Inft1004 Lec4 Selection

7/27/2019 Inft1004 Lec4 Selection

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INFT1004Visual Programming

Lecture 4

Selection

(Guzdial & Ericson chapter 5)


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INFT1004 - SEMESTER 1 - 2012

Week 1

Week 2

Week 3

Week 4

Week 10

Week 11

Week 9

Week 12

Mar 4

Mar 11

Mar 18

Mar 25

May 6

May 13

May 20

May 27

Assignment due3:00 Tuesday May 21

Programs, Arrays and Iteration

Working with x and y coordinates

Selection

Web, Representations, Steganography

Lists, Files and Modules

Turtles and Other Classes

Introduction

Week 13 Jun 3

Revision and Look Ahead

No formal classes

Mid Year Examination Period - MUST be available normal & supplementary period

Recess Apr 1 - Apr 7 Mid Semester Recess Period

Practical Test 2in Lab class

Week 8 Apr 29

More Sound and ArraysWeek 7 Apr 22

Program Design and Strings

Lecture Topics and Lab topics are the same for each week

LECTURE TOPICS

Week 5 Apr 8 Practical Test 1in Lab class

More Picture Techniques

Week 6 Apr 15 Sound and Arrays


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INFT1004 - SEMESTER 1 - 2012

Week 1

Week 2

Week 3

Week 4

Week 10

Week 11

Week 9

Week 12

Mar 4

Mar 11

Mar 18

Mar 25

May 6

May 13

May 20

May 27

Assignment due3:00 Tuesday May 21

Programs, Arrays and Iteration

Working with x and y coordinates

Selection

Web, Representations, Steganography

Lists, Files and Modules

Turtles and Other Classes

Introduction

Week 13 Jun 3

Revision and Look Ahead

No formal classes

Mid Year Examination Period - MUST be available normal & supplementary period

Recess Apr 1 - Apr 7 Mid Semester Recess Period

Practical Test 2in Lab class

Week 8 Apr 29

More Sound and ArraysWeek 7 Apr 22

Program Design and Strings

Lecture Topics and Lab topics are the same for each week

LECTURE TOPICS

Week 5 Apr 8 Practical Test 1in Lab class

More Picture Techniques

Week 6 Apr 15 Sound and Arrays

This week in the lab there will be a

practice practical so you can practice forweek 5s exam.

But dont forget you still need to do the tut

material for this week


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Revision Simple & Object Types

With simple types (eg int, float, string)

Assignment gives the value of the expression on

the right to the variable on the left

number = 8

sum = 10 + number


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Revision Simple & Object Types

With object types

Assignment makes the variable on the left a name

(or another name) for the object on the right

myFile = newFile

newFile

myFile


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Revision - Functions

Programs are made up of one of more functions

If we want a function to take arguments, we includecorrespondingparameters when defining it

Whenever we find the same piece of code appearingseveral times in a program, we should extract it, define it as

a function, and replace the multiple occurrences withmultiple calls to the function

If there are variations between the occurrences, we useparameters and arguments to deal with the variations


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Revision - sequence, iteration

The statements in a function or a loop will beexecuted in the order theyre written

When working out what the code does, be sure tonote the effect of each statement before going on

to consider the next

sequence


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Revision - sequence, iteration

Within a sequence of statements, a loop iterates

(repeats) the statements in its body, as manytimes as specified

sequence loop


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Revision - Comments

Your programs must include three kinds of comments:

1.A comment at the start of every program (collection offunctions), saying who wrote it, and when, and why

2.A comment at the start of every function, explaining brieflywhat it does3.A comment with every bit of code that another programmermight find easier to understand if its explained

Do not write comments to explain what will be

obvious to a reasonable programmer!


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Sequence, selection, iteration

Programming has three essential building blocks

Weve already met sequence and iteration

Selection determines which code to execute

depending on specified conditions

sequence loop

selection?


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Sequence, selection, iteration

For example, we might want to change the colourof some pixels:

not all the pixels in a picture

not all the pixels in a predefined block

but all the pixels satisfying a particular

condition, such as all the red pixels,

for red-eye reduction


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The ifstatement

Selection is done with the if statement. At itssimplest, the statement takes the form

if

selection?


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The ifstatement

Selection is done with the if statement. At itssimplest, the statement takes the form

where means some condition to be tested

and is one or more statements suitably indented

if

selection?


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The ifstatement

if

selection?

