Semester 2. Structured Programming. Modular Programming Top-down design. Essentials covered. More complex programming topics.

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1 Semester 2 Essentials covered Basis of problem solving More complex programming topics Modular programming Lists, tuples, strings & arrays File handling Dr. Hugh Melvin, Dept. of IT, NUI,G 1 Structured Programming Well defined algorithm Correct and efficient Easy-to-follow logic Implemented using three basic control structures.. Sequence Decision/selection Repetition.. and code organised to minimise complexity Modular Programming Dr. Hugh Melvin, Dept. of IT, NUI,G 2 Modular Programming Top-down design Divide and conquer approach Complex program divided into simpler tasks (and simpler again..and again ) Main program Holds overall program together Complexity hidden in subprograms Dr. Hugh Melvin, Dept. of IT, NUI,G 3 1

2 Sub program Each subprogram can tackle a logically separate section Advantages Easier to design, build and error check Easier to understand Easier to modify Conducive to team working Code reusability Dr. Hugh Melvin, Dept. of IT, NUI,G 4 Approach to Modular Programming Divide program into logical tasks Create a hierarchy chart Tasks and their relationships Write each module individually Once complete, tie together Test completed program Verify correct operation of each procedure Dr. Hugh Melvin, Dept. of IT, NUI,G 5 Hierarchy Chart Main program manipulates subprograms to solve overall problem Main One.1 One One.2 Two Two.1 Three.1 Three Three.2 One.2.1 One.2.2 Dr. Hugh Melvin, Dept. of IT, NUI,G 6 2

3 Modular Programming: terminology Different terminology across different languages Functions/Subroutines/Procedures etc Python Functions Built-in functions (BIF) Also Library / Factory functions User Defined Functions (UDF) Modules File containing related Python functions/statements/definitions that can be imported into your Python program No need to write your own! Have already seen examples of import math Code re use is key concept Dr. Hugh Melvin, Dept. of IT, NUI,G 7 Python Built-in Functions Have already seen quite a few.. input() & raw_input() chr(num) Converts ASCII num to character ord(chr) Takes character chr and returns ASCII value type(x) Returns data type of x len(list1) Returns length of list list1 range(10) Creates list of numbers Dr. Hugh Melvin, Dept. of IT, NUI,G 8 abs(num) Other built in functions Returns absolute value of argument num int(num) Chops off decimal point round(num) Rounds num to nearest integer pow(n1,n2,[optional]) Performs exponentiation n1 n2 and other calculations Similar to ** but latter is an operator and less powerful Dr. Hugh Melvin, Dept. of IT, NUI,G 9 3

4 Modules & built in functions Other built in functions are included in modules Need to import the module before you can use them Each module contains related functions and definitions Example Modules math random decimal array Dr. Hugh Melvin, Dept. of IT, NUI,G 10 Modules Math module Have already seen math.pi floor() Rounds to next smaller integer.. Compare with int() Random module Contains a series of pseudo random number generators eg.. randint(num1,num2) Returns an integer between num1 and num2 inclusive uniform(num1,num2) Similar but returns a float and inclusive of smaller number only Dr. Hugh Melvin, Dept. of IT, NUI,G 11 Dr. Hugh Melvin, Dept. of IT, NUI,G 12 4

5 Examples Note difference between floor, int and round 3 rd argument of pow()applies modulus operator % i.e. 8 % 7 = 1 Dr. Hugh Melvin, Dept. of IT, NUI,G 13 Lotto? Dr. Hugh Melvin, Dept. of IT, NUI,G 14 Dr. Hugh Melvin, Dept. of IT, NUI,G 15 5

6 Lotto 2? Dr. Hugh Melvin, Dept. of IT, NUI,G 16 Dr. Hugh Melvin, Dept. of IT, NUI,G 17 Random Number Generators What would be required to truly create a lotto machine? What other uses for such functions? Password/PIN no generation Random drug testing Statistical analysis Dr. Hugh Melvin, Dept. of IT, NUI,G 18 6

7 Password Generation Dr. Hugh Melvin, Dept. of IT, NUI,G 19 Dr. Hugh Melvin, Dept. of IT, NUI,G 20 Class Exercise 1. Write a program to generate a 4 digit PIN number ASCII code for 0-9 is 48 to How would you generate a password with a combination of letters and numbers? ASCII ASCII A-Z Cf Code : How long will it take to run? WCET = infinity? Dr. Hugh Melvin, Dept. of IT, NUI,G 21 7

