Programming for Engineers

in Python

Sawa 2015

Lecture 2: Lists and Loops

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Plan for today• While loop• Lists• For loop

While Loop

Used to repeat the same instructions until a stop criterion is met

while expression:statement1statement2…

expression

true

false

statement(s)

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Example - factorial

#factorialn = 7

fact = 1i = 1while i <= n:

fact = fact * ii = i + 1

print n, "! = ", fact

Example – smallest divisor

# Find the smallest divisor

n = 2015div = 2while n % div != 0: div = div + 1print "Smallest divisor of", n, "is", div

Can the while loop above be infinite?

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Infinite Loops

i = 1while i < 4:print i

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Plan for today• Highlights from last week• While loop• Lists• For loop

Lists

A list is an ordered sequence of elements.

Create a list in Python:

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

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Lists are Indexable

Remember this?

The same indexing + slicing works for lists!

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H e l l o

0 1 2 3 4 5

-5 -4 -3 -2 -1

Lists are Indexable

>>> my_list = [2, 3, 5, 7, 11]>>> my_list[0]2>>> my_list[4]11>>> my_list[-3]5>>> my_list[5]Traceback (most recent call last):File "<pyshell#7>", line 1, in <module>my list[5]IndexError: list index out of range

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11 7 5 3 2

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] # forward/backward indexing[1, 2, 3, 4, 5, 6, 7, 8, 9] >>> my_list[::2] # add a step[1, 3, 5, 7, 9]

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1 2 3 4 5 6 7 8 9 10

0 1 2 3 4 5 6 7 8 9 10

-10 -9 -8 -7 -6 -5 -4 -3 -2 -1

Slicing# reverse>>> my_list[::-1][10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

# output is an empty list. This is NOT an error>>> my_list[3:8:-2][]

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

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ListsLists can contain strings:

>>> days = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"]

>>> days[3] 'Wed' >>> len(days)7

Lists can mix different types:>>> pi = ['pi', 3.14159, True]

# student: name, age, height, SAT>>> student = ['Roi', 21, 1.83, 782]

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Lists – DynamicMaintain a list of the students either by name or by id:

>>> students = ['Itay',9255587, 'Alon', 'Zohar',744554887]

>>> 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']

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Lists – Dynamic

• Alon wants to leave the course:

>>> students.remove('Alon')>>> students['Itay', 9255587, 'Zohar', 744554887, 'Michal']

remove removes only the first occurrence of a value.

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Nested Lists

>>> mat = [ [1, 2, 3],[4, 5, 6] ]>>> mat[1][4,5,6]>>> mat[1][2]6

• What is len(mat) ?

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Nested Lists

>>> family = [‘Meir‘, [‘Yossi‘,

[‘Yuval‘, [‘Elisha‘]] ],

[‘Yitzhak‘, [‘Meir‘], [‘Orna‘], [‘Evyatar‘] ],

[‘Gavri‘, [’Uri‘], [‘Boaz‘] ],]

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RangeAn ordered list of integers in the range.

>>> range(10)[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

range(from, to) contains all integers k satisfying from ≤ k < to.

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

>>> range(2,10)[ 2, 3, 4, 5, 6, 7, 8, 9]>>> range(-2,2)[-2, -1, 0, 1]>>> range(4,2)[]

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Range>>> type(range(3))<type 'list'>

Step size:

range(from, to, step) returns:

from, from+step, from+2*step,…, from+i*step

until to is reached, not including to itself.

>>> range(0,10,2)[0, 2, 4, 6, 8]>>> range(10,0,-2)[10, 8, 6, 4, 2]

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Range

>>> 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

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Plan for today• Highlights from last week• While loop• Lists• For loop

For Loopfor element in iterable:

statement1

statement2

…

Run over all elements in the iterable (list, string, etc.)

Iteration 0: Assign element = iterable[0]• Execute the statements

Iteration 1: Assign element = iterable[1]• Execute the statements

…

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For Loop

Indentation determines the scope of the iteration.

Note

No infinite lists in Python

No infinite for loops!

