Core Elements

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PART I I I : ITERATIONS

Core Elements

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Overview

Tuples

Mutable, Immutable object

For Loops

While Loops

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Lists in Python

Lists are mutable objects, e.g. we can modify their contents.

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>>> my_list = [1, 10, 4, 5] # List creation [ ]>>> my_list[2] # Accessing the third element4>>> my_list[1, 10, 4, 5]>>> my_list[2] = 9 # Modifying the third element>>> my_list # The modified list[1, 10, 9, 5]>>>

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Tuples in Python

Tuples are immutable objects, e.g. we CANNOT modify their contents.

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>>> my_tuple = (1, 10, 4, 5) # tuple creation ( ,)>>> my_tuple[2] # Accessing the third element4>>> my_tuple (1, 10, 4, 5)>>> my_tuple[2] = 9 # Modifying the third element

Traceback (most recent call last): File "<pyshell#15>", line 1, in <module> my_tuple [2] = 9TypeError: 'tuple' object does not support item assignment >>>

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Tuples in Python

Tuples are immutable objects, e.g. we CANNOT modify their contents.

Note that numbers and strings are immutable too.

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>>> my_t = (1, 10, 4, 5) # tuple creation ( ,)>>> my_t = my_t(:2) + (9,) + my_t(3:) # Modifying the third element>>> my_t(1, 10, 9, 5)

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Be Careful

Example:

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>>> lst1 = lst2 = [2,4,6,8]>>> t1 = t2 = (1,3,7,9)>>> lst1[2, 4, 6, 8]>>> lst2[2, 4, 6, 8]>>> t1(1, 3, 7, 9)>>> t2(1, 3, 7, 9)>>>

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Be Careful

Example:

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>>> t2 = t2[:2] + (5, ) + t2[3:] # Modifying the third element t2>>> lst2[2] = 5 # Modifying the third element of lst2>>> t1(1, 3, 7, 9)>>> t2(1, 3, 5, 9)>>>

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A change in t2 does not affect t1

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Be Careful

Example:

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>>> t2 = t2[:2] + (5, ) + t2[3:] # Modifying the third element t2>>> lst2[2] = 5 # Modifying the third element of lst2>>> t1(1, 3, 7, 9)>>> t2(1, 3, 5, 9)>>> lst1[2, 4, 5, 8]>>> lst2[2, 4, 5, 8]>>>

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A change in t2 does not affect t1

A change in lst2 does affect lst1

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Be Careful

State Diagram:

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>>> lst1 = lst2 = [2,4,6,8]>>>

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lst1

lst2

0 1 2 3

2 4 6 8

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Be Careful

State Diagram:

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>>> lst1 = lst2 = [2,4,6,8]>>> t1 = t2 = (1,3,7,9)>>>

Code

t1

t2

lst1

lst2 0 1 2 3

1 3 7 9

0 1 2 3

2 4 6 8

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Be Careful

State Diagram:

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>>> lst1 = lst2 = [2,4,6,8]>>> t1 = t2 = (1,3,7,9)>>> t2 = t2[:2] + (5, ) + t2[3:]>>>

Code

t1

t2

lst1

lst2 0 1 2 3

1 3 7 9

0 1 2 3

2 4 6 8

0 1 2 3

1 3 5 9

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Be Careful

State Diagram:

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>>> lst1 = lst2 = [2,4,6,8]>>> t1 = t2 = (1,3,7,9)>>> t2 = t2[:2] + (5, ) + t2[3:]>>> lst2[2] = 5>>>

Code

t1

t2

lst1

lst2 0 1 2 3

1 3 7 9

0 1 2 3

2 4 6 8

0 1 2 3

1 3 5 9

5

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Be Careful

State Diagram:

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>>> lst1 = lst2 = [2,4,6,8]>>> t1 = t2 = (1,3,7,9)>>> t2 = t2[:2] + (5, ) + t2[3:]>>> lst2[2] = 5>>> >>> lst2 = [2, 4, 5, 8]>>>

Code

t1

t2

lst1

lst2 0 1 2 3

1 3 7 9

0 1 2 3

2 4 5 8

0 1 2 3

1 3 5 9

0 1 2 3

2 4 5 8

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Be Careful

State Diagram:

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>>> lst1 = lst2 = [2,4,6,8]>>> t1 = t2 = (1,3,7,9)>>> t2 = t2[:2] + (5, ) + t2[3:]>>> lst2[2] = 5>>> >>> lst2 = [2, 4, 5, 8]>>> lst2[2] = 0

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t1

t2

lst1

lst2 0 1 2 3

1 3 7 9

0 1 2 3

2 4 5 8

0 1 2 3

1 3 5 9

0 1 2 3

2 4 5 80

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Swapping Values

State Diagram:

