Recitation 2

Programming for Engineers in Python

Plan

Range

For loop

While loop

Lists

Modules

Operations Arithmetic Operations:

+ plus

- minus

* multiply

/ divide (int / float)

% modulo (remainder)

** power

Comparison Operators (Return True or False):

== equal

!= not equal

> greater than

< less than

>= greater than or equal to

<= less than or equal to

Range Creates an ordered collection of all integers in a given range.

range(a, b) contains all integers k satisfying a ≤ k < b.

range(b) is a shorthand for range(0, b).

>>> range(0,10)

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> range(10)

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> range(-2,2)

[-2, -1, 0, 1]

>>> range(4,2)

[]

>>> type(range(3))

<type 'list„> # coming soon

Range – Cont.

range(a,b,d), returns the list: a, a+d, a+2d,…, a+id until

b is reached.

>>> range(0,10,2)

[0, 2, 4, 6, 8]

>>> range(10,0,-2)

[10, 8, 6, 4, 2]

>>> range(0, 10, -1)

[]

>>> range(0,10,0)

Traceback (most recent call last):

File "<pyshell#21>", line 1, in <module>

range(0,10,0)

ValueError: range() step argument must not be zero

For Loop for elem in list (TBD):

statement1

statement2

…

Compute 1 + 2 + … + 100:

partialSum = 0

for i in range(1,101):

partialSum = partialSum + i

print partialSum

5050

determines the scope of the iteration.

“Cheat”:

sum(range(1,101))

For Loop – Cont.

Iterate over strings:

name = “Assaf”

for letter in name.upper():

print 'Give me', letter

print 'What did we get?', name

Give me A

Give me S

Give me S

Give me A

Give me F

What did we get? Assaf

While Loop # Find the smallest divisor

n = 9

div=2

while n % div != 0:

div = div+1

print "Smallest divisor of“,n,"is",div

is the while loop above infinite?

While Loop – Cont. # Find prime

if n < 2:

print “Out of range"

If div != n:

print div," is a divisor of “,n

else:

print n," is prime"

While Loop

while [expression]:

statement1

statement2

…

Infinite loop:

i = 1

while i < 4:

print i

expr

statement(s)

true

false

# Find smallest divisor using For:

for div in range(2, n+1):

if n % div == 0:

break

print div

For / While Equivalence

Break - Aborting Iteration

break terminates the nearest enclosing loop, skipping the code that follows the break inside the loop.

Useful for getting out of loops when some predefined condition occurs.

>>> for i in range(1,10):

print(i)

if i % 3 == 0:

break

1

2

3

Lists

A list is an ordered sequence of elements.

The simplest way to create a list in Python:

Enclose its elements in square brackets.

>>> my_list = [2,3,5,7,11]

>>> my_list

[2,3,5,7,11]

Lists are Indexable Remember this?

The same indexing + slicing works for lists!

o

l

l e

H

4

3

2

1

0

-1 -2 -3 -4 -5

Slicing

>>> my_list = [1,2,3,4,5,6,7,8,9,10]

>>> my_list[1:5] # slicing

[2,3,4,5]

>>> my_list[0:-1:2] # slicing an arithmetic progression

[1,3,5,7,9]

>>> my_list[::2] # shorthand for previous slicing

[1,3,5,7,9]

Slicing – Cont. >>> my_list[::-1] # going backwards

[10,9,8,7,6,5,4,3,2,1]

>>> my_list[3:8:-2]

[ ] # output is an empty list. This is NOT an error

>>> my_list

[1,2,3,4,5,6,7,8,9,10] # slicing does NOT change

original list

Lists are Indexable – Cont. 0 1 2 3 4

>>> my_list = [2, 3, 5, 7, 11]

>>> my_list[0]

2

>>> my_list[4]

11

>>> my_list[5]

Traceback (most recent call last):

File "<pyshell#7>", line 1, in <module>

my list[5]

IndexError: list index out of range

>>> my_list[2:4]

[5, 7]

Lists – Cont. Iterate:

>>> sum_list = 0

>>> for k in my_list:

sum_list += k # shortcut for sum_list = sum_list + k

>>> sum_list

28

Create lists:

range(…) creates a list

>>> new_list = [i**2 for i in my_list]

>>> new_list

[4, 9, 25, 49, 121]

Length of Lists

len returns the length (number of elements) of a list.

>>> len(my_list)

5

>>> len([])

0

Quite often, the length is a useful quantity to have.

