Programming Seminar 2009

Night 0

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.

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

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

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

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.

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 +

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

Assignments

• Try telling Python>>> sentence = “Arma virumque cano”>>> print(sentence)

• This does exactly what you might expect>>> print(“Arma virumque cano”)

• 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!

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))

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?

…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

Assignment is storage

• Remember when I said that assignment tells the program to remember a value?>>> x = 5tells Python “x has the value 5”– And x holds the number 5

• But this value can change.>>> x = x + 1increases the value of x from 5 to 6.

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.

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)

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 + “!”)

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

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 isif condition:

do_something()else:

do_something_else()

For statements>>> for item in (2,3,5,7,11,13):>>> print(item * item, “ has and odd number of factors.” )

• The general form isfor var in collection:

do_something()

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)

…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”)

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.

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))

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

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.

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

Defining a Function

>>> def name(arguments):>>> …actions…

• Here is an example:>>> def square(n):>>> print(n*n)

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.