lilian blot core elements selection & functions lecture 3 autumn 2014 tpop 1

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CORE ELEMENTSSELECTION

&FUNCTIONS

Lecture 3

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Last Week

What a program is

What a statement is

What an expression is

How to do repetitions/iterations

Introduced the notion of Functions

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Overview

Branching with Selection Structure nested if-elif-else

Functions and Parameters

Void and Non-Void Function

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Today’s Problem

We would like to write a program taking a measurement in meters (resp. Feet, inches) as input and convert it to Feet, inches (resp. meters). We need to find a way for the user to tell us which

conversion he/she want to do Depending on user SELECTION we need to do one

operation or the other. We haven’t seen such a structure yet

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IF-ELIF-ELSE

A Selection Structure

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Motivation

Straight line programs: sequence of commands executed one after the other one, and only once. Cannot do many interesting things if

any.

Branching: A program can decide which statements to execute based on a condition. If condition is True, execute A1 to An

If condition is False, execute B1 to Bk

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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Motivation


any.



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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Motivation


any.



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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Motivation


any.



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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Motivation


any.



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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Motivation


any.



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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Motivation


any.



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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Motivation


any.



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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Motivation


any.



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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Motivation


any.



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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Motivation


any.



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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Motivation


any.



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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Motivation


any.



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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Motivation


any.



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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Python Code

Schema Python Code

……if condition : Statement A1 … Statement Anelse : Statement B1 … Statement Bk…

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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Python Code

Schema Python Code


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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Python Code

Schema Python Code


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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Python Code

Schema Python Code


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Statement B1…

Statement Bk

Statement A1…

Statement An

True FalseCondition

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Simple if-statement

Schema Python Code

……if condition : Statement A1 … Statement An

statement Xstatement Y…

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Statement A1…

Statement An

True

False

Condition

Statement XStatement Y…

Note the change of indentation

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if-elif-else statement

Schema Python Code

……if cond_1 : Statement Aelif cond_2 : Statement B…elif cond_n : Statement Celse : Statement D…

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Statement A

True Falsecond_1

cond_2

cond_n

Statement B

Statement C

Statement D

True

True

False

False

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Nested if-else statements

Schema Python Code

……if cond_1 : Statement Aelse : Statement B1 Statement B2 if cond_2 : Statement C else : Statement D…

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Statement A

True Falsecond_1

cond_2

Statement B1

Statement B2

Statement C

Statement D

True False

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Why using Functions?

Advantages code reuse facilitate team work modularisation maintainability

monolithic code huge collection of statement

no modularisation no code reuse (cut & paste is not code reuse!) no parallel implementation

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Implementation

See code snippet 1

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What is a Function?

math function f(x) = 2 * x +1

kind of a sub-program function definition when use, we say that the definition is ‘called’ or

‘invoked’

may or may not have argument

may or may not return a value/object

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Void and non-void function

Void function doesn’t return anything (meaningful) help(…) function None keyword <type ‘NoneType’>, value to represent

the “nothingness”.

non-void function returns something other than None input(…) value raw_input(…) string len(…) int

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Declaring a Function

template:

def <function_name> (<formal_parameters>): <body>

<function_name>: an identifier <formal_parameters>: comma-separated identifiers <body>: any number of indented statements A non-void function must have a return statement as

the last statement of the <body>.

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Implementation

See code snippet 2

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• Key point 1: Separation of concerns

• Key point 2: A function should do one and only one thing (most of the time)

• Key point 3: Decide which information is needed by the function to do its computation… (parameters)

• Key point 4: Does the function need to return zero, one or more values.

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non-void functions

Must have a return statement

Function with no parameter (see head_tail())

Function with one or more parameters (see feet_to_meters(feet, inches) in snippet 3)

Function returning multiple values (see meters_to_feet(number_meters) in snippet 3)

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Be Careful

print x, y never executed

In some language this would generate a compile error

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def f(x): y = 3 * x + 7 return y print x, y

Code

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Void function

Aimed at changing a state or display information

No return statement

Technically, all function return something

A void function will return the None value even if there is no return statement in the body

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Function Call

When Python comes to a function call, it initiate a four-step process:

1. the calling program suspends execution at the point of call

2. the formal parameter of the function get assigned the value supplied by the actual parameters in the call

3. the body of the function is executed

4. control returns to the point just after where the function was called

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Function Call

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def head_tail():rand = random.random()if(rand < 0.5):

return 'head'else :

return 'tail'

Code

>>> heads = 0>>> tails = 0>>> for i in range(1, 10000):

if (head_tail() == 'head'):heads += 1

else :tails += 1

>>>

Interpreter

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Todays’ New Keywords

Selection If, if-else, if-elif-else,

Functions def, return, None.

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Summary

Sometime we need to execute some statements while ignoring others depending on some condition Use the selection structure: if-elif-else

When testing we must ensure that all branches are traversed by the test suit (difficult).

When designing a function, we define what information is needed (parameters) and what should be returned after computation.

Separation of concerns is key for reuse of a function

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lilian blot core elements selection & functions lecture 3 autumn 2014 tpop 1

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