03 Variables - Chang.pptx
- 1. Variables and Expressions CMSC 201 Chang (rev. 2015-02-05)
- 2. Today we start Python! Two ways to use python: You can write a program, as a series of instructions in a file, and then execute it. You can also test simple python commands in the python interpreter.
- 3. Variables Variables are names of places that store information. Making variables in Python: someVariable = 10 someVariable is a variable. someVariable has value 10. someVariable can have a different value later. The = is called the assignment operator. It allows you to change the value of a variable.
- 4. Variables The right hand side of the assignment can be any mathematical expression. someVariable = 10 anotherVariable = 5 + 9 aThirdVariable = 10 * anotherVariable someVariable = someVariable + anotherVariable Whenever a variable is on the right hand side of an expression, imagine it being replaced by whatever value is currently stored in that variable.
- 5. Types There are many different types of values ! Numbers (integer or floating point) True / False values (called booleans) Strings (collections of characters) aString = ‘Hello everyone’ x = 1.12 bool = True
- 6. Rules for Naming Variables Variable names are case sensitive. Hello different from hello Only may contain alphabetic letters, underscores or numbers. Should not start with a number. Cannot be any other python keyword (if, while, def, etc). Give your variables meaningful names!
- 7. Output We’d like to see what’s stored in our variables! Python can print things for us: print("Hello world") You can also output the contents of variables: someVariable = 10 print(someVariable) You can even do combinations! print("Your variable is ", someVariable)
- 8. Exercise What will the following code snippet print? a = 10 b = a * 5 c = "Your result is: " print(c, b)
- 9. Exercise What will the following code snippet print? a = 10 b = a a = 3 print(b) There are two possible options for what this could do! Any guesses?
- 10. Exercise a = 10 b = a a = 3 print(b) It will print out 10. When you set one variable equal to another, they don’t become linked; b is set to 10 and no longer has anything else to do with a.
- 11. Input Sometimes, we’d like the user to participate! We can also get input from the user. userInput = input("Please enter a number ") print(userInput) The output will look like this: Please enter a number 10 10
- 12. Input This line: userInput = input("Please enter a number") Takes whatever the user entered and stores it in the variable named "userInput" You can do this as many times as you like! userInput1 = input("Enter first number.") userInput2 = input("Enter second number.")
- 13. Input Note: All input vlaues are strings. a = "10" b = 10 Variables a and b are different. To turn an input into a number: stringInput = input("Enter a number: ") aNumber = int(stringInput) int stands for integer
- 14. Operator Precedence Like algebra, multiply and divide before adding and subtracting: 3 * 4 + 5 is 17, not 27. Python has many more operators. Precedence allows this expression to make sense without parentheses: 3 * 4 + 5 > 2 + 2 * 4 and 17 + 9 > 31 - 22
- 15. Associativity Associativity does not hold (4.05 - 4.05) + 0.0000000000000001 does not equal 4.05 – (4.05 + 0.0000000000000001) are not the same. Most operations associate left to right by default: a + b + c means (a + b) + c
- 16. More operators Exponentiation: 2**3 is 8 2.5**2 is 6.25 Integer division (rounds down): 5//2 is 2 Modulus (gives the remainder): 28 % 5 is 3
- 17. Exercise Write, on paper or on your computer, a program that asks the user for two numbers a prints out the average.
- 18. Exercise Pretend you’re writing a program to compute someone’s weighted grade. You have so far: hwWeight = 0.4 examWeight = 0.5 discussionWeight = 0.1 Write a program starting with these three lines that asks the user for their homework grade, exam grades, and discussion grades and prints out their total grade in the class.