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Conditionals and Booleans

CMSC 201

Fall 2012

Instructor: John Park

Lecture Section 01

Discussion Sections 02-08, 16, 17 1

Learning Outcomes

• Will know what a block of code is.

• An understanding of if statements and boolean expressions used with them.

• An ability to use if-else statements

• An ability to use if-elif-else statements

• An understanding of conditionals used with strings

• Be familiar with boolean operators

• Be familiar with boolean algebra, including DeMorgan's law

• Be familiar with Truth tables

• Understand the use of logical and to replace nested ifs

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Control Structures

• Control Structures allow different lines of code to be executed depending on an evaluation of some expression.

• The most common conditional structure is the if structure.

• An if structure contains an expression and a block or body of code.

• When the expression is true, the block of code associated with the if statement is executed, otherwise it's skipped.

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“if” Statements and Control Blocks

• If statements take the form : if <condition>:

<block>

• Take note of the ':' after the <condition>.

• In Python, the ':' character, colon, is used to denote the start of a code block.

• All code in the block must be tab indented after the ':'.

(For the rest of the lecture, when we refer to “tabs”, we mean “an additional level of indentation”, not an actual TAB character)

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“if” Statements and Control Blocks

• The first line that is not tabbed-in is the first line not included in that code block.

if <condition>:

block-line-1

block-line-2

block-line-3

out-of-block-line-1

out-of-block-line-2

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“if” Statements and Control Blocks

• Many other structures in Python also use code blocks in this same form, where all lines after the ':' are tabbed in to be part of that block. This includes functions and loops.

• The indentation formatting is critical: the lines in the block must all be indented exactly the same amount.

• Although it's possible to indent in any way (1 space, 3 spaces, 1 tab, etc..), it must be consistent, or python will return an error.

• Our CMSC 201 Coding Standard mandates an indentation of 4 spaces for each level

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“if” Statements and Control Blocks

• If statements and other structures that use blocks may be nested.

– This means the number of tab indents must increase with the level of nesting.

if <condition1>:

block1-line-1

block1-line-2

if <condition2>:

block2-line-1

block2-line-2

out-of-block2-line-1-in-block1-line-3

out-of-block2-line-2-in-block1-line-4

out-of-block1-line-1

out-of-block1-line-2

out-of-block1-line-3 7

Managing Indentation

• Emacs automatically handles Python indentation for you:

– Emacs analyzes your code as you type, looking for special tokens like the ‘:’ . It then infers the level of indentation

– When you start a line by hitting the <TAB> key, emacs automatically indents that line by inserting the correct number of leading spaces.

– To finish a block, hit <TAB> to indent to the current block depth, then hit <Backspace> to “unindent” a level.

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Managing Indentation

• Since emacs indents consistently for us, using it will keep us from having this kind of error when our code is in a file. You'll still have to be careful about indentation when using the interactive interpreter though.

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Managing Indentation

>>> if True:

... x = 1

... y = 2

File "<stdin>", line 3

y = 2

^

SyntaxError: invalid syntax

>>>

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Boolean Expressions

• The <condition> part of the if statement represents a boolean expression that is either true or false.

• Expressions can be formulated using mathematical symbols to compare different values.

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Python Comparison Operators

Operation Python Operator

is less than <

is less than or equal to <=

is greater than >

is greater than or equal to >=

is equal to ==

is not equal to !=

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Python Comparison Operators

• The same 6 operators can be applied to strings, in which case it tests for equality or alphabetical order.

• Equality test (“==“) is strict, case-sensitive test

• For ordering, comparison is done position-by-position, stopping when a difference is found

– “less than”, (i.e., “earlier””) determined by comparing ASCII codes.

– So, upper-case letters come before lower-case

“ab” < “ac”, “ab” < “abc”, and “Abc” < “aBC” (why?) 13

Boolean Expressions

• When an expression is evaluated, it returns a boolean value in Python

• The Python boolean values are True and False (capitalization is important!)

• Examples:

>>> x = 6

>>> y = 4

>>> x < y

False

>>> x <= y

False

>>> x == y

False

>>> x >= y

True

>>> x > y

True

>>> x != y

True

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Boolean Expressions

• Like any other type in Python, variables can store boolean values.

>>> aBoolean = x > y

>>> print aBoolean

True

• Boolean values themselves are boolean expressions.

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Converting to Boolean

• Other data types can be converted to booleans by using bool(value); the conversion rules are:

– Integer or Float 0 becomes False, any other number becomes True.

– An empty string becomes False, any other string becomes True.

– An empty list becomes False, any other list becomes True.

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Converting to Boolean

>>> bool(0)

False

>>> bool(3)

True

>>> bool(-1)

True

>>> bool(0.0)

False

>>> bool(0.1)

True

>>> bool("")

False

>>> bool("hi")

True

>>> bool([])

False

>>> bool([1, 2])

True

# Some strange e.g.s:

>>> bool(“False")

True

>>> bool([0])

True

>>> bool([False])

True

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“if” Statement Execution

• When the condition of an if statement evaluates to true, the code block for that if statement is executed, otherwise it is skipped.

>>> x = 6

>>> y = 4

>>> if x > y:

... print "x is greater than y“

...

x is greater than y

>>> if x <= y:

... print "x is less than or equal to y“

...

