Chapter 1  Why should you learn to write programs?

python words.py
Enter file:words.txt
to 16

1.1  Creativity and motivation

1.2  Computer hardware architecture

• The Central Processing Unit (or CPU) is that part of the computer that is built to be obsessed with "what is next?". If your computer is rated at 3.0 Gigahertz, it means that the CPU will ask "What next?" three billion times per second. You are going to have to learn how to talk fast to keep up with the CPU.
  
  
• The Main Memory is used to store information that the CPU needs in a hurry. The main memory is nearly as fast as the CPU. But the information stored in the main memory vanishes when the computer is turned off.
  
  
• The Secondary Memory is also used to store information, but it is much slower than the main memory. The advantage of the secondary memory is that it can store information even when there is no power to the computer. Examples of secondary memory are disk drives or flash memory (typically found in USB sticks and portable music players).
  
  
• The Input and Output Devices are simply our screen, keyboard, mouse, microphone, speaker, touchpad, etc. They are all of the ways we interact with the computer.
  
  
• These days, most computers also have a Network Connection to retrieve information over a network. We can think of the network as a very slow place to store and retrieve data that might not always be "up". So in a sense, the network is a slower and at times unreliable form of Secondary Memory

1.3  Understanding programming

• First you need to know the programming language (Python) - you need to know the vocabulary and the grammar. You need to be able spell the words in this new language properly and how to construct well-formed "sentences" in this new languages.
  
  
• Second you need to "tell a story". In writing a story, you combine words and sentences to convey an idea to the reader. There is a skill and art in constructing the story and skill in story writing is improved by doing some writing and getting some feedback. In programming, our program is the "story" and the problem you are trying to solve is the "idea".

1.4  Words and sentences

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   ï¬ nally   in   print 

print 'Hello world!'

1.5  Conversing with Python

Python 2.6.1 (r261:67515, Jun 24 2010, 21:47:49) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 

>>> I come in peace, please take me to your leader
  File "<stdin>", line 1
    I come in peace, please take me to your leader
         ^
SyntaxError: invalid syntax
>>> 

>>> print 'Hello world!'
Hello world!

>>> print 'You must be the legendary god that comes from the sky'
You must be the legendary god that comes from the sky
>>> print 'We have been waiting for you for a long time'
We have been waiting for you for a long time
>>> print 'Our legend says you will be very tasty with mustard'
Our legend says you will be very tasty with mustard
>>> print 'We will have a feast tonight unless you say
  File "<stdin>", line 1
    print 'We will have a feast tonight unless you say
                                                     ^
SyntaxError: EOL while scanning string literal
>>> 

>>> good-bye
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'good' is not defined

>>> if you don't mind, I need to leave
  File "<stdin>", line 1
    if you don't mind, I need to leave
             ^
SyntaxError: invalid syntax

>>> quit()

1.6  Terminology: interpreter and compiler

01010001110100100101010000001111
11100110000011101010010101101101
...

>>> x = 6
>>> print x
6
>>> y = x * 7
>>> print y
42
>>> 

^?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
....

C:\Python27\python.exe

1.7  Writing a program

csev$ cat hello.py
print 'Hello world!'
csev$ python hello.py
Hello world!
csev$

1.8  What is a program?

the clown ran after the car and the car ran into the tent 
and the tent fell down on the clown and the car 

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 is None or count > bigcount:
        bigword = word
        bigcount = count

print bigword, bigcount

1.9  The building blocks of programs

input:

Get data from the the "outside world". This might be reading data from a file, or even some kind of sensor like a microphone or GPS. In our initial programs, our input will come from the user typing data on the keyboard.

output:

Display the results of the program on a screen or store them in a file or perhaps write them to a device like a speaker to play music or speak text.

sequential execution:

Perform statements one after another in the order they are encountered in the script.

conditional execution:

Check for certain conditions and execute or skip a sequence of statements.

repeated execution:

Perform some set of statements repeatedly, usually with some variation.

reuse:

Write a set of instructions once and give them a name and then reuse those instructions as needed throughout your program.

