20

3. Assigning the spam variable to 3 (15 gets

overwritten)

4. spam + 5 is same as 3 + 5 = 8

>>> spam = 15>>> spam + 5>>> spam = 3

>>> spam + 5


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Explanation:1. Assigning 15 to the spam variable

2. spam + spam is same as 15 + 15 = 30

3. spam spam is same as 15 15 = 0

>>> spam = 15>>> spam + spam>>> spam - spam


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Explanation:

1. Assigning 15 to the variable spam

2. First, adds spam with spam (15 + 15) and then stores

the result (30) back in spam.

3. First, multiplies spam by 3 (3*30) and then stores the

result (90) back in spam.

>>> spam = 15>>> spam = spam + spam>>> print(spam)

>>> spam = 3*spam>>> print(spam)


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Variable Names

One Rule:

-variable names cannot include a space

mattsAge = 26

matts age = 26


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Variable Names

One Rule:


mattsAge = 26

matts age = 26


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Variable Names

One Rule:


mattsAge = 26

matts age = 26

Two conventions:

-variable names should be descriptive

stuff = 26

mattsAge = 26


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Variable Names

One Rule:


mattsAge = 26

matts age = 26

Two conventions:


stuff = 26

mattsAge = 26


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Variable Names

One Rule:


mattsAge = 26

matts age = 26

Two conventions:


stuff = 26

mattsAge = 26-use Caps or underscores to separate words

mattsAge = 26

mattsage = 26

matts_age = 26


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Variable Names

One Rule:


mattsAge = 26

matts age = 26

Two conventions:


stuff = 26

mattsAge = 26-use Caps or underscores to separate words

mattsAge = 26

mattsage = 26

matts_age = 26


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Variable Names

IN PYTHON, VARIABLE NAMES ARE CASE SENSATIVE.

mattsAge = 26

MattsAge = 26


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Variable Names

IN PYTHON, VARIABLE NAMES ARE CASE SENSATIVE.

mattsAge = 26

MattsAge = 26

(you have just created two separate variables, one called

mattsAge and one called MattsAge)


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Explanation:

1. Store the number 15 in myNumber

2. Store the number 30 in yourNumber3. Puts whatever is in yourNumber (30) into myNumber

(15). Now, myNumber is 30. 15 is overwritten.

4. Put whatever is in myNumber (30) into yourNumber

(30). Now, yourNumber is 30.

>>> myNumber = 15>>> yourNumber = 30>>> myNumber = yourNumber>>> yourNumber = myNumber>>> print(myNumber)

>>> print(yourNumber)

10/13/2011 CS Variables and Expressions in Python

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