COMP 364: Conditional Statements Control Flow

Transcription

1 COMP 364: Conditional Statements Control Flow Carlos G. Oliver, Christopher Cameron September 15, /28

2 Outline 1. New midterm date Tuesday October 24 19:00-21:00 ENGMC Recap 3. Conditionals (if, else, elif) 4. User input 2/28

3 ipython vs command line The interactive Python interpreter is not the same as the Terminal/Command Prompt Interactive Python prints the result of evaluating an expression. (more later) 1 >>> "hello" 2 "hello" #interactive python automatically prints result of expression 3 >>> print("hello") #this is the same as above 4 "hello" 5 >>> s = "hello" #will not print anything 3/28

4 Refresher: the OS and file system The Terminal / Command line lets you execute programs (e.g. ls, cd, dir, python, mkdir) How to open terminal in Mac and in Windows Syntax: $ [program name] [program options] These functions include Launch the Python interpreter $ python List files in your current directory $ dir $ ls Change directories $ cd [destination path] A program you are executing can only see files in your current directory. If you want to access a file outside of your current directory you must specify its path in [program options]. (quick demo) e.g. $ python /Users/carlos/Desktop/hello.py will work from anywhere because it has the full path to the file. 4/28

5 Functions are objects A function is also an Object which can execute some commands given an (optional) input to produce an output. function name(input) e.g. >>> print("hello world") Input Function Output 5/28

6 Names, Namespaces, Objects class / type o1 o2 o3 Objects bob alice eve Names Namespace 6/28

7 Making decisions: conditional statements I I I Programs often have to make decisions that depend on some conditions. For example, depending on what you type into a Google search, it executes a different set of commands Python lets us accomplish this using conditional statements 1 1 Rick and Morty 7/28

8 An example:finding genes Genes are DNA sequences made up of codons that get converted into proteins. We know that they always start with the letters: AUG We can use the if statement to check if an object is a start codon. 1 codon = "AAG" 2 3 if codon == "AUG": 4 print("this is a start codon!") 5 else: 6 #if we have a start codon, this NEVER executes 7 print("this is not a start codon!") 8/28

9 A little terminology: Expressions and Operators Expressions are any line of code that can be evaluated to some value and stored as an object. Operators do computations on expressions (e.g. +,-,==, <=, >=, %) to produce new expressions. You can think of operators as special tokens for functions. Life Hack 1 Don t worry too much about all this terminology. It s just so we can have a common language when talking about code. 9/28

10 A little more terminology: Statements Statements are special instructions that tell Python how to execute code. (e.g. x = 2, if, else) They do not evaluate to a value. (e.g. if is a statement, x = 4 is also a statement) 1 >>> x = 3 #statement 2 >>> y = 9 # statement 3 >>> x + y # expression 4 12 Life Hack 2 If you can print it or assign a name to it it s an expression, otherwise it is a statement. 10/28

11 Back to the if statement Python executes line by line going down your code. However, if statments (and others) let us intervene. The if statement lets us split our commands into separate units that execute based on the value of some boolean expression. Code belonging to an if/else is denoted by a tab (4 spaces) Syntax: if (boolean expression) :, else: Note: here, the else is not mandatory but can come in handy. 11/28

12 Small example 1 sequence = "AACGAAgU" 2 3 if not s.isupper(): 4 #inside first case 5 #DNA sequence not capitalized 6 print("you forgot to capitalize") 7 sequence = sequence.upper() 8 print("i fixed it") 9 else: 10 #inside second case 11 # DNA sequence correct 12 print("sequence OK") #outside if statement 15 print(sequence) 12/28

13 Control Flow s = "AAgU" if not s.upper() s.upper() print("ok") print("done") 13/28

14 Control flow s = "AAGU" if not s.upper() s.upper() print("ok") print("done") 14/28

15 What if we have more than two cases? elif Let s try to figure out if we have a natural disaster. 1 ground_shaking = True 2 flooding = False 3 4 if(ground_shaking and not flooding): 5 print("earthquake!") 6 elif(not ground_shaking and flooding): 7 print("hurricane!") 8 elif(ground_shaking and flooding): 9 print("end of the world!") 10 else: 11 print("everything is okay.") Note: as soon as ONE of the cases is True the rest does not execute (even if they happen to be True as well. Try it!). 15/28