Either way, any

following statementsare then executed

If the condition is true when tested, the body will beexecuted; otherwise, the body will be ignored

loop


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ConditionsA condition is something that, when evaluated,

gives a value of true or false

Technically, it is a boolean expression, which

simply means an expression whose value is true

or false

boolean is another simple type to add to int, float,

and string

if


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Conditions

Unfortunately, Python implements booleans asintegers, which can lead to some confusion:

The int 0 is regarded as the boolean falseany other int is regarded as the boolean true

false 0

true any other int (e.g. 1,6,10)


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Boolean expressions

Expression Value

3 > 5 false

3 < 5 true

3 == 5 false

3 5 true

3 >= 5 false

3


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Boolean expressions

Expression Value

3 > 5 false

3 < 5 true

3 == 5 true

3 5 false

3 >= 5 false

3


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Boolean expressions

Note that the check for equality

uses a double = sign - becausethe single one is reserved forassignment

Expression Value

3 > 5 false

3 < 5 true

3 == 5 true

3 5 false

3 >= 5 false

3


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Distance Between Colours

This is a function that measures how close together two

colours are

It goes up to about 450

Experiment with different colours to see what sorts of

values it produces

distance(colour1, colour2)


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Lets enhance the green of colours close to white

(we will need to work out the distance between a pixelcolour and the colour white)


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Lets enhance the green of colours close to white

(we will need to work out the distance between a pixelcolour and the colour white)

For all the pixels in the picture,

if the colour of that pixel is close to white enhance the green of a pixel

?


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Irradiating a swan

This method works well with swan.jpg

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def irradiate(picture):

# Take pixels that are near enough to

# white and make them very green

for px in getPixels(picture):

colour = getColor(px)if distance(colour, white) < 270:

# Found distance by trial & error

setBlue(px, getBlue(px) / 2)

setRed(px, getRed(px) / 2)

setGreen(px, 190)repaint(picture)


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Irradiating a swan

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Irradiating a swan

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For all the pixels in the picture, if the colour of that pixel is close to white enhance the green of a pixel

?


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Not as easy as it looks

This approach is of limited value

With most pictures, more (or fewer) pixels than youwould expect are within a given distance of a given

colour

It works best where the colours in a picture are quite

distinct

While it does show a new way of processing a picture,

the real point is to show you how the if statement canspecify that a sequence of statements should be carried

out only when some particular condition applies


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Red-eye reductionTo reduce the red that a camera flash puts in the eyes,

without reducing the red in the rest of the picture, we need tofirst note the range of pixels in which the eyes are located.


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Red-eye reduction

Instead of

reducing the redin the wholepicture - we

reduce it just in

the range with

the eyes in it

To reduce the red that a camera flash puts in the eyes,

without reducing the red in the rest of the picture, we need tofirst note the range of pixels in which the eyes are located.


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Red-eye reductionfor x in range(startX, endX):

for y in range(startY, endY):pix = getPixel(pic, x, y)

if distance(red, getColor(pix) < 165):

setColor(pix, replacementColour)

endXstartX

startY

endY


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Red-eye reductionfor x in range(startX, endX):

for y in range(startY, endY):pix = getPixel(pic, x, y)

if distance(red, getColor(pix) < 165):

setColor(pix, replacementColour)

endXstartX

startY

endY

usually a

blackish

colour


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ifwith else

Weve seen the simplest form of the ifstatementif :

?

true


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ifwith else

What if we want to do something when the condition is true

and something different if the condition is false?

For example, we want to turn the white pixels green and the

other pixels purple?

if :

else:

?true

false


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ifwith else

What if we want to do something when the condition is true

and something different if the condition is false?

For example, we want to turn the white pixels green and the

other pixels purple?

if :

else:

?true

false


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ifwith else

When we follow an ifand its body with an else and its

body..

the body of the ifwill be done if the condition is true, and

the body of the else will be done if the condition is false

?true

false

if distance(colour, white) < 270:

else:


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ifwith else

Guzdial & Ericson prefer to achieve the same effect with

two separate ifs


if distance(colour, white >= 270:

?

true

?

true


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ifwith else


else # distance(colour, white >= 270:

But most programmers prefer the proper use ofelse

?true

false


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The oddly named elif

A common use of selection is to test for several mutually

exclusive conditions

if value < 64:

value = 31

elif value < 128:

value = 95elif value < 192:

value = 159

else:

value = 223

elifis short forelse if; be sure that you understand theelse bit


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The oddly named elif

A common use of selection is to test for several mutually

exclusive conditions

if value < 64:

value = 31

elif value < 128:

value = 95elif value < 192:

value = 159

else:

value = 223

elifis short forelse if; be sure that you understand theelse bit

Note that only the first

condition to be true will be

executed.