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>>> num_one = 5>>> num_two = 9>>>

Code

num_two

num_one

5

9

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Swapping Values

State Diagram: First Attempt

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>>> num_one = 5>>> num_two = 9>>> num_one = num_two>>> num_two = num_one>>>

Code

num_two

num_one

5

9

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Swapping Values

State Diagram: Second Attempt

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>>> num_one = 5>>> num_two = 9>>> temp = num_two>>> num_two = num_one>>> num_one = temp>>>

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num_two

num_one

temp

5

9

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DEFINITE&

INDEFINITE LOOPS

Iteration

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Iteration

We need a structure to execute a sequence of statements multiple times in succession

So Far, we have to write the sequence of statement n times to achieve n repetition

How can we proceed if we are not sure how many times it needs to be repeated?

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Definite Loop: For statement

The simplest kind of loop

It will execute a definite amount of times

The for loop statementfor <var> in <iterable>: <body>

Iterable can return its element one at a time

The body of the loop can be any sequence of Python statements

The variable after the keyword for is called the loop index.

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Example

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print "Before the for loop"

for val in [1,2,3]: square_val = val * val print "--> inside loop: square of", str(val), "is", str(square_val)

print "After the for loop"

Code

Before the for loop--> inside loop: square of 1 is 1--> inside loop: square of 2 is 4--> inside loop: square of 3 is 9After the for loop

Python shell

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Example

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for val in [1,2,3]: square_val = val * val print . . .


Code

Python shell

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Example

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Code

Before the for loop

Python shell

1 2 3

val

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Example

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Code

Before the for loop

Python shell

1 2 3

1

valsquare_val

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Example

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Code

Before the for loop--> inside loop: square of 1 is 1

Python shell

1 2 3

1

valsquare_val

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Example

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Code


Python shell

1 2 3

1

valsquare_val

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Example

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Code


Python shell

2 3

4

valsquare_val

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Example

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Code

Before the for loop--> inside loop: square of 1 is 1--> inside loop: square of 2 is 4

Python shell

2 3

4

valsquare_val

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Example

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Code


Python shell

2 3

4

valsquare_val

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Example

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Code


Python shell

3

9

valsquare_val

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Example

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Code

Before the for loop--> inside loop: square of 1 is 1--> inside loop: square of 2 is 4--> inside loop: square of 3 is 9

Python shell

3

9

valsquare_val

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Example

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Code

Before the for loop--> inside loop: square of 1 is 1--> inside loop: square of 2 is 4--> inside loop: square of 3 is 9After the for loop

Python shell

3

9

valsquare_val

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Indefinite Loop: While Statement

An indefinite loop keeps iterating until certain condition are met (conditional loop)

There is no guarantee ahead of time regarding how many times the loop will go around zero time x-times indefinitely

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The while loop statementwhile <condition>: <body>

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<condition>

<body>

False

True

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<condition>

<body>

False

True

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<condition>

<body>

False

True

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<condition>

<body>

False

True

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<condition>

<body>

False

True

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<condition>

<body>

False

True

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<condition>

<body>

False

True

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<condition>

<body>

False

True

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<condition>

<body>

False

True

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<condition>

<body>

False

True

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<condition>

<body>

False

True

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<condition>

<body>

False

True

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<condition>

<body>

False

True

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<condition>

<body>

False

True

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<condition>

<body>

False

True

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The <body> usually contains statements that modify the evaluation of <condition> at some point

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<condition>

<body>

False

True

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The <body> usually contains statements that modify the evaluation of <condition> at some point

The <body> may never be executed!

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<condition>

<body>

False

True

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General Form

The while loop general form Two new Keywords:

1. break 2. continue

while <condition>: <statements K> if<test A>: break if<test B>: continue <statements M>

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<condition>

<statements K>

False

True

if<test A>: break if<test B>: continue <statements M>

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General Form




Test A is True

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<condition>

<statements K>

False

True


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General Form




Test A is False & Test B is True

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<condition>

<statements K>

False

True


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General Form

The for loop general form

for <val> in <iterable>: <statements K> if<test A>: break if<test B>: continue <statements M>

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Summary

We have seen mutable and immutable data type consequences when modifying data

Definite iteration for loops

Indefinite iteration (conditional loop) while loops

General form using break and continue

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Exercises

Consider a problem where we want to record and store the marks of many students, each students having more than one mark (several modules with same credits). what form the data structure would look like how would you write a program to enter the marks of

one student how would you proceed to enter the marks of more

than one students how would you calculate the average mark of each

student what if modules don’t have the same credits (e.g. 10,

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Exercises

Calculate the average of values in a list

Calculate the average of values entered by a user, we don’t know how many values the user want to enter. The user should enter an empty value to signal the end of data input, then a result must be returned.

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Core Elements

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