1. Introduction to Python
Introduction to Computing Science and Programming I

2. Literals
Literals
These are values in the code that represent themselves
1, 22.8, -45, “Chris”, ‘-356’

3. The print Command
Use print to output a line of information to the screen
print “Hello World”
To print multiple items separated by single spaces use commas. The output for this command is identical to the one above
print “Hello”,”World”
The command will also output numbers

4. Calculations
An expression is any kind of calculation that produces a result
7 – 3
“Combine two “ + “strings”

5. Calculations Basic Math
+,-,*,/ work as you’d expect. Python follows the normal order of operations
5+2*2 is 9
Parentheses () also work as they do in mathematics
(5+2)*2 is 14

6. Calculations String “math” To combine (concatenate) two strings use +
“Chris “ + “Schmidt” is “Chris Schmidt”
You’ll be using this quite a bit for output to the screen
If you want to repeat a string, use *
“Chris ” * 3 is “Chris Chris Chris ”

7. Data Types
Different types of information have different data types. Python will behave differently depending on what type of information it is working with
Data Types
Integers
9, 34, -5
Floats
23.45, 12.0, 15.3
Strings
“Chris”, “This is a string”
We’ll see other data types in the future

8. Data Type You need to be careful
“123” is a string, 123 is an integer, is a float
In mathematical operations if both operands are integers, the result is given as an integer
5 / 3 is 1
Notice that the result has the decimal portion removed instead of being rounded
5.0 / 3 is …
The ‘.0’ makes the first operand of type float and therefore the result is given as a float

9. The type Function
Python provides a function to tell you what data type a piece of information is
type(24) returns <type ‘int’>

10. Type Conversion
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
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”)

11. Variables
Variables
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
name = “Chris”
id = 123
height = 1.7
If you want to use the value stored in the variable, simply use the variable’s name
print name

12. Variables Allowed names
Variable name start with a letter and can contain letters, numbers, and underscores
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
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

13. Variables
Point of good coding: Give your variables intelligent descriptive names
BAD: n=“Chris” d=5
GOOD: name=“Chris” numberOfDays=5
This makes it easier for anyone, including you, to understand what is going on in the code

14. Functions
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

15. Functions A couple simple built in functions
len takes a string as its only argument and returns the length
len(“Chris”) returns 5
len(“”) returns 0
round can take one or two arguments
If the only argument is a number round returns the number rounded to the nearest integer
round(22.3) returns 22.0
A second optional argument indicates how many decimal places to round to
round( ,2) returns 12.34
round( ,3) returns

16. Functions Functions can be part of an expression
x = 12 + round(22.6)
Variables can be used as arguments for a function
name=“Chris”
nameLength=len(name)
Any expression can be uses as an argument
round( ) is 14.0

17. The raw_input Function
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.

18. A Bit More On Strings
There are a couple things you can do with strings to help format output or display certain characters
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
print “Descartes said “I think, therefore I am””
Python will think the string has ended when you hit the quotation mark before I
print “Descartes said \”I think, therefore I am\””
This will print correctly, the \” represents the quotation mark within the string

19. A Bit More On Strings Other escape characters
\\ to represent a backslash in a string
Since the backslash is used to indicate a special character
\n represents a newline
“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

20. A Bit More On Strings
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