Programming Seminar 2009 Night 1

Review: Last Time We covered variables, for loops, functions, modules, and how programming isn’t math – Actually, we just scratched the surface for most of these This time, we’ll be focusing more on functions – And functionality – With any luck, we will work in Python 3.

For loops and if statements, revisited >>> str = input() >>> words = 1 >>> for c in str: >>>if c == ‘ ’: >>>words = words + 1 What does this do?

Defining a function >>> def cube(n): >>>return n*n*n Remember, return tells Python what value the function should “evaluate” to. Similar to math functions defined in piecewise fashion: abs(x) = {x if x >= 0; -x if x < 0}

Functions take action! Calling a function says: “please do this” – E.g. input() asks for user input, waits for it to be entered, then returns it as the “value” of the function. Most functions are created to make doing a repetitive task easy.

Okay, so what should these “functions” do? Let’s take our example of a calculator program – Inputs two numbers and an operation, the outputs the result Break up this idea into smaller pieces – Which can be accomplished in, say, a couple statements

You tell me First, Then, we Last,

We can turn this into a high-level, function-based overview in = read_input() op = pick_operation(in) a,b = pick_numbers(in) result = carry_out_operation(a,b) print(result) Then we just have to implement each part – Correctly…

(Your calculator program, written collaboratively) >>>

What Python can already do for you We now (formally) introduce the data type Data Type: A combination of information and functionality given a specific name. Now, for a pile of examples

Standard Python Data Types str – a string list – a list of items tuple – a list of items (with a few more restrictions) – All entries have to be of the same type – Length cannot change set – a collection of unique elements dictionaries – a mapping between one set of data and another

Python Strings >>> s = “This is a string” Ask for the 5 th character >>> print(s[5]) Ask for the 3 rd through 8 th characters >>> print(s[3:8]) Ask for the 2 nd character from the end >>> print(s[-2])

Strings, continued Ask for all characters after (and including) the 4 th >>> print(s[4:]) All of these are called slices Can pull the same neat tricks with lists – Plus a few more

Lists >>> l = [4,8,15,16,23,42] Ask for the last three elements >>> print(l[-3:]) Remove the middle two elements >>> del l[2:-2] >>> print(l) Assign the first element to 108 >>> l[0] = 108

Strings and lists can work together >>> print(“Please enter a list of names (first and last).” >>> l = [] # empty list >>> s = “First Last” >>> while s: # so long as s is not empty >>>s = input(“Name please: ”) >>>l.append(s) >>> print(“You entered ” + len(l) + “ items.”)

One more interaction Remember that program from last time which counted the number of words in a sentence? >>> s = input(“Please enter a sentence.”) >>> l = s.split(“ ”) >>> print(len(l)) s.split(t) gives a list of all substrings of s broken around t “This is a sentence.”.split(“ ”) gives [“This” “is”, “a”, “sentence.”] “This is a sentence.”.split(“s”) gives [“Thi ”, “ i”, “ a ”, “entence.”]

Your turn, again! Write a program that asks for a bunch of s as input. – Output a list of all the domains (whatever is after Given a list of numbers, find the list of numbers that are relatively prime to the first one. Given a word, print its Scrabble score – aeioulnrst are 1 point, dg are 2, bcmp are 3, fhvwy are 4, k is 5, jx are 8, and qz are 10

On Files Repeating input is annoying. – You have to type everything every time We can put input into a file. >>> file = open(“README.txt”) >>> for line in file: >>>print(line) >>> file.close()

Files, continued What’s up with open() and close()? – Python has to ask the operating system for help. Details in CIS 240 (more details in CIS 380) or by demand Useful for processing information – Read in from a file, output to a file

Next Time Project Euler Classes? – Not the kind you’ll be attending in a few days Tuples Fancier variations on simple constructs