Why Program? Computers want to be helpful... Programmers Anticipate Needs. Chapter 1. Computers are built for one purpose - to do things for us

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1 Why Program? Chapter 1 Unless otherwise noted, the content of this course material is licensed under a Creative Commons Attribution 3.0 License. Copyright 2010, Charles Severance Python for Informatics: Exploring Information Computers want to be helpful... Programmers Anticipate Needs Computers are built for one purpose - to do things for us But we need to speak their language to describe what we want done Users have it easy - someone already put many different programs (instructions) into the computer and users just pick the ones we want to use iphone Applications are a market iphone Applications just reached 3 Billion downloads Programmers have left their jobs to be full-time iphone developers Programmers know the ways of the program Pick Me! Pick Me! Pick Me! Pick Me! Pick Me! Pay Me!

2 Users.vs. Programmers Users see computers as a set of tools - word processor, spreadsheet, map, todo list, etc. Programmers learn the compter ways and the computer language Programmers have some tools that allow them to build new tools Programmers sometimes write tools for lots of users and sometimes programmers write little helpers for themselves to automate a task Data User Computer Hardware + Software Information... Networks From a software creator s point of view, we build the software. The end users (stakeholders/actors) are our masters - who we want to please - often they pay us money when they are pleased. But the data, information, and networks are our problem to solve on their behalf. The hardware and software are our friends and allies in this quest. Programmer Why be a programer? To get some task done - we are the user and programmer Clean up survey data Fix a performance problem in the Sakai software Add guestbook to a web site To produce something for others to use - a programming job is Code? Software? A Program? A set of stored instructions It is a little piece of our intelligence in the computer It is a little piece of our intelligence we can give to others - we figure something out and then we encode it and then give it to someone else to save them the time and energy of figuring it out A piece of creative art - particularly when we do a good job on user experience

3 Writing programs (or programming) is a very creative and rewarding activity. You can write programs for many reasons ranging from making your living to solving a difficult data analysis problem to having fun to helping someone else solve a problem. This book assumes that everyone needs to know how to program and that once you know how to program, you will figure out what you want to do with your newfound skills. We are surrounded in our daily lives with computers ranging from laptops to cell phones. We can think of these computers as our ``personal assistants'' who can take care of many things on our behalf. The hardware in our current-day computers is essentially built to continuously ask us the question, `` would you like me to do next?''. Our computers are fast and have vasts amounts of memory and could be very helpful to us if we only knew the language to speak to explain to the computer what we would like it to ``do next''. If we knew this language we could tell the computer to do tasks on our behalf that were reptitive. Interestingly, the kinds of things computers can do best are often the kinds of things that we humans find boring and mind-numbing. name = raw_input("enter file:") handle = open(name, "r") text = handle.read() words = text.split() counts = dict() for word in words: counts[word] = counts.get(word,0) + 1 bigcount = None bigword = None for word,count in counts.items(): if bigcount == None or count > bigcount: bigword = word bigcount = count print bigword, bigcount python words.py Enter file: words.txt to 16 Software Generic Computer Hardware Architecture Input and Output Devices Central Processing Unit Secondary Memory Main Memory

4 Definitions Software Generic Computer Central Processing Unit: Runs the Program - The CPU is always wondering what to do next? Not the brains exactly - very dumb but very very fast Input Devices: Keyboard, Mouse, Touch Screen Output Devices: Screen, Speakers, Printer, DVD Burner Main Memory: Fast small temporary storage - lost on reboot - aka RAM Secondary Memory: Slower large permanent storage - lasts until deleted - disk drive / memory stick Input and Output Devices Central Processing Unit Main Memory if x< 3: print Secondary Memory Software Terms Input and Output Devices Central Processing Unit Secondary Memory Source code - the programs we humans write - and read - written in a programming language - source code is generally portable across systems Main Memory Machine Language Machine code - what really runs on the machine - not very readible - produced by a compiler - machine code is unique to hardware and operating system - Windows machine code is different from Macintosh machine code.

5 Programmer Tools Programmer Tools Since the Central Processing Unit must ran Machine Code, we must translate our source code to machine code Compiler - Takes our source code and makes a machine code version of our program Interpreter - Reads our source code and runs it directly - Python is an interpreted language - Python is an interpreter Input Data Input Data A programmer develops a program. If a compiler is used the compiler translated the source to machine code for distribution. If an interpreter is used, the programmer simply distributes the source code. Compiler.vs. Interpreter Only the programmer needs to have the compiler - once the compiler is done - the machine code program is self-contained The programmer keeps the source code and distributes the machine code - different machine code is needed for Mac, PC, etc. Both the programmer and user need to have the interpreter installed on their system Generally the programmer distributes the source code of the program