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For Example

Compute 1 + 2 + … + 100:

The sum is 5050

Shortcut:

sum(range(1,101))

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For Example

# factorial

n = 8

fact = 1

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

fact = fact * i

print n, "! = ", fact

Syntactic sugar:

fact *= i is equivalent to fact = fact * i

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Fibonacci series

Fibonacci series

0, 1, 1, 2, 3, 5, 8, 13, 21, 34

Definition

fib(0) = 0

fib(1) = 1

fib(n) = fib(n-1) + fib(n-2)

en.wikipedia.org/wiki/Fibonacci_number Leonardo Fibonacci

1170-1250, Italy

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סלט פיבונאצ'י

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Fibonacci series

Write a program that for an integer n > 0,

prints the nth Fibonacci number.

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Fibonacci series - code

n = 10

if n < 2:

curr = n

else:

prev = 0

curr = 1

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

new = prev + curr

prev = curr

curr = new

print "The nth Fibonacci number is", curr

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Fibonacci – Another Coden = input(“Insert a non-negative number")

fibs = [0, 1]

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

fibs.append(fibs[-1] + fibs[-2])

print fibs[n]

• Is shorter code necessarily better?

• What about memory consumption?

For loop – Average

# Average

ages = [19, 22, 20]

sum_ages = 0

for age in _____:

____________

sum_ages = sum_ages/len(ages)

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ages

sum_ages += age

For loop – Average Distance

start_points = [5, 2, 8]

end_points = [15, 14, 21]

num_points = len(end_points)

sum_dist = 0

for i in ____________:

____________________

sum_dist = sum_dist / num_points

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range(num_points)

sum_dist += end_points[i] – start_points[i]

For Loop and Strings

Iterate over strings:

name = "Kobe"for letter in name: print "Give me", letterprint "What did we get?", name

Give me K

Give me o

Give me b

Give me e

What did we get?

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Example: Change Elements in Listlst = [1, 4, 3, 6, 8] # we want to zero every odd element.

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# take 2

inds_lst = range(len(lst))

for ind in inds_lst:

if lst[ind] % 2 == 0:

lst[ind] = 0

print lst

# take 1

for elem in lst:

if elem % 2 == 0:

elem = 0

print lst

For loop – Think First!

• Given a sequence seq, we’ve seen 2 ways to go over it using a for loop:1. Go over the elements of seq directly.

for elem in seq: … # Do something with elem.

2. Go over the indexes of seq:for i in range(len(seq)): … # Do something with seq[i].

Before writing a for loop – choose the form which works better for you.

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Break – breaking loops

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

Used for getting out of loops when a condition occurs.

Example:

for elem in lst:if elem < 0:

print "First negative number is", elem

break

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# Find smallest divisor using for loop:

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

if n % div == 0:break

print div

Break Example

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Example - Prime

n = 2013

for div in range(2,int(n**0.5)): if n % div == 0:

breakif n % div == 0:

print n, "is not prime"else:

print n, "is prime"

range must accept argument of the type int so we perform casting on the result of the power operation.

Continue

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The continue statement, continues with the next iteration of the loop.

Example - create a list of unique elements:

>>> lst = [1,4,5,8,3,5,7,1,2]>>> uniques = []>>> for x in lst:

    if x in uniques:          continue

    uniques.append(x)

>>> print uniques[1,4,5,8,3,7,2]

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for or while?

• In most cases it is more natural to use for• In some cases it is better to use while• for:

• Predefined number of iterations• No need to initialize or advance the loop

variable• while:

• Unknown number of iterations• Can specify a stop condition

Let’s Play!

secret = input(“Choose a number: ”))

while True:

guess = input("Enter guess: "))

if _______________:

break

elif ______________:

print "Try a higher number"

else:

print "Try a lower number"

print "Right!"

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guess == secret

guess < secret

Player 1

Player 2

Hint (sample output)

Choose a number: 34Enter guess: 100Try a lower numberEnter guess: 50Try a lower numberEnter guess: 25Try a higher numberEnter guess: 35Try a lower numberEnter guess: 34Right!

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Programming Style

• Comments: #• Meaningful variables names

Why is it important?

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Exercise – Integer Division

• Input:• Two positive integers – A and B

• Output:• How many times A contains B (the result of the

integer division A/B)

• Do not use the operators ‘/’, ‘*’

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Exercise – Power of Two

• Input: a positive integer A• Output:

If there is an integer N such that

A == 2N

then print N, otherwise print a suitable message.

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Write a program that prints an upside-down triangle of *.

Input: The height of the triangle. *****

*****

****

*

Exercise – Triangle Printout