Download Programming for Engineers in Python

Programming for Engineers in Python
Recitation 1
Plan
 Administration:
 Course site
 Homework submission guidelines
 Working environment
 Python:
 Variables
 Editor vs. shell
 Homework 0
 Python – Cont.
 Conditional Statements (if/else)
 Discuss Homework 1
Administration
Yoav Ram
 Email: yoavram@post.tau.ac.il
 Office hours: by appointment only
 Location: Room 409, Britannia building
 Noga Levy
 Email: noga@post.tau.ac.il
 Office hours: by appointment only
 Location: Room 405a, Shenkar building
Course Site - Moodle
 http://moodle.tau.ac.il
 All relevant material: Slides for the lectures +




recitations, homework, solutions, code examples.
Automatic homework submission, (manual) grades.
Forum – anything you want to clarify from the lecture,
recitations and homework.
Announcements
Instructions (how to set a working environment at
home)
Homework
 Very important when learning to program!
Therefore:
 Weekly “hands-on” assignments.
 Strict submission dates.
 15-20% of the final grade.
Note that:
 Grades: 0 / 60 / 80 / 100.
 No appeals.
Where can I work?
 Computer lab 06, open: 8:00 – 20:00,
use email/disk-on-key
Submission Guidelines
 Submission in singles!
 Should work on Python 2.7
 No cheating!
 Guidelines in the course site.
How to handle “doesn’t work” situation:
 Go over the code
 Consult course slides
 Google (a useful and legitimate source)
 Check the forum in moodle
 Submit a question to forum (reply within 48 hours)
Working Environment
Install (at home):
 Windows 32 bit:
http://www.python.org/ftp/python/2.7.2/python2.7.2.msi
 Continue as in class
Open:
Start Menu 
All Programs 
Python 2.7 
IDLE (Python GUI)
IDLE Editor
We want to save a sequence of commands and run it
later in a new Python session.
The Editor:
- Write Python commands
- Execute them in one key-press.
Open the editor from the Shell:
File 
New Window
IDLE Editor – Cont.
The new window is Untitled. First – choose a
title:
In the new window:
File  Save as…
Chose a folder and a name. The name must
end with ‘.py’
IDLE Editor – Cont.
Run Python:
The output appears in the Shell window (a new Shell
might open)
What are Variables ?
A location in the computer’s memory. A variable
- has a name (for access)
- holds a value
- has type – according to its value
- This is how data is handled
Numbers and their Types
>>> 4
4
>>> type(4)
<class 'int'> # integers type
>>> 3.14159
3.14159
>>> type(3.14159)
<class 'float'> # floating point ("reals") type
Arithmetic operations: +, -, *, / , % (modulo), ** (power)
What type is 8/5 ? And 8/5.0 ?
Let’s check…
Variables and Assignments
>>> n = 10
>>> m=(10+4)*5
The left hand side is a variable. The right hand side is
an expression.
The interpreter evaluates the expression and assigns its
value to the variable.
The variable's name is a sequence of letters and digits,
starting with a letter.
n
m
10
70
Variables and Assignments:
An Example
Changing the value of a variable:
>>> n=11
>>> n
11
Changing the type of a variable:
>>> n=1.3141
>>> n
1.3141
Variables can be used in expressions:
>>> pi = 3.14159
>>> pi*2 + 1
7.28318
Variables and Assignments – Cont.
Referring to undefined variables leads to runtime error
>>> check_this
Traceback (most recent call last):
File "<pyshell#16>", line 1, in <module>
check_this
NameError: name 'check_this' is not defined
Documentation and Variable Names
Real computer programs include thousands of code lines,
lots of variables.
Readability is very important for code maintenance,
it is a requirement in this course!
Hence:
 Choose informative variable names:
 Not informative: i, j, m, n, aString, doesSomething …
 Informative: sumOfExpenses, studentName, countWords …
 Documentation – add remarks (‘#’) before:
 Functions
 ‘Logical units’ of code
 complex implementation
Strings
 We already met Strings in class.
 Strings are text sequences. They are actually an
ordered list of characters
Strings – Cont.
>>> mssg1="Let there"
>>> mssg2=" be light"
>>> mssg1+mssg2
'Let there be light‘
What will the next expression print?
>>> mssg1 + mssg2*2
Strings Access - Reminder
>>> a=‘Hello’
>>> a[1:3]
'el'
>>> a[1:]
'ello'
>>> a[-4:-2]
'el'
>>> a[:-3]
'He'
>>> a[-3:]
'llo’
H
e
l
l
o
0
1
2
3
4
-5
-4
-3
-2
-1
Homework 0
Let the work begin:
 Download Homework 0 from the course website.
 Follow instructions
 Submit the file [your_name]_a0_q1.py through the
course website.
Boolean Variables
Comparison :
 Numbers – by their order
 Strings – by lexicographical order
Returns boolean variabels: True or False.
Comparison types:
 =
 !=
 >
 <
 >=
 <=
Python Comparison Operators
>>> 5.0 == 5
True
>>> 6 != 2*3
False
>> not(-2 >= 1)
True
>>> (-2 >= 1) or (-2 <= -1)
True
>>> (-2 >= 1) and (-2 <= -1)
False
Variables - Status
 We saw the classes ‘int’, 'float', 'str‘, ‘bool’.
 Some operations allow “mixing" variables of




different types.
Assignments: variable name = expression
Subsequent assignments to the same variable
can change its value and even its type.
‘int’ – integer numbers, ‘float’ – real numbers.
True and False are Boolean constants
Conditional Statements
if <condition>:
do something
[else:
do something else]
Conditional Statements - Examples
>>> if 54% 18 == 0: # the remainder of 54 divided by 18
print “54 is divisible by 18"
else:
print “54 is not divisible by 18"
54 is not divisible by 18
Indentation:
 Following the if statement:
Open a new scope = one tab to the right.
 Indicates the commands within the scope of this if.
 else - outside that scope.
Note: tab != four spaces, even if it looks identical!
Functions - Reminder
def function_name(input1, input2, …):
command1
command2
…
Exercise
Donuts:
Input: count - an int representing the number of donuts
Output: a string of the form 'Number of donuts: <count>’.
However, if the count is 10 or more, use the word 'many‘
instead of the actual count.
Examples:
>>> donuts(5)
Number of donuts: 5
>>> donuts(23)
Number of donuts: many
Function prototype:
def donuts(count):
# +++your code here+++
Solution
def donuts(count):
if count < 10:
return 'Number of donuts:‘,str(count)
else:
return 'Number of donuts: many‘
Return instead of print – explanation in the next slide.
Exercise – Unit Testing
Test
(1) Run my code
with some input
(2) Check the output
My Code
Exercise – Unit Testing
Use the given implementation:
- Download hw1.py
- Fill in the necessary code instead of the remarks
# +++your code here+++
- Instead of printing the code to the shell, return it to the testing function.
- “Run Module”
Example’s output:
OK got: 'Number of donuts: 4' expected: 'Number of donuts: 4'
OK got: 'Number of donuts: 9' expected: 'Number of donuts: 9'
OK got: 'Number of donuts: many' expected: 'Number of donuts: many'
OK got: 'Number of donuts: many' expected: 'Number of donuts: many'