16 Else if: elif elif always comes after an if or another elif and the last statement must be an else If the previous if or elif did not evaluate to True AND this elif is True execute this block of code. Useful for when you have multiple cases but only one should execute. Important: vertical alignment is crucial. 16/28

17 Nested if/else/elif statements We can put if/else/elif statements inside other if/else/elif statements. Note: can lead to repetitive code so elif is preferred when applicable. 2 2 http: //wallpoper.com/images/00/21/28/83/movie-inception_ jpg 17/28

18 Nested if statements 1 ground_shaking = True 2 flooding = False 3 4 if ground_shaking: 5 if flooding: 6 print("end of the world!") 7 else: 8 print("earthquake!") 9 else: 10 if flooding: 11 print("hurricane!") 12 else: 13 print("everything is okay.") 18/28

19 An interlude: interacting with the user Code is not very useful if it can t receive information from a user. Receiving information input Holiday+Store+opmeLl_yL8ql.jpg 19/28

20 Interactive user input: input() The input() function lets us store values into string objects based on text given to us while the program is running. a.k.a at runtime Synatx: x = input("message to user ") Convert the input to the appropriate data type using: int(), float(), bool() 1 flooding = input("is there flooding? (Y/N)") 2 if flooding == "Y": 3 flooding = True 4 elif flooding == "N": 5 flooding = False 6 else: 7 print("incorrect input format, enter Y/N") 20/28

21 Example 1 flooding = input("is there flooding? (True/False) ") 2 ground_shaking = input("is the ground shaking? ") 3 4 #leaving out the type conversions for brevity (see prev slide for example) 5 6 if(ground_shaking and not flooding): 7 print("earthquake!") 8 elif(not ground_shaking and flooding): 9 print("hurricane!") 10 elif(ground_shaking and flooding): 11 print("end of the world!") 12 else: 13 print("everything is okay.") 21/28

22 Sanity checks: assert statements Often you want to make sure your code doesn t do something that makes no sense. This comes up a lot in user input since we can t predict what the user will input. The assert statement takes a boolean expression. If it evaluates to false, the program terminates. 1 #this code divides two user input numbers 2 numerator = int(input("give me a number: ")) 3 denominator = int(input("give me a number: ")) 4 assert denominator!= 0 5 print(numerator / denominator) 22/28

23 In-class problem: mini medical diagnosis program Let s write a program that takes info on a patient s symptoms and outputs a diagnosis /28

24 In class problem: house.py Since it may get a little long and we would like to reuse it, we should save our code in a file house.py (or a notebook). Input: age, sex, temperature, coughing, headaches, nausea Output: "healthy", or "infection", or "hypothermia", or "pregnant", or "food poisoning" 24/28

25 RTFM: Documentation Everything I am showing you and much much more is documented in the python docs for bulit-in functions. The entire set of default python commands and data types is here How to read documentation syntax (live demo) _646849b145.jpg 25/28

26 Collections The world is full of collections of things. We would like to be able to work with such things efficiently. 6 6 Rick and Morty 26/28

27 How can we keep track of all the Jerrys? We know how to store data as objects. So let s make an object for each Jerry. 1 jerry1 = "Original" 2 jerry2 = "First clone" 3 jerry3 = "Second clone" This doesn t seem like a very efficient way of doing things. Thankfully Python lets us store collections of things very easily. 27/28

28 Lists Just like everything in Python, a list is an Object of type list. We store a list using square brackets [] and separate objects with commas (,). 1 >>> jerrys = ["Original jerry", "Clone one", "Clone two"] 2 >>> type(jerrys) 3 <class list > 4 >>> id(jerrys) /28