Transcript Chapter 2 (Math Operations) Powerpoint

MATH OPERATIONS
9/19/16
Math operations
You can add, subtract, multiply, divide numbers like this:
◦ addition = 72 + 23
◦ subtraction = 108 – 204
◦ multiplication = 108 * 1.5
◦ division = 108 / 9
You try it: Set the variable count_to equal to the sum of two big numbers
Then print count_to
Exponentiation
All that math can be done on a calculator, so why use Python? Because you can
combine math with other data types (e.g. variables, strings, and booleans…which we
will get to) and commands to create useful programs. Calculators just stick to
numbers.
Now let’s work with exponents.
eight = 2 ** 3
In the above example, we create a new variable called eight, and set it to 8, or the
result of 2 to the power to 3 (2^3)
Notice that we use ** instead of * or the multiplication operator.
You try it: Create a new variable and use exponents to set that variable equation to
100. Try raising 10 to the power of 2.
Modulo
Our final operator is modulo. Modulo returns the remainder
from a division. So, if you type 3 % 2, it will return 1,
because 2 goes into 3 evenly once, with 1 left over.
You try it: use modulo to set a variable equal to 1. You can
use any two numbers that will leave a remainder of 1 to do
this.
Useful math operators
Operator
Description
Example
Evaluates to
+
Addition
7+3
10
-
Subtraction
7-3
4
*
Multiplication
7*3
21
/
Division (True)
7/3
2.33333333333333
33335
//
Division (Integer)
7//3
2
%
Modulus
7%3
1
More about variables
◦ Recap: a variable is a way to label and access information. Instead of having to
know exactly where in the computers memory something is store, you use a variable
to get at it.
◦ name = “Larry”
◦ This line is called an assignment statement.
◦ Technically, an assignment statement stores the value on the right side of the equal
sign into your computers memory while the variable on the left side only refers to
the value
Using Variables
◦ Once the variable has been created, it refers to some value. The convenience of
the variable is that it can be used just like the value to which it refers.
◦ print (name)
◦ Prints the string “Larry” just like the statement print (“Larry”) does.
◦ And the line print (“Hi, “, name)
◦ Prints the string “Hi” followed by a space, followed by “Larry”
You try it
◦ Write a single-line comment on line 1. It can be anything! (Make sure
it starts with a #)
◦ Set a variable python equal to 1.234
◦ Set a second variable to monty_python equal to python squared
◦ Print monty_python
The Meal
Now let's apply the concepts from the previous section to a real world
example.
You've finished eating at a restaurant, and received this bill:
•Cost of meal: $44.50
•Restaurant tax: 6.75%
•Tip: 15%
You'll apply the tip to the overall cost of the meal (including tax).
Instructions
First lets declare the variable meal and assign it to 44.50
The Tax
◦ Now let's create a variable for the tax percentage.
◦ The tax on your receipt is 6.75%. You'll have to divide 6.75 by 100 in
order to get the decimal form of the percentage.
Instructions
◦Create the variable tax and set it equal to the decimal value
of 6.75%
The Tip
◦ You received good service, so you'd like to leave a 15% tip on top of
the cost of the meal, including tax.
◦ Before we compute the tip for your bill, let's set a variable for the tip.
Again, we need to get the decimal form of the tip, so we divide 15.0
by 100.
Instructions
◦On the next line, set the variable tip to the
decimal value of 15%
Reassign in a Single Line
◦ We've got the three variables we need to perform our calculation, and
we know some arithmetic operators that can help us out.
◦ We saw in last week’s lesson that we can reassign variables. For
example, we could say spam = 7 and then later change out minds and
say spam = 3
Instructions
◦On the next line, reassign meal to the value of itself +
itself * tax. And yes, your allowed to reassign a
variable in terms of itself.
◦We’re only calculating the cost of the meal and tax
here. We will get to the tip next.
The Total
◦ Now that meal has the cost of the food plus tax, let’s introduce a new variable on
the next line, total, equal to meal +meal *tip
Instructions
Assign the variable total to the sum of meal + meal * tip on the next line. Now you have the total
cost of the meal.
Then add: print("%.2f" % total)
The code on the end formats and prints to the console the value of total with exactly two numbers
after the decimal. (We'll learn about string formatting, the console, and print in the next unit!)