2. First Program  Open a file  In Shell  Type into the file: 3  You did it!!! You wrote your first instruction, or code, in python!

3. Atomic Data  the lowest level of detail from which aggregate data is computed.  the smallest unit of data you can do something with  if we give Python atomic data, Python spits it back at us  Try :  42  128.4  “puddle”

4. Expressions  Use atomic data to build compound expressions.  Compound expressions consist of two expressions (either atomic or compound), with an operator between them 3 + 2 3 - 2 3 * 2 3 / 2 3 ** 2 (3 + 2) ** 3 (3 * 4) / 2  We’re starting to acquire TOOLS!

5. Order of operators: 1. (x+y)  parentheses 2. x ** y  Power (right associative) 3. x * y, x / y, x // y, x % y  Multiplication, division, floor division, modulo 4. x + y, x - y  Addition, subtraction Shall we try this?  5 + 4 / 2 + 1  7//3  -7//3  5*2**3  7%2  18%4  3 + 2**3  (3 + 2) ** 3  12/2 ** 2

6. Files 1. Open a file  In IDLE, go to File->New Window 2. In New Window:  Type: 3 + 2 3. File->Save As  Save the file as first.py 4. Run the file:  Run->Run Module Now you’ve saved your work so you can run it later When saving a python program, it must have a.py extension!!!  So interpreter knows it’s python code.

7. Functions  We have a file: we can save our work.  Now, let’s create “functions” to name code we might want to use again  Math: function takes a number or numbers and transforms it to another number  E.g., f(x) = 2x f(3) = 6 f(5) = 10 g(x) = x 3 + 1 g(2) = 9 g(5) = 126

8. Creating a function: Function (mathematical) Consists of 3 parts and a name: -> name: g (not a good name! Tells us nothing about what this function does) -> input parameter in the example, integers 2 or 5 -> instructions (code) in the example, x**3 + 1 -> output in the example, the integer 9 or 126 g(x) = x 3 + 1 g(2) = 9 g(5) = 126

9. Function (in Python) def g(x): return(x ** 3 + 1) g(x) = x 3 + 1 g(2) = 9 g(5) = 126

10. Creating a function: Function (programming)  Function: a set of instructions (code) identified with a name  Every function in a program has a unique name  The instructions inside the function do not get executed until the function’s name is called  Can be called again and again  Or can never be called  …which is kind of pointless

11. Function (in Python) def g(x): return(x ** 3 + 1) To Call the Functions (to make them run): g(2) g(5) To see what the function calculates (returns): print (g(2)) print (g(5)) g(x) = x 3 + 1 g(2) = 9 g(5) = 126

12. Input Values: Parameters values into the function are known as parameters 3,2 addfunc 5 7,4 addfunc11 9,8 addfunc 17 Code: def addfunc(value1,value2): return (value1 + value2) print(addfunc(3,2)) print(addfunc(7,4)) print(addfunc(9,8))  We should know what we want to come out of the function, so we can check to make sure the function is working correctly  Print allows us to check the value that is coming out of the function.

13. Function:  Calculate the area of a rectangle? 1. Name of function? 2. Input? 3. Output? 4. Test cases? 5. Calculations? Can we now write the function? def arearectangle(len,width): return(len*width) func(x,y) = x*y func(2,7) = 14 func(5,4) = 20

14. Function names:  Naming Rules:  Must start with a letter  not a number!  Can only contain letters and numbers  NO SPACES!!!!  NO SPECIAL CHARACTERS!  Anything that isn’t a letter or a number  * & ^ % $ $ # @ ! `~ + = - ) ( “ ‘ : ; ><?/, etc.  Other than _ (the underscore)  cat@yahoo  4Cats  Cat_n_Mouse  Cat-tail  Cat123  cats4sale  Cat n Mouse

15. Comments #This function calculates the square of the input value #and returns that squared value #input: an integer #output: an integer #Test Cases: # print(newfunc(3)) -> 9 # print(newfunc(5)) -> 25 # print(newfunc(2))-> 4 #Author: Debra Yarrington #Feb 6, 2016 def newfunc(par1): return(par1**2) # returns the square  Comments aren’t executed (aren’t converted to machine language). Python’s compiler ignores them. They’re for people who are reading your code.  They also can be used to help you (and others) understand what you are doing and why

16. What we’ve learned about writing functions:  We should come up with test cases first  How many parameters go into the function in order to get the output?  We should include comments that clearly describe how the function works.  These comments should include our test cases  After we’ve got the test cases and the function description, then we write the function.  Basically, you have to think it through before you write the function.