1.10  What could possibly go wrong?

>>> primt 'Hello world!'
  File "<stdin>", line 1
    primt 'Hello world!'
                       ^
SyntaxError: invalid syntax
>>> primt 'Hello world'
  File "<stdin>", line 1
    primt 'Hello world'
                      ^
SyntaxError: invalid syntax
>>> I hate you Python!
  File "<stdin>", line 1
    I hate you Python!
         ^
SyntaxError: invalid syntax
>>> if you come out of there, I would teach you a lesson
  File "<stdin>", line 1
    if you come out of there, I would teach you a lesson
              ^
SyntaxError: invalid syntax
>>> 

Syntax errors:

These are the first errors you will make and the easiest to fix. A syntax error means that you have violated the "grammar" rules of Python. Python does its best to point right at the line and character where it noticed it was confused. The only tricky bit of syntax errors is that sometimes the mistake that needs fixing is actually earlier in the program than where Python noticed it was confused. So the line and character that Python indicates in a syntax error may just be a starting point for your investigation.

Logic errors:

A logic error is when your program has good syntax but there is a mistake in the order of the statements or perhaps a mistake in how the statements relate to one another. A good example of a logic error might be, "take a drink from your water bottle, put it in your backpack, walk to the library, and then put the top back on the bottle."

Semantic errors:

A semantic error is when your description of the steps to take is syntactically perfect and in the right order, but there is simply a mistake in the program. The program is perfectly correct but it does not do what you intended for it to do. A simple example would be if you were giving a person directions to a restaurant and said, "... when you reach the intersection with the gas station, turn left and go one mile and the restaurant is a red building on your left.". Your friend is very late and calls you to tell you that they are on a farm and walking around behind a barn, with no sign of a restaurant. The you say "did you turn left or right gas station?" and they say, "I followed your directions perfectly, I have them written down, it says turn left and go one mile at the gas station.". Then you say, "I am very sorry, because while my instructions were syntactically correct, they sadly contained a small but undetected semantic error.".

1.11  The learning journey

1.12  Glossary

bug:

An error in a program.

central processing unit:

The heart of any computer. It is what runs the software that we write; also called "CPU" or "the processor".

compile:

To translate a program written in a high-level language into a low-level language all at once, in preparation for later execution.

high-level language:

A programming language like Python that is designed to be easy for humans to read and write.

interactive mode:

A way of using the Python interpreter by typing commands and expressions at the prompt.

interpret:

To execute a program in a high-level language by translating it one line at a time.

low-level language:

A programming language that is designed to be easy for a computer to execute; also called "machine code" or "assembly language."

machine code:

The lowest level language for software which is the language that is directly executed by the central processing unit (CPU).

main memory:

Stores programs and data. Main memory loses its information when the power is turned off.

parse:

To examine a program and analyze the syntactic structure.

portability:

A property of a program that can run on more than one kind of computer.

print statement:

An instruction that causes the Python interpreter to display a value on the screen.

problem solving:

The process of formulating a problem, finding a solution, and expressing the solution.

program:

A set of instructions that specifies a computation.

prompt:

When a program displays a message and pauses for the user to type some input to the program.

secondary memory:

Stores programs and data and retains its information even when the power is turned off. Generally slower than main memory. Examples of secondary memory include disk drives and flash memory in USB sticks.

semantics:

The meaning of a program.

semantic error:

An error in a program that makes it do something other than what the programmer intended.

source code:

A program in a high-level language.

1.13  Exercises

>>> primt 'Hello world!'
  File "<stdin>", line 1
    primt 'Hello world!'
                       ^
SyntaxError: invalid syntax
>>> 

x = 123

x = 43
x = x + 1
print x

1

http://xkcd.com/231/