Download Seminar 0 - STWing@UPenn

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 = 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.
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 is
if 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 is
for 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.