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First Program Open a file In Shell Type into the file: 3 You did it!!! You wrote your first instruction, or code, in python! 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” 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! Order of operators: 1. (x+y) 2. x ** y 3. Power (right associative) x * y, 4. parentheses x // y, x%y Multiplication, division, floor division, modulo x + y, 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 Files 1. Open a file 2. In IDLE, go to File->New Window In New Window: Type: 3 + 2 3. File->Save As 4. Save the file as first.py 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. 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) = x3 + 1 g(2) = 9 g(5) = 126 Creating a function: Function (mathematical) g(x) = x3 + 1 Consists of 3 parts and a name: g(2) = 9 -> name: g(5) = 126 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 Function (in Python) def g(x): return(x ** 3 + 1) g(x) = x3 + 1 g(2) = 9 g(5) = 126 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 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) = x3 + 1 g(2) = 9 g(5) = 126 Input Values: Parameters values into the function are known as parameters 3,2 7,4 9,8 Code: addfunc addfunc addfunc 5 11 17 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. 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 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) [email protected] 4Cats Cat_n_Mouse Cat-tail Cat123 cats4sale Cat n Mouse 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 What we’ve learned about writing functions: We should come up with test cases first We should include comments that clearly describe how the function works. How many parameters go into the function in order to get the output? 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.