Python

November 14, Unit 7

Python

• Hello world, in class

Strings

• A string is a bunch of characters• In >>>print “Hello, world!”, the string is:

Hello, world!

• We can have numbers inside strings

>>>print “Today is my friend’s 35th birthday!”

>>>print “4+4”• Strings are one type of value that python can

manipulate

Values and Type

• Values are some of the basic items that your program can manipulate

• “Hello, world!” is a value of the type string

• >>>print 1+1, the value of this is 2 and is of the type integer

• So what is a type?– Values are categorized according to their type

Types

• It’s easy to see that “Hello, world!” and the value of 4+4 are of different types– “Hello, world!” is a string– 4+4 produces 8, which is a number

• They have different types• 1.2 and 4 also have different types

– 4 is an integer (whole number can be positive or negative)– 1.2 is called a float (or floating-point number)

• Can also be positive or negative• If it has a decimal point it’s usually a float• Like real numbers

• “4+4” is of the type string– If it’s in quotes, it is a string

What’s so Important about Types?

• The type of value dictates what you can do with it• >>>print 8-4

– Has a value of 4

• >>>print “Hello, world!”-H– We may think this means take away the “H” in “Hello, world!”– But the interpreter has no idea what we mean– We can’t subtract a letter from a string in this way

• One of the reasons for using types is that different types of information are stored differently– Characters are not stored the same way as integers– Integers and floating point numbers aren’t stored the same way

Mathematical Expressions

• Should all be familiar with a mathematical expression– 4+4– 8 * 2– 24 / 2 + 6 * 3

• An expression is any kind of calculation that returns a result

Expressions, in General

• We can have things other than mathematical expressions

• A value by itself is considered an expression– >>>4– >>>”Hello, world!”

• Expressions are any combination of values, variables, and operators– We’ll get more into this later

Mathematical Expressions, cont.

• Mathematical expressions have two parts:– Operators– Operands

• Operators are the symbols we use for math– + addition– - subtraction– / division– * multiplication– ** power (exponent)– ()

• Operands are the values we use these operators on– 4 + 8 / 6 * 3

• Operands are: 4, 8, 6, 3• Operators are: + , /, *

– Operandscan also be variables

Evaluating Mathematical Expressions

• In python (like in most programming languages), mathematical expressions are evaluated like we learned in grammar school

• PEMDAS– Parenthesis, Exponents, Multiplication, Division,

Addition, and Subtraction– Please Explain My Dear Aunt Sally– Multiplication and Division have the same precedence

• As do Subtraction and Addition• Evaluated left to right• Doesn’t really matter 3+4-2

– 7-2 = 5– 3 + 2 = 5

Mathematical Expressions

• In class example• Integer division:

– When you divide one integer by another– Produces another integer– Always rounds down– 2/3 = 0– 4/3 = 1

• Floating point division:– If we want the decimal information at least one of the two

operands must be a float• 2.0 /3• 2 / 3.0• 2.0 / 3.0

In-Class Examples

• Playing with mathematical expressions

Variables

• Variables are a way to store data so that we can use it over and over>>>x = 4 + 8>>>print x12– x is the variable

• We can store data of any type in a variable>>>name=“Sarah Brown”>>>print name

Variables, cont.

• Let’s say we have the expression “8 *2 +4” and we want to store it in a variable called “x”>>>x = 8 *2 + 4

• The value of 8 *2 + 4 is calculated, resulting in 20

• This is then assigned to the variable x using the “=“ operator

• This is called an assignment statement

Variables, cont.

• We can assign the value of one variable to another variable>>> y = x

• We can assign an expression using a variable to another variable>>>y = x*2 + 7

• Variables do not store the expression– They store the value of the expression– If x = 4, y has the value of 15– y does not have the value “x *2 + 7”

User Input

• Now that we have a way to store data, we can get input from users and manipulate it

• To prompt the user for information we are going to use raw_input(“Whatever you want to ask for”) and assign that to a variable

• Example:>>>name = raw_input(“What is your name”)>>>print name, “is such a pretty name”

raw_input, cont.

• Another example:>>>age = raw_input(“How old are you? “)>>>print “you are “, age,”years old! That is SO old!”

• So when printing out the above statement we want to put the value for age in between two strings– One way to do this is to use a “,”– Items in python separated by a comma are

considered items in a list– Just prints them in order

raw_input, cont.

• One of the issues with using raw_input is that all input from the user is considered to be a string

• Take the following:>>>base = raw_input(“Enter a number”)>>>print “ 2 times base is: “, base

• The out put would be :“2 times base is: basebase”

• If the user entered the number 4“2 times base is 44”

• The user could have entered a name if they had wanted and it would print it twice– If base = “pizza”– “2 times base is pizzapizza”

Another Example with raw_input

• Let’s say instead of multiplying base by 2 we want to output base+1>>>base = raw_input(“Enter a number”)>>>print base+1

• This would cause an error– We can’t add the number 1 to a string

• We need to convert the value in base from a string to a number– We are going to use that concept of types

Type Function

• We can use the function type to determine the type of a variable

>>>base = raw_input (“enter a number”)>>>type(base)

Will output <type ‘str’>>>>type(4) will output <type ‘int’>>>>type(4.0) will output <type ‘float’>

Type Conversion

• We can use different functions to convert from one type to another– int() converts whatever is inside the () to an integer– float() converts the contents of the () to a floating

point number– str() converts contents of () to a string

• We can use these function to convert the input from the user to the type that we need

Type Conversion, example

>>>base = int(raw_input(“enter a number”))

>>>print base +1

If the user enters the number 5,

6 will be output

In Class Example

• Getting input from the user– Storing input in a variable

• Using multiple variables

• Converting from one type to another

Concatenation

• So far when we want to print multiple things we’ve been using a comma to separate the items

• Better way is to concatenate the items• Concatenation is like string addition• >>>print “Hello,” + “world!”

– Outputs “Hello, world!”• We can only concatenate strings

>>>x = 4+8>>>print “4 + 8 = “ + xCauses an error

Using str()

• Like we can change the type of input from the user from a string to a number, we can go back in the other direction

>>>x = 4 + 8

>>>print “4 + 8 = “ + str(x)

Would output:

“4 + 8 = 12”

In Class Examples

• Putting it all together

• Converting input to number then back to a string for printing

Questions

• The main points from today’s lecture:– Types (int, float, string)– Mathematical Expressions

• PEMDAS

– Variables• Assignment statements

– User Input (raw_input())• Always produces a string

– Type conversion