Functions. Python Part 3

Transcription

1 Functions Python Part 3 1

2 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

3 Function Calls Commonly takes an argument returns a result Result called the return value. 3

4 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

5 Type conversion functions >>>float (32) 32.0 >>>float ( ) >>> str (32) 32 >>> str ( )

6 Math functions Math module provides most of the familiar mathematical functions Module a file that contains a collection of related functions 6

7 Math Functions Import the module before using it >>> import math Creates a module object named math >>> print math <module math (build-in)> Prints some information about it 7

8 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

9 Math functions >>> degrees=45 >>> radians=degrees/360.0*2*math.pi >>>math.sin(radians) >>>math.sqrt(2)/

10 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

11 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

12 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

13 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

14 Adding new functions Defining a function creates a variable with the same name >>> print_lyrics() def repeat_lyrics(): print_lyrics() print_lyrics() 14

15 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 15

16 Parameters and Arguments >>>print_twice ( spam ) spam spam >>>print_twice(17)

17 Parameters and Arguments >>>print_twice( spam *2) spam spam spam spam Argument evaluated before function is called 17

18 Parameters and Arguments Can also use a variable as an argument >>>number=17 >>>print_twice(number)

19 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

20 Local variables (cont d ) >>> line1 = 'Bing >>> line2 = 'bang.' >>> cat_twice(line1, line2) Bing bang. Bing bang. 20

21 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

22 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

23 Void functions >>> result = print_twice('bing') Bing Bing >>> print result None 23

24 End Part 3 24