Lists, Strings, and Beyond

Lists can contain strings:

days = [“Sunday”, “Monday", "Tuesday”, “Wednesday”,

“Thursday”, “Friday”, “Saturday”]

>>> s = “”.join(days)

'SundayMondayTuesdayWednesdayThursdayFridaySaturday„

>>> s.lower()

'sundaymondaytuesdaywednesdaythursdayfridaysaturday„

>>> s.find('y')

5

>>> first = s[:s.find('y')+1] # what does first contain?

>>> first = first[::-1] # and now?

Nested Lists >>> mat = [ [1, 2, 3], [4, 5, 6], [7, 8, 9] ]

>>> mat[1]

[4, 5, 6]

>>> for row in mat: # swap rows and columns

for col in row:

print col, # , - continue the same line

print

1 2 3

4 5 6

7 8 9

What is len(mat) ?

Lists – Dynamic Mixing types: assume you want to maintain a list of the

students in this course either by name or by id:

students = ]„Itay‟,9255587, „Alon‟, „Zohar‟,744554887]

print students[2]

>>> „Alon‟

Michal decided to join the course, so we update the list:

# append - add an element to the end of the list

>>> students.append('Michal')

>>> students

['Itay', 9255587, 'Alon', 'Zohar', 744554887, 'Michal']

Lists – Dynamic But Alon wants lo leave the course:

>>> students.remove('Alon')

>>> students

['Itay', 9255587, 'Zohar', 744554887, 'Michal']

remove removes only the first occurrence of a value.

Functions def function_name(argument1, argument2,...):

statement1

statement2

…

return result1, result2, … # optional

Calling func:

[var1, var2,…[ = function_name(val1, val2,...)

A function is:

A named code scope

Can receive input parameters

Can return output parameters

Palindromes

Palindromes are read identically from right and left.

Examples:

ילד בדלי

A Man, A Plan, A Canal, Panama

Never odd or even

…

Pseudo-code: Translate from definition to code

- For every index in the string:

- Check if word[i] == word[len – i – 1] # why -1?

- False: this is not a palindrome

- If all indices “survived” – this is a palindrome

ו נ ש ר פ

נ ת ב ע ר

ש ב ד ב ש

ר ע ב ת נ

ף ר ש נ ו

Palindromes – Code

def is_palindrome(word):

middle = len(word)/2

for i in range(middle):

if word[i] != word[-i-1]: # no palindrome

return False

return True # matches all the way

Is there a shorter code for identifying

palindromes?

Functions – Scope Consider the following function, operating on two

arguments:

def linear_combination(x,y):

y=2*y

return (x+y)

The formal parameters x and y are local, and their “life

time" is just the execution of the function. They

disappear when the function is returned.

linear_combination

x, y

3, 4 11

Functions – Scope >>> a=3

>>> b=4

>>> linear combination(a,b)

11 # this is the correct value

>>> a

3

>>> b

4 # b has NOT changed

The change in y is local - inside the body of the

function.

Functions – Scope >>> x=3

>>> y=4

>>> linear_combination(x,y)

11 # this is the correct value

>>> x

3

>>> y

4

The y in the calling environment and the y in the

function are not the same variable!

Exercise – Unit Testing

My Code

Test

(1) Run my code

with some input (2) Check the output

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'

Module

Python Code Hierarchy

Statement

function

Package

Modules

All modules and their contents (functions, constants) can be found at

http://docs.python.org/modindex.html

>>> import math # mathematical functions

>>> dir(math) ['__doc__', '__name__', '__package__', 'acos', 'acosh', 'asin', 'asinh', 'atan',

'atan2', 'atanh', 'ceil', 'copysign', 'cos', 'cosh', 'degrees', 'e', 'erf', 'erfc', 'exp',

'expm1', 'fabs', 'factorial', 'floor', 'fmod', 'frexp', 'fsum', 'gamma', 'hypot',

'isinf', 'isnan', 'ldexp', 'lgamma', 'log', 'log10', 'log1p', 'modf', 'pi', 'pow',

'radians', 'sin', 'sinh', 'sqrt', 'tan', 'tanh', 'trunc']

>>> math.pi # a constant

3.141592653589793

>>> math.sqrt(16) # a function

4.0 >>> math.log(8, 2)

3.0

Modules – Cont. >>> import os #operating system interfaces

>>> os.rename(„my_file.txt‟, „your_file.txt‟)

>>> os.mkdir(„new_dir‟)

>>> for root, dirs, files in os.walk('lib/email'):

print root, "consumes",

print sum(getsize(join(root, name)) for name in files),

print "bytes in", len(files), "non-directory files"

>>> os.times()[0] # user time

10.1244649