Python – Part 3 Functions 1

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. Prepared by Department of Preparatory year 2

Modulus Operator Yields the remainder when first operand is divided by the second. >>>remainder=7%3 >>>print (remainder) 1

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

Boolean expressions Type bool – True and False >>>type (True) >>>type (False)

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

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

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. 9 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) Prepared by Department of Preparatory year

Type conversion functions >>>float (32) 32.0 >>>float (‘ ’) >>> str (32) ’32’ >>> str ( ) ‘ ’ 11 Prepared by Department of Preparatory year

Math functions Module – a file that contains a collection of related functions Math module – provides most of the familiar mathematical functions 12 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 13 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 14 Prepared by Department of Preparatory year

Math functions >>> degrees=45 >>> radians=degrees/360.0*2*math.pi >>>math.sin(radians) Can use functions to compose more complex expressions x=math.sin(degrees/360.0*2*math.pi) 15 Prepared by Department of Preparatory year

Composition >>>math.sqrt(2)/ >>>minutes=hours*60 #right >>>hours*60=minutes #wrong! SyntaxError: can’t assign to operator 16 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." ) 17 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 18 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) 19 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() 20 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 21 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 23 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. 25 Prepared by Department of Preparatory year

Local variables (cont’d…) >>> line1 = 'Bing‘ >>> line2 = 'bang.' >>> cat_twice(line1, line2) Bing bang. 26 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 27 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 28 Prepared by Department of Preparatory year

Void functions >>> result = print_twice('Bing') Bing >>> print result None 29 Prepared by Department of Preparatory year

Part 3 End 30