Programming Seminar 2009

1. Programming Seminar 2009 Night 0

2. Why we’re holding this seminar • You’re probably not a computer science major • Or even anything remotely close (e.g. EE) • We think that programming is an important skill • Useful (sometimes) • Interesting (often) • This will be a combination of practice, theory, and examples.

3. What I’m assuming about you • You have (almost) no experience writing code of any sort. • The words [“syntax”, “variable”, ”function”] might mean something. • If you took certain math or linguistics courses. • The methodology for building a program (or parts of it) is somewhat mysterious

4. So what is a “program” anyway? • A number • A set of instructions and data • For a computer to execute • Human-readable source code • Readable for some humans, anyway • A (hopefully) consistent way to accomplish some task

5. Enter Python • Python is a new, friendly, language with an amazing library of functionality • Most of the text is English, with a few abbreviations and some other characters • We will be using Python 3 • Unless the computers really don’t want to, in which case we will be using Python 2.6 • Or in the case they don’t have that either, Python 2.5

6. Let’s get started! • The first item for action is running the Python interpreter. • Or perhaps IDLE, a Python IDE • You can do this by finding Python in the system menu, or by starting a terminal and typing “python” • If you see a line beginning with >>>, you are fine. • All together? Good.

7. As tradition dictates… • Type >>> print(“Hello World!”) • And Python prints “Hello World!” to the console window. • Don’t you feel accomplished? • Here’s what you’ve learned: >>> print(“(something)”) prints (something) • You can put two strings together with +

8. On Statements • In general, code falls into one of four categories: • Declaration – saying that something exists • Assignment – telling the program to remember a value • Definition – detailing how something works • Control – ordering the program what to do next

9. Assignments • Try telling Python >>> sentence = “Armavirumquecano” >>> print(sentence) • This does exactly what you might expect >>> print(“Armavirumquecano”) • Both print the first few words of the Aeneid • If you didn’t expect this, what did you expect? • Every time I say you might have expected something, please speak up if you didn’t!

10. Assignments, continued • Try >>> x = 3 >>> print(x * x) • You might have guessed that this prints the number 9 to the console • What do you think these lines do? >>> x = 4 >>> y = 3 >>> print(x * y – (x + y))

11. So Python can do math… • Congratulations! Your computer can be used as a (very expensive) calculator. • Most arithmetic operators make sense • + - / * • This works for real numbers (e.g. 1.56), as well. • Quick! What is 1234*9876?

12. …But programming math is (generally) not real math • If you say “x = 5” when proving that the natural numbers are in bijection with the rationals, x is always 5. • In Python (and most other programming languages) >>> x = 5 >>> x = 6 >>> print(“x = “ + x) prints 6

13. Assignment is storage • Remember when I said that assignment tells the program to remember a value? >>> x = 5 tells Python “x has the value 5” • And x holds the number 5 • But this value can change. >>> x = x + 1 increases the value of x from 5 to 6.

14. Wait – what!? • x = x + 1 doesn’t even make sense! • Subtract x from both sides and you get 0 = 1. • But remember, programming “math” is not math • x = x + 1 can be read • “Increment x by 1” • Find the value I previously assigned to x. Add 1 to it. Now assign the new value to x.

15. Other ways to assign >>> word = “coconut” tells Python to remember that word represents the value “coconut” • Which variables represent what ? • What is the output? >>> pi = 3.14 >>> radius = 5 >>> area = pi * radius * radius >>> print(area)

16. So what good is any of this? • It gets a little more interesting when we have user input • We do this with the input() function. • In previous versions it was raw_input() • We can make programs interactive! >>> name = input(“What is your name?”) >>> print(“Hello, ” + name + “!”)

17. Okay, but no program I’ve even used does only one thing every time I run it • Good point. • Enter Control statements • Conditional execution: • If condition is true, do something, otherwise do something else • While condition, do something, and continue to do that something until condition is false. • For each item in a collection, do something

18. If statements >>> number = int(input(“Enter a number: ”)) >>> if number > 0: >>> print(“A square with side length”, number, “ has area ”, number * number) >>> else: >>> print(“I need a positive number!”) The general format is if condition: do_something() else: do_something_else()

19. For statements >>> for item in (2,3,5,7,11,13): >>> print(item * item, “ has and odd number of factors.” ) • The general form is for var in collection: do_something()

20. A few more examples… >>> limit = int(input(“Give me an upper limit: ”)) >>> if limit < 0: >>> print(“Too low!”) >>> elif limit > 50: >>> print(“Too high!”) >>> else: >>> for i in range(limit): >>> print(i, “squared is ”, i*i)

21. …examples >>> sentence = input(“A sentence, please”) >>> words = 1 >>> for c in sentence: >>> if c == ‘ ’: >>> words = words + 1 >>> print(“The sentence you entered had”, words, “words”)

22. Your turn! • Write a program to count the number of vowels in a sentence which the user inputs • Write a guessing game that gives the player 5 chances to guess a number correctly • We will make the number’s selection random later • Write a simple calculator; that is, a program which asks for two numbers and whether it should add, subtract, multiply, or divide them. The program should then output the calculation and its result.

23. More about modules • As previously stated, Python has an extensive, useful library of functionality • Some of this functionality is included by default • E.g. abs(), print(), range(),… • The rest is in a number of modules • Math-related stuff is in the math module. >>> import math >>> print(math.sqrt(5))

24. Modules, continued • Can also import specific functions >>> import cos from math >>> from math import atan • There is a module for almost everything • Want to enter passwords? Import getpass • Want OS functionality? Import os, sys • Want to handle options passed on the command line? Import optparse

25. On Functions • Certain pieces of code that are used repeatedly in a succinct fashion • Input(), split(), abs(), print() • Functions run specified code every time we “call” them.

26. Programming is not Math, revisited • Programming functions are not like math functions! • The return value can change • Input() rarely returns the same result • Functions often do some action each time we “call” them • Print() prints output to the screen • We don’t evaluate functions, we “call” them to do their job

27. Defining a Function >>> def name(arguments): >>> …actions… • Here is an example: >>> def square(n): >>> print(n*n)

28. Your turn, once more • Create a function that will print a row of n stars (*), with n passed as an argument • Use this function to create a function that will print an isosceles right triangle of stars • Write a function fib(n), which prints the nth Fibonacci number. The first and second Fibonacci numbers are both considered to be 1 • Do not use recursion, if you know what it is.