Python – Part 3 Functions 1

Function Calls Function – A named sequence of statements that performs a computation – Name – Sequence of statements “call” function by name >>> type(32) – Function name – type – Argument Prepared by Department of Preparatory year

Function Calls Commonly “takes” an argument “returns” a result Result called the return value. 3 Prepared by Department of Preparatory year

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 4 Prepared by Department of Preparatory year

Type conversion functions >>>float (32) 32.0 >>>float (‘ ’) >>> str (32) ’32’ >>> str ( ) ‘ ’ 5 Prepared by Department of Preparatory year

Math functions Math module – provides most of the familiar mathematical functions Module – a file that contains a collection of related functions 6 Prepared by Department of Preparatory year

Math Functions Import the module before using it >>> import math Creates a module object named math >>> print math Prints some information about it 7 Prepared by Department of Preparatory year

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 8 Prepared by Department of Preparatory year

Math functions >>> degrees=45 >>> radians=degrees/360.0*2*math.pi >>>math.sin(radians) >>>math.sqrt(2)/ Prepared by Department of Preparatory year

Composition Can use functions to compose more complex expressions x=math.sin(degrees/360.0*2*math.pi) >>>minutes=hours*60 #right >>>hours*60=minutes #wrong! SyntaxError: can’t assign to operator 10 Prepared by Department of Preparatory year

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." ) 11 Prepared by Department of Preparatory year

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 12 Prepared by Department of Preparatory year

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) 13 Prepared by Department of Preparatory year

Adding new functions Defining a function creates a variable with the same name >>> print_lyrics() def repeat_lyrics(): print_lyrics() 14 Prepared by Department of Preparatory year

Parameters and Arguments math.pow(2,3) 8.0 def print_twice(bruce): print bruce Assigns the argument to a parameter named bruce 15 Prepared by Department of Preparatory year

Parameters and Arguments >>>print_twice (‘spam’) spam >>>print_twice(17) Prepared by Department of Preparatory year

Parameters and Arguments >>>print_twice(‘spam’*2) spam Argument evaluated before function is called 17 Prepared by Department of Preparatory year

Parameters and Arguments Can also use a variable as an argument >>>number=17 >>>print_twice(number) Prepared by Department of Preparatory year

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. 19 Prepared by Department of Preparatory year

Local variables (cont’d…) >>> line1 = 'Bing‘ >>> line2 = 'bang.' >>> cat_twice(line1, line2) Bing bang. 20 Prepared by Department of Preparatory year

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 21 Prepared by Department of Preparatory year

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 22 Prepared by Department of Preparatory year

Void functions >>> result = print_twice('Bing') Bing >>> print result None 23 Prepared by Department of Preparatory year

Part 3 End 24