>>>

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“if” Statement Execution

Restating the obvious:

– The conditional "x > y" tests whether x is a greater value than y and if that is true, the string "x is greater than y" is printed.

– It is not a mathematical assertion that x is greater than y

• The condition expression of an if statement is implicitly converted to a boolean:

>>> str = ""

>>> if str:

... print ’string "str" is not empty’

...

>>>

– Notice nothing was printed out

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if-else

• If you want to handle cases when a condition fails, as well as when it succeeds, use the else

statement in conjunction with the if.

• else statements also contain a code block and are executed when the condition in the if statement is false, otherwise it is skipped.

• if-else statements take the form of : if <condition>:

<true-block>

else:

<false-block> 20

if-else

• We can revise the previous example as : >>> x = 6

>>> y = 4

>>> if x >= y:

... print "x is greater than or equal to y“

... else:

... print "x is less than y“

...

x is greater than or equal to y

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if-else

• If we swap the values of x and y and rerun the if-else we get:

>>> x = y

>>> y = x

>>> if x >= y:

... print "x is greater than or equal to y“

... else:

... print "x is less than y“

...

x is greater than or equal to y

• Something went wrong!

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Digression: Swapping Variables

• So here's what we should have written : >>> x = 6

>>> y = 4

>>> temp = x

>>> x = y

>>> y = temp

>>> if x >= y:

... print "x is greater than or equal to y“

... else:

... print "x is less than y“

...

x is less than y

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Digression: Swapping Variables

• Most languages require the use of a third variable (conventionally called temp) to perform a swap

• Python has a short-cut built into the language for this, called simultaneous assignment:

var1, var2 = var2, var1

• You can even rotate a set of variables: a, b, c, d = b, c, d, a

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Digression: Swapping Variables

• So here's what we should have written : >>> x = 6

>>> y = 4

>>> x, y = y, x

>>> if x >= y:

... print "x is greater than or equal to y“

... else:

... print "x is less than y“

...

x is less than y

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Nested if Statements

• You can think of an if structure as a fancy statement: you can insert it anywhere where you would put a simple statement.

• So, you can even put it inside another if: >>> x = 1000001

>>> if x > 0:

... print "x is positive“

... if x > 1000000:

... print “Boy, x is REALLY positive!!!“

... else:

... print "x is not positive“

...

x is positive

Boy, x is REALLY positive!!!

>>> 27

Nested if Statements

• You can also put it inside the else block: >>> x = 0

>>> if x > 0:

... print "x is positive“

... else:

... print "x is not positive“

... if x < 0:

... print “In fact, x is negative“

... else:

... print “In fact, x is zero“

...

x is not positive

In fact, x is zero

>>>

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Nested if Statements

• You can use this to make a n-way selection >>> x = 3

>>> if x == 1:

... print “You selected 1“

... else:

... if x == 2:

... print “You selected 2“

... else:

... if x == 3:

... print “You selected 3“

... else:

... if x == 4:

... print “You selected 3“

... else:

... print “Invalid selection“

...

You selected 3

>>>

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if-elif-else

• That was tedious; is there a better way? Yes: the if—else-if—else chain

– Python actually uses keyword “elif”—more terse >>> x = 3

>>> if x == 1:

... print “You selected 1“

... elif x == 2:

... print “You selected 2“

... elif x == 3:

... print “You selected 3“

... elif x == 4:

... print “You selected 4“

... else:

... print “Invalid selection“

...

You selected 3

>>>

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Boolean Operators

• How do you do complex comparisons?

– E.g.: “x is greater than 0, but less than 10”

or: “x is either less than -10, or greater than 10”

or: “x is not in a given list”

• Python provides the boolean operators “and”, “or”, and “not”

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Boolean or

X Y X or Y

T T T

T F T

F T T

F F F

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Boolean and

X Y X and Y

T T T

T F F

F T F

F F F

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Boolean not

X not X

T F

F T

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Combining Boolean Operators

• Order of operator precedence, from high to low is: not >> and >> or

• Expressions may use parentheses to change the order of operations

• These basic principles can be used to construct more complex operators:

– E.g.: exclusive-or: (P or Q) and not (P and Q)

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Combining Boolean Operators

X Y not X and Y

T T F

T F F

F T T

F F F

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Combining Boolean Operators

X Y not (X and Y)

T T F

T F T

F T T

F F T

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Boolean Algebra

• You can use Boolean algebra to manage and reduce complex expressions.

• The and and or operators distribute over each other.

– a or (b and c) == (a or b) and (a or c)

– a and (b or c) == (a and b) or (a and c)

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Boolean Algebra

• DeMorgan's Law distributes an outer 'not' by negating (not-ing) each of the components and flipping ands to ors and ors to ands.

– not(a or b) == (not a) and (not b)

– not(a and b) == (not a) or (not b)

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Example

• So now, combining several things we just learned:

>>> str = raw_input(“Enter a char: “)

>>> ch = str[0]

>>> if str >= ‘a’ and str <= ‘z’:

... print “Lower-case letter“

... elif str >= ‘A’ and str <= ‘Z’:

... print “Upper-case letter“

... else:

... print “Non-alphabetic character“

...

Enter a char: q

Lower-case letter

>>>

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