6 main() { printf("hello world\n"); } ^@^@^@^@ nl_symbol_ ptr^@ DATA^@^@^@^@^@^@^@^@^@^@ ^@^@0t^@^@^@^P^@^@ t^@^@^@^b^@^@^@^@^@^@^@^@^@^@^@ ^F^@^@^@^Q^@^@^@^@ la_symbol_p tr^@ lib/ libmx.a.dylib^@^@^@^@^@^l^@^@^@ 4^@^@^@^XC½m ^@X^A^C^@^A^@^@/ usr/lib/libsy Hello world Your Source Code in the C Language The C Language Compiler Reads your source code and produces your machine code. Your Machine Code ^?ELF^A^A^A^@^@^@^@^@^@^@^@^@^B^@^C^@^A^@^@^@\xa0\x82^D^H4^@^@^@\x90^]^@^@^@^@^@ ^@4^@ ^@^G^@(^@$^@!^@^F^@^@^@4^@^@^@4\x80^D^H4\x80^D^H\xe0^@^@^@\xe0^@^@^@^E^@^@ ^@^D^@^@^@^C^@^@^@^T^A^@^@^T\x81^D^H^T\x81^D^H^S^@^@^@^S^@^@^@^D^@^@^@^A^@^@^@^A\ ^D^HQVhT\x83^D^H\xe8\xb7\xff\xff\xff\xf4\x90\x90U\x89\xe5S\xe8^@^@^@^@[\x81\xc3_ ^R^@^@P\x8b\x83\xfc\xff\xff\xff\x85\xc0t^B\xff\xd0\x8b]\xfc\xc9\xc3\x90\x90\x90\ x90\x90\x90\x90\x90\x90\x90u\x89\xe5\x83\xec^h\x80=l\x95^d^h^@t^l\xeb^\\x83\xc0^ D\xa3H\x95^D^H\xff\xd2\xa1H\x95^D^H\x8b^P\x85\xd2u\xeb\xc6^EL\x95^D^H^A\xc9\xc3\ x90u\x89\xe5\x83\xec^h\xa1\\x94^d^h\x85\xc0t!\xb8^@^@^@^@\x85\xc0t^x\xc7^d$\\x94 ^D^H\xe8\xbc \xfb\xf7\x8d\xb6^@^@^@^@\x8d\xbf^@^@^@^@\xc9\xc3\x90\x90u\x89\xe5\x print hello world Hello world è^@^@^qp^@^@^@^b^@^@^@^@^@^@^@^@<80>^@^d^@^@^@^@^@^@^@^@^@ picsy mbol_stub TEXT^@^@^@^@^@^@^@^@^@^@^@^@,X^@^@^@^@^@^@^ \X^@^@^@^B^@^@^@^@^@^@^@^@<80>^@^@^H^@^@^@^@^@^@^@ $ symbol_stub^@^@^@ TEXT^@^@^@^@^@^@^@^@^@^@^@^@,X^@^@^@^@^@^@^ \X^@^@^@^B^@^@^@^@^@^@^@^@<80>^@^@^H^@^@^@^@^@^@^@^T picsymbolst ub1 TEXT^@^@^@^@^@^@^@^@^@^@^@^@,`^@^@^B ^@^@^ \`^@^@^@^E^@^@^@^@^@^@^@^@<80>^@^D^H^@^@^@^@^@^@^@ cstring^@^@^@^@^@^@^@ TEXT^@^@^@^@^@^@^@^@^@^@^@^@.<80>^@^@^AX Machine ^@^@^^<80>^@^@^@^B^@^@^@^@^@^@^@^@^@^@^@^B^@^@^@^@^@^@^@^@^@^@^@^ A^@^@^AÐ DATA^@^@^@^@^@^@^@^@^@^@^@^@0^@^@^@^P^@^@^@ Code ^@^@^@^P^@^@^@^@^G^@^@^@^C^@^@^@^F^@^@^@^@ DATA^@^@^@^@^@^@^@^@^@^@^@^@0^@^@^@^@q^@^@ for the ^@^@^@^@^B^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@ nl_symbol_ptr ^@ DATA^@^@^@^@^@^@^@^@^@^@^@^@0t^@^@^@^P^@^@ Python t^@^@^@^b^@^@^@^@^@^@^@^@^@^@^@^f^@^@^@^q^@^@^@^@ DATA^@^@^@^@^@^@^@^@^@^@^@^@0<84>^@^@^@D^@^@ Interpreter <84>^@^@^@^B^@^@^@^@^@^@^@^@^@^@^@^G^@^@^@^U^@^@^@^@ DATA^@^@^@^@^@^@^@^@^@^@^@^@0È^@^@^@^\^@^@ È^@^@^@^B^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@ DATA^@^@^@^@^@^@^@^@^@^@^@^@0ä^@^@^@^D^@^@^@^@^@^@^@ ^B^@^@^@^@^@^@^@^@^@^@^@^A^@^@^@^@^@^@^@^@ Python is an Interpreter To run Python programs, users must install Python on their computers Development is quick and easy - we simply make a change to our program and run it again in a single step For data analysis - Python is just a tool that you keep on your computer Interpreters are more convenient when the user and programmer are the same person

