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Programming for
Engineers in
Python
Lecture 2: More Basics
Autumn 2011-12
1
Lecture 1: Highlights
• Functions
• Control structures (if/else)
• Strings
2
Today
• Leftovers from last week
• Compiler, interpreter
• Variables, types, operators, scope
3
Installing and Running Python 2.7
• Python 2.7 is already installed in the computers’
classroom
• Install:
• Windows 32 bit: http://www.python.org/ftp/python/2.7.2/python-2.7.2.msi
• A “Python 2.7” folder will be created in your Start Menu:
• IDLE (Python GUI) - recommended working environment
• Python (command line) - textual Python interpreter (a little faster,
less convenient)
• Working with IDLE video: http://www.youtube.com/watch?v=lBkcDFRA958
4
String Type
http://docs.python.org/tutorial/introduction.html#strings
5
Strings Structure
6
Slicing
H
e
l
l
o
0
1
2
3
4
-5
-4
-3
-2
-1
7
Lists
8
Iterating Over Lists
for var in list:
value in list
print(var)
while loop in tirgul
9
Lists Built In Methods
10
Range, Sum
11
Abstruse Goose, Computer Programming 101,
12
Abstruse Goose, Under The Hood
13
Machine Code (Language)
• Computers understand only machine language
• Basically looks like a sequence of 1’s and 0’s
• Very inconvenient to work with and non intuitive
• All other computer languages were created for
human convenience
• The computer does not understand C/C#/Java/Scala
• Must be “translated” into machine language
14
Programming Languages Basics
• A computer program is a sequence of instructions
(texts) that can be “understood" by a computer and
executed by it
• A programming language is a machine-readable
artificial language designed to express computations
that can be performed by a computer
15
Compiler
16
Hello World (in C) ;-)
17
Interpreter
18
Interpreter
• Python is an interpreted programming
language
• The Read/Evaluate/Print Loop
• Read an expression
• Compute its value
• Print the result
• Repeat the above
19
Language Selection and Python
• Goal
• Runtime vs. Development time
• Operating systems
• Platforms
• Python is appropriate for quick development of
small-medium tasks:
•
•
•
•
Experiments
Data analysis
Small projects
Short development-execution rounds
20
Memory
21
Memory (Cont.)
• The computer memory is composed of a long
list of bits
• Bits are grouped into bytes and words
• Every byte is numbered sequentially
• This number is called an address
22
‫‪Variables and Types‬‬
‫•‬
‫•‬
‫•‬
‫•‬
‫משתנה (‪ :)variable‬יחידת מידע המאכסנת ערך במהלך ריצת‬
‫התכנית‬
‫ניתן ל"גשת" למשתנה‪ :‬לשנות (לא תמיד) או לקבל את ערכו‬
‫ניתן לבצע פעולות חשבוניות (ואחרות) בעזרת משתנים‬
‫לכל משתנה מוגדר טיפוס (‪ )type‬שקובע את אופן ביצוע‬
‫הפעולות על המשתנה‬
‫‪23‬‬
Define Variables
variable assignment value
name
operator
Variable x “is of type”
Int and is assigned the
value 4
24
Why do We Need Variables?
• Computer programs manipulate data
• Data is given as input or calculated throughout
the program
• To be later accessed, variables must be
remembered
• Thus, variables are stored in the memory
• Variable name  memory address
25
Types
• Determine number of bytes in memory
(“locker size”)
• Determine the operations that are defined for a
given variable
26
Some Basic Types
27
Variables and Types
28
Why Do We Need Different Types?
•
•
•
•
Saving memory
Execution speed
Defined operations
Makes compiler “life” easier (not relevant in
Python)
29
Arithmetic Operators
30
Example 1
31
Types and Casting
32
Example 3
33
Comparisons
34
Example 1
35
Example 2
36
Logical Operators
Truth Tables (1 – True, 0 - False)
and
or
0
1
0
0
0
1
0
1
0
1
not
0
0
1
1
1
1
0
1
1
0
37
Scope
38
Next Week – A Big Jump!
A cape will not help – work hard!