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Python – Part 3

Python – Part 3. Functions. 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. Function Calls. Commonly “takes” an argument

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Python – Part 3

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  1. Python – Part 3 Functions

  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 - 32 Prepared by Department of Preparatory year

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

  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 Prepared by Department of Preparatory year

  5. Type conversion functions >>>float (32) 32.0 >>>float (‘3.14159’) 3.14159 >>> str (32) ’32’ >>> str (3.14159) ‘3.14159’ Prepared by Department of Preparatory year

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

  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 Prepared by Department of Preparatory year

  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 Prepared by Department of Preparatory year

  9. Math functions >>> degrees=45 >>> radians=degrees/360.0*2*math.pi >>>math.sin(radians) 0.707106781187 >>>math.sqrt(2)/2.0 0.707106781187 Prepared by Department of Preparatory year

  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 Prepared by Department of Preparatory year

  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.") Prepared by Department of Preparatory year

  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 Prepared by Department of Preparatory year

  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) Prepared by Department of Preparatory year

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

  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 Prepared by Department of Preparatory year

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

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

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

  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. Prepared by Department of Preparatory year

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

  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 Prepared by Department of Preparatory year

  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 Prepared by Department of Preparatory year

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

  24. Part 3 End

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