7 Python Interactive Running Python Interactively Since Python is interpreted we can just type programs directly into Python See Also x=1 print x x = x + 1 >>> x = 1 >>> print x 1 >>> x = x + 1 >>> next? csev$ python Python 2.5 (r25:51918, Sep , 08:49:13) [GCC (Apple Computer, Inc. build 5341)] on darwin Type "help", "copyright", "credits" or "license" for more information. >>> x = 1 >>> print x 1 >>> x = x + 1 >>> print x 2 >>> exit() This is a good test to make sure that you have Python correctly installed. Python Scripts Interactive Python is good for experiments and programs of 3-4 lines long But most programs are much longer so we type them into a file and tell python to run the commands in the file. In a sense we are giving Python a script As convention, we add.py as the suffix on the end of these files to indicate they contain Python

8 Python as a Language Parseltongue is the language of serpents and those who can converse with them. An individual who can speak Parseltongue is known as a Parselmouth. It is a very uncommon skill, and may be hereditary. Nearly all known Parselmouths are descended from Salazar Slytherin. Python is the language of the Python Interpreter and those who can converse with it. An individual who can speak Python is known as a Pythonista. It is a very uncommon skill, and may be hereditary. Nearly all known Pythonistas use software inititially developed by Guido van Rossum. Early Learner: Syntax Errors We need to learn the Python language so we can communicate our instructions to Python. In the beginning we will make lots of mistakes and speak gibberish like small children. When you make a mistake, the computer does not think you are cute. It says syntax error - given that it *knows* the language and you are just learning it. It seems like Python is cruel and unfeeling. You must remember that you are intelligent and *can* learn - the computer is simple and very fast - but cannot learn - so it is easier for you to learn Python than for the computer to learn English...

9 Elements of Python Vocabulary / Words - Variables and Reserved words (Chapter 2) Sentance structure - valid syntax patterns (Chapters 3-5) Story structure - constructing a program for a a purpose name = raw_input("enter file:") handle = open(name, "r") text = handle.read() words = text.split() counts = dict() for word in words: counts[word] = counts.get(word,0) + 1 bigcount = None bigword = None for word,count in counts.items(): if bigcount == None or count > bigcount: bigword = word bigcount = count print bigword, bigcount A short Story about how to count words in a file in Python. python words.py Enter file: words.txt to 16 Reserved Words You can not use reserved words as variable names / identifiers and del for is raise assert elif from lambda return break else global not try class except if or while continue exec import pass yield def finally in print Chapter 2 Sentances or Lines x = 2 Assignment Statement x = x + 2 Assignment with expression print x Print statement Variable Operator Constant Reserved Word

10 Program Steps or Program Flow Programming Paragraphs Like a recipe or installation instructions, a program is a sequence of steps to be done in order Some steps are conditional - they may be skipped Sometimes a step or group of steps are to be repeated Sometimes we store a set of steps to be used over and over as needed several places throughout the program Sequential Steps x = 5 Conditional Steps x = 1 print x x = x + 1 print x Program: x = 2 print x x = x + 2 print x Output: 2 4 X < 10? X > 20? Yes print Smaller Yes print Bigger Program: x = 5 if x < 10: print "Smaller" if x > 20: print "Bigger" Output: Smaller Finis When a program is running, it flows from one step to the next. We as programmers set up paths for the program to follow. Chapter 2 print Finis print Finis Chapter 3

11 Done i = print Bye print i Repeated Steps Program: for i in range(5) : print i print Bye Output: Bye Loops (repeated steps) have iteration variables that change each time through a loop. Often these iteration variables go through a sequence of numbers. Chapter 5 def hello() print Zip hello() Stored (and reused) Steps hello(): print Hello print Fun Program: def hello(): print "Hello" print "Fun" hello() print Zip hello() We call these reusable pieces of code functions. Output: Hello Fun Zip Hello Fun Chapter 4 name = raw_input("enter file:") handle = open(name, "r") text = handle.read() words = text.split() counts = dict() for word in words: counts[word] = counts.get(word,0) + 1 bigcount = None bigword = None for word,count in counts.items(): if bigcount == None or count > bigcount: bigword = word bigcount = count print bigword, bigcount Sequential Repeated Conditional An Animated Short Story... Finding the largest number in a list of numbers...

12 is the Largest Number? is the Largest Number? is the Largest Number? is the Largest Number? largest_so_far

13 name = raw_input("enter file:") handle = open(name, "r") text = handle.read() words = text.split() counts = dict() for word in words: counts[word] = counts.get(word,0) + 1 bigcount = None bigword = None for word,count in counts.items(): if bigcount == None or count > bigcount: bigword = word bigcount = count print bigword, bigcount A short Story about how to count words in a file in Python. A word used to read data from a user. A sentance about updating one of many counts. A paragraph about how to find the largest item in a list. Tao of Programming... In the first phase, you hate the computer and you hate Python. It seems that no matter what you do, Python always tells you Syntax error or Trace Back. In the second phase, you realize that when Python is telling you Syntax error or Traceback, it means that Python has become confused and does not know what to do next. Python is kind of like a puppy. In the third phase, you realize that Python is actually a great help to you and that Python represents the collective intelligence of hundreds of programmers who have given you some powerful and brilliant tools and capabilities. When you face a problem you are not alone. Summary This is a quick overview of Chapter 1 Focus on the big picture We will revisit these concepts throughout the course