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Python – Part 3

Functions

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Getting help

• Start the Python interpreter and type help() to start the online help utility.

• Or you can type help(print) to get information about the print function.

• When finished with the help utility, type quit at the prompt to return to the interpreter.

Modulus Operator

• Yields the remainder when first operand is divided by the second.

• >>>remainder=7%3• >>>print (remainder)• 1

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Boolean expressions

• An expression that is either true or false• Operator ==• >>>5==5• True• >>>5==6• False

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Boolean expressions

• Type bool – True and False• >>>type (True)• <type ‘bool’>• >>>type (False)• <type ‘bool’>

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Boolean expressions

Other operators:x != y # x is not equal to y x > y # x is greater than y x < y # x is less than y x >= y # x is greater than or equal to y x <= y # x is less than or equal to y

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Logical operators

• And, or, not• Semantics similar to their meaning in English• x>0 and x<10• not(x>y)• Any nonzero number in Python is interpreted

as “true”• >>> 17 and True• True

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Function Calls

• Function– A named sequence of statements that performs a

computation– Name– Sequence of statements

• “call” function by name>>> type(32)<type ‘int’>– Function name – type– Argument - 32

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Function Calls

• Commonly “takes” an argument• “returns” a result• Result called the return value.

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Type conversion functions

• Built-in functions that convert values from one type to another

>>>int (’32’)32>>>int (‘Hello’)ValueError: invalid literal for int(): Hello>>>int (3.9999)3>>> int (-2.3)-2

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Type conversion functions

>>>float (32)32.0>>>float (‘3.14159’)3.14159>>> str (32)’32’>>> str (3.14159)‘3.14159’

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Math functions

• Module – a file that contains a collection of related functions

• Math module – provides most of the familiar mathematical functions

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Math Functions

• Import the module before using it>>> import mathCreates a module object named math>>> print math<module ‘math’ (build-in)>Prints some information about it

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Math functions

>>> ratio = signal_power / noise_power>>> decibels = 10 * math.log10(ratio) #computes logarithm base 10 of ratio

>>> radians = 0.7 >>> height = math.sin(radians) #computes sine of radians

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Math functions

>>> degrees=45>>> radians=degrees/360.0*2*math.pi>>>math.sin(radians)0.707106781187

• Can use functions to compose more complex expressions

x=math.sin(degrees/360.0*2*math.pi)

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Composition

>>>math.sqrt(2)/2.00.707106781187

>>>minutes=hours*60 #right>>>hours*60=minutes #wrong!SyntaxError: can’t assign to operator

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Adding new functions

• Function definition– Name of a new function– Sequence of statements that execute when

function is called

def print_lyrics(): print ("I'm a lumberjack, and I'm okay.“) print ("I sleep all night and I work all day." )

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Adding new functions

• def – keyword for function definition• print_lyrics – name of the function• () – no arguments

• Function name –same rules as for variables• Avoid using variable and function with same

name

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Adding new functions

• Header – first line of the function definition– Ends in colon

• Body – the rest– Has to be indented (always four spaces)

• Empty line to end the function (not necessary in a script)

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Adding new functions

• Defining a function creates a variable with the same name

>>> print_lyrics()

def repeat_lyrics(): print_lyrics() print_lyrics()

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Parameters and Arguments

• math.pow(2,3)• 8.0

• def print_twice(bruce):• print bruce• print bruce

• Assigns the argument to a parameter named bruce

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Parameters and Arguments

>>>print_twice (‘spam’)spamspam

>>>print_twice(17)1717

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Parameters and Arguments

>>>print_twice(‘spam’*2)spam spamspam spam

Argument evaluated before function is called

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Parameters and Arguments

• Can also use a variable as an argument

>>>number=17>>>print_twice(number)1717

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Local variables and parameters

• Variable inside a function is local• Exists only inside the function• def cat_twice(part1, part2):• cat = part1 + part2 • print_twice(cat)

• This function takes two arguments, concatenates them, and prints the result twice.

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Local variables (cont’d…)

>>> line1 = 'Bing‘>>> line2 = 'bang.' >>> cat_twice(line1, line2) Bing bang. Bing bang.

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Local variables (cont’d …)

When cat_twice terminates, the variable cat is destroyed. If we try to print it, we get an exception:

• >>> print cat NameError: name 'cat' is not defined

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Void functions

• print_twice • Perform and action but does not return a

value

• If function returns value, almost always use it as part of an expression:

• x = math.cos(radians) • golden = (math.sqrt(5) + 1) / 2

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Void functions

• >>> result = print_twice('Bing')• Bing • Bing • >>> print result • None

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Part 3

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