Transcript Intro To Python

Introduction to Python
Introduction to Computing
Science and Programming I
Literals
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Literals
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These are values in the code that represent
themselves
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1, 22.8, -45, “Chris”, ‘-356’
The print Command
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Use print to output a line of information to
the screen
print “Hello World”
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To print multiple items separated by single
spaces use commas. The output for this
command is identical to the one above
print “Hello”,”World”
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The command will also output numbers
Calculations
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An expression is any kind of calculation
that produces a result
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34 + 12
7–3
“Combine two “ + “strings”
Calculations
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Basic Math
+,-,*,/ work as you’d expect. Python
follows the normal order of operations
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5+2*2 is 9
Parentheses () also work as they do in
mathematics
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(5+2)*2 is 14
Calculations
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String “math”
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To combine (concatenate) two strings use +
“Chris “ + “Schmidt” is “Chris Schmidt”
 You’ll be using this quite a bit for output to the
screen
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If you want to repeat a string, use *
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“Chris ” * 3 is “Chris Chris Chris ”
Data Types
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Different types of information have different
data types. Python will behave differently
depending on what type of information it is
working with
Data Types
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Integers
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Floats
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23.45, 12.0, 15.3
Strings
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9, 34, -5
“Chris”, “This is a string”
We’ll see other data types in the future
Data Type
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You need to be careful
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“123” is a string, 123 is an integer, 123.0 is a float
In mathematical operations if both operands are
integers, the result is given as an integer
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5 / 3 is 1
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Notice that the result has the decimal portion removed
instead of being rounded
5.0 / 3 is 1.66666…
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The ‘.0’ makes the first operand of type float and therefore
the result is given as a float
The type Function
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Python provides a function to tell you
what data type a piece of information is
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type(24) returns <type ‘int’>
Type Conversion
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It is very common to need to convert one data
type to another. Python provides simple
operations to do so.
To convert use the int, float, and str functions
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int(6.8) returns 6, int(“123”) returns 123
float(6) returns 6.0, float(“34.6”) returns 34.6
str(123) returns “123”, str(4.5) returns “4.5”
If the conversion isn’t possible, there will be an
error. E.g. int(“narf!”), int(“one”)
Variables
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Variables
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To store a value use a variable to reserve a small
piece of the computer’s memory
An assignment statement is used to store a value in a
variable
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name = “Chris”
id = 123
height = 1.7
If you want to use the value stored in the variable,
simply use the variable’s name
name = “Chris”
print name
Variables
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Allowed names
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Variable name start with a letter and can contain
letters, numbers, and underscores
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id, student_name, number6 are examples of allowed variable
names
Python keywords that are part of the programming language
cannot be used as a variable name. E.g. print
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and del from not while as elif global or with assert else if pass
yield break except import print class exec in raise continue
finally is return def for lambda try
Python is Case-Sensitive so ‘counter’ and ‘cOUNteR’
will not access the same variable
Variables
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Point of good coding: Give your variables
intelligent descriptive names
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BAD: n=“Chris”
GOOD: name=“Chris”
d=5
numberOfDays=5
This makes it easier for anyone, including
you, to understand what is going on in the
code
Functions
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Functions in python are similar to
functions in mathematics, they take
arguments as input and return a value
Python has many built in functions and we
will learn how to write our own functions
later
Functions
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A couple simple built in functions
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len takes a string as its only argument and returns
the length
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len(“Chris”) returns 5
len(“”) returns 0
round can take one or two arguments
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If the only argument is a number round returns the number
rounded to the nearest integer
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round(22.3) returns 22.0
A second optional argument indicates how many decimal
places to round to
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round(12.3456,2) returns 12.34
round(12.3456,3) returns 12.346
Functions
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Functions can be part of an expression
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x = 12 + round(22.6)
Variables can be used as arguments for a
function
name=“Chris”
nameLength=len(name)
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Any expression can be uses as an
argument
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round(12.1+1.6) is 14.0
The raw_input Function
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This is a function that allows you to take
input from the user.
It takes one argument, a string that will
be printed to the screen as a prompt to
the user.
The function returns the user’s input as a
string.
A Bit More On Strings
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There are a couple things you can do with strings to help
format output or display certain characters
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You can escape a character by using a backslash \ in a string.
The backslash and the character after it combine to signify one
character in the string.
The start and end of a string are denoted by single or double
quotation marks. Therefore you can’t just drop them in the
middle of a string without using the backslash
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print “Descartes said “I think, therefore I am””
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Python will think the string has ended when you hit the quotation mark
before I
print “Descartes said \”I think, therefore I am\””
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This will print correctly, the \” represents the quotation mark within the
string
A Bit More On Strings
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Other escape characters
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\\ to represent a backslash in a string
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\n represents a newline
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Since the backslash is used to indicate a special
character
“This is the first line \n This is the second line”
\t represents a tab
There are others, but just be aware of the
idea of escape charaters
A Bit More On Strings
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Python provides a way to simplify
formatting of a string and avoid using
escape characters.
If you begin and end a string with 3
quotation marks, Python will handle all of
this for you and format the string as you
wrote it