Hello Python! Maryam Tavakol Machine Learning Group Winter - - PowerPoint PPT Presentation

Hello Python!

Maryam Tavakol

Machine Learning Group

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Winter semester 2016/17

Programs & Programming

• A program is a set of instructions
• Every program is written in terms of a few basic
• perations that its reader already understands
• Defining new operations and combining them to do

useful things is the heart and soul of programming

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Programming Languages

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Compiler vs. Interpreter

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*http://www.c4learn.com/c-programming/compiler-vs-interpreter/

Run a Python Program

• The Python program is

saved in a file

• The program is executed

by Python interpreter

• Interacting with it is

possible in a program called a shell

Expressions: Arithmetic

>>> 4+13 17

• Unary &

binary operators

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Expressions: Arithmetic

• Operator precedence
• A collection of rules which defined the order of operations

>>> 212 - 32 * 5 / 9 194.2223 >>> (212 - 32) * 5 / 9 100.0

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Expressions: Arithmetic

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Expressions: Comparison

>>> 2 == 2 #Equality True >>> 2 != 2 #Inequality False >>> 0 < 2 < 5 #Chained inequality True >>> 2 > 2 False

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Comments!

Expressions: Boolean Operators

• The operator not yields True if its argument is false, False otherwise

>>> not 1 == 1 False

• The expression x and y first evaluates x; if x is false, its value is returned;
• therwise, y is evaluated and the resulting value is returned

>>> 2 == 2 and 1 != 1 True

• The expression x or y first evaluates x; if x is true, its value is returned; otherwise, y

is evaluated and the resulting value is returned

>>> 2 == 2 or 1 != 1 False

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Variables

• A variable holds information (1.343, ‘hi’, [1,3,6,7])
• In Python this is simple:

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Variables

• Their values can vary as the program executes
• (no more knowledge
• f old values)

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Variables

>>> degree_celsius = 26.0 >>> 9 /5 * degree_celsius + 32 78.8 >>> difference = 100 - degree_celsius >>> diference 74

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Types

• Type consists of:
• set of possible values
• set of operations that can be applied to those values
• Unlike C/C++ and Java, variables can change
• types. (Python internally keeps track of the type.)

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Computer Memory

>>> degree_celsius = 26.6

• Value 26.0 has the memory address: id1
• The object at the memory address id1 has type float and the

value 26.0

• Variable “degrees_celsius” contains the memory address id1
• Variable “degrees_celsius” refers to the value 26.0

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Assignment Statement

«variable» = «expression»

• Executes as follows:
• 1. Evaluate the expression to produce a value.

(with a memory address)

• 2. Store the memory address of the value in the
• variable. (Create a new variable otherwise, or

reuse the existing variable)

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Reassigning to Variables

Consider this code:

>>> difference = 20 >>> double = 2 * difference >>> double 40 >>> difference = 5 >>> double 40

Augmented Assignment

• To create shorthand notation:

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>>> score = 50 >>> score 50 >>> score = score + 20 >>> score 70 >>> score = 50 >>> score 50 >>> score += 20 >>> score 70

Augmented Assignment

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Bugs

• Something went wrong…

>>> 3 + moogah Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'moogah' is not defined >>> 3 + ^ SyntaxError: invalid syntax

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Statement Split

• Two possible ways:
• 1. Make sure your line break occurs inside

parentheses

• 2. Use the line-continuation character,

backslash, \

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Summary

• Programs and Python interpreter
• Expressions; arithmetic, comparison, boolean
• Variables and types
• Computer memory

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