If none of the conditions aretrue then the else part will be

executed.

(There doesnt need to be an

else part but usually there is)


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Posterising

A posterised picture is one that uses a highly reduced set

of colours (suitable for poster printing)


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Posterising

if-elif-else statements form the basis of a function that can

be used to posterise a picture

It divides the possible values into four ranges, then

replaces every value with the midpoint of that range

0-63

2231599531

64-127 128-191 192-255


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Posterising

The posterise function is typically used for each colourchannel (red, blue, green)

def posterise(picfile):

# Posterise a picture by replacing each channel of every pixel

# with the middle of the range of values it lies inpicture = makePicture(picfile)

for px in getPixels(picture):

setRed(px, midRange(getRed(px)))

setGreen(px, midRange(getGreen(px)))

setBlue(px, midRange(getBlue(px)))

repaint(picture)


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Posterisingdef midRange(value):

# See which of four ranges a colour channel value lies in,# and return the midpoint of that range

if value < 64:

value = 31

elif value < 128:

value = 95

elif value < 192:

value = 159

else:

value = 223

return value

0-63

2231599531

64-127 128-191 192-255


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if value < 64:

value = 31

elif value < 128:

value = 95

elif value < 192:

value = 159

else:

value = 223

return value

0-63

2231599531

64-127 128-191 192-255


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if value < 64:

value = 31

elif value < 128:

value = 95

elif value < 192:

value = 159

else:

value = 223

return value

0-63

2231599531

64-127 128-191 192-255


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if value < 64:

value = 31

elif value < 128:

value = 95

elif value < 192:

value = 159

else:

value = 223

return value

0-63

2231599531

64-127 128-191 192-255


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if value < 64:

value = 31

elif value < 128:

value = 95

elif value < 192:

value = 159

else:

value = 223

return value

0-63

2231599531

64-127 128-191 192-255


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Posterising

Our approach (using elif) is basically the same as the

version in the textbook, but our program is a lot smallerand neater

Lecture4demo.py shows this posterise function and the

one from the book


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The program in the book

The one using elifonly needs to test the upper end

of each range; the one in the book has to test both

ends of each range

if value < 64:

value = 31

elif value < 128:

value = 95

elif value < 192:

value = 159

else:

value = 223

if redVal < 64:

setRed(p, 31)

if redVal > 63 and redVal < 128:

setRed(p, 95)


setRed(p, 159)


setRed(p, 223)

Be sure you understand why


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See how the book function repeats the same code

(with variations) three times.

This should clearly be rewritten as a function,

which should be called three times.


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And the book function uses the names red, green,

and blue as variables, which is not good

(there are already defined constants called red,

green and blue)


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Return or not return

In the lecture demo program, some picture-processing functions simply repaint the picture,

while others repaint and return it; why the

difference?

repaint(picture) repaint(picture)

return(picture)


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Return or not return

If all we want to do is see the resulting picture,repaint is fine

If we might want to do something else, such as

explore it or save it, return is better, because we

can then assign it to a variable and do these things.

With big pictures on a small screen, repaint makes

them too big to see properly with no scroll bars!

This is one reason we might want a different

approach.


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Combining conditions

Sometimes a simple condition isnt enough

We might want to combine multiple conditions

in various ways

We can do this with the logical operatorsand, or, and not


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and

,or

, andnot

mean pretty much what they mean in English

if age > 16 and < 25


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and

,or

, andnot

mean pretty much what they mean in English

if age > 16 and < 25

An important difference is that they must joincomplete conditions, not partial ones

if age > 16 and age < 25


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Edge detection

We can turn a colour picture into a black-and-white

line drawing if we can find edges and make them black

An edge is where theres a sharp difference in

luminance (brightness) between neighbouring pixels


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Edge detection

Carefully read and understand the edge detection

program in the book

Carefully read and understand the edge detection

program in lecture4demo.py

Try to understand the differences; for example,

theres no need to have two copies of the picture,

as in the book

And the proper use ofelsemeans we dont need to

use not

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Inft1004 Lec4 Selection