8 User Defined Functions So far, we have been using library/built-in functions within our code What about writing your own functions to carry out a specific set of tasks i.e. user defined (UDF) Dr. Hugh Melvin, Dept. of IT, NUI,G 22 UDF Should be Easy to read Manageable length Perform specific and well defined subset of overall problem Code reusability optimised Need to think carefully about overall code design How many UDFs needed? What does each do? What information does it need to do its work? Often need to transfer information between Main program and UDF or between UDFs Dr. Hugh Melvin, Dept. of IT, NUI,G 23 Function Definition Specifies what function does Syntax def UDF_name (list of arguments): STATEMENTS Keyword def followed by UDF name Cannot use keyword for UDF name Arguments specify what info needed for UDF to do its work (may be null) Can be any number of indented statements in body Calling program invokes UDF via function call return statement may be used to return info to calling program Dr. Hugh Melvin, Dept. of IT, NUI,G 24 8

9 Dr. Hugh Melvin, Dept. of IT, NUI,G 25 Simple UDF Note UDF has no arguments UDF is defined at start but code does not run until UDF is called from calling program Note order of outputted print statements UDF must be defined before it is invoked This is termed the flow of execution of the program Dr. Hugh Melvin, Dept. of IT, NUI,G 26 Arguments and Parameters Argument Used to pass info between the calling program and UDF CalAverage(Num1, Num2, Num3) CalNetSal(Hours,Rate,Taxrate) Parameter Dummy variable in the UDF that receives the argument CalAverage(X, Y, Z) CalNetSal(Hrs, Rt,Tax) Dr. Hugh Melvin, Dept. of IT, NUI,G 27 9

10 Dr. Hugh Melvin, Dept. of IT, NUI,G 28 Note that x,y,z are only known in UDF Could have used same names.. but would be separate variables Relates to Variable Scope.. Complex issue Dr. Hugh Melvin, Dept. of IT, NUI,G 29 Arguments and Parameters Number must match Eg. Num1.. X Num2. Y Num3. Z Names can be different or same..local scope Dr. Hugh Melvin, Dept. of IT, NUI,G 30 10

11 Local variables & Scope Local variable One that is used only in the main program or the UDF in which it is defined not known elsewhere We have used local variables in programs so far Can have local variables in main or UDF with the same name without any side effects..useful for team programming Can also have variables with wider scope Any examples where this might be useful? Dr. Hugh Melvin, Dept. of IT, NUI,G 31 Dr. Hugh Melvin, Dept. of IT, NUI,G 32 x,y or z are not known in main code error Dr. Hugh Melvin, Dept. of IT, NUI,G 33 11

12 Dr. Hugh Melvin, Dept. of IT, NUI,G 34 Note Variable x is changed to 10 in UDF.. But num1 unchanged at 3 in calling program num3 is visible in UDF and can be referenced BUT cannot be changed once referenced Dr. Hugh Melvin, Dept. of IT, NUI,G 35 Dr. Hugh Melvin, Dept. of IT, NUI,G 36 12

13 Attempt to change num3 after referencing from calling program is illegal Can assign value to variable of same names in UDF but will not be referencing them from calling program Dr. Hugh Melvin, Dept. of IT, NUI,G 37 Dr. Hugh Melvin, Dept. of IT, NUI,G 38 num3 in UDF is a separate variable num3 is calling program unchanged Dr. Hugh Melvin, Dept. of IT, NUI,G 39 13

14 Dr. Hugh Melvin, Dept. of IT, NUI,G 40 num1,num2,num3 are separate variables in UDF from calling program General Rule: Treat as separate.. Cf Last example Dr. Hugh Melvin, Dept. of IT, NUI,G 41 Dr. Hugh Melvin, Dept. of IT, NUI,G 42 14

15 Code works ok.. no arguments sent UDF references variables from calling program Cannot reassign BUT UDF programmers must know exact variable names Avoid generally Dr. Hugh Melvin, Dept. of IT, NUI,G 43 Class Exercise Write a UDF that calculates the net income of an employee given number of hrs worked, rate per hour and tax rate. In UDF definition CalNetSal(Hrs, Rt,Tax) In main code Read in data and call UDF CalNetSal(Hours,Rate,Taxrate) Dr. Hugh Melvin, Dept. of IT, NUI,G 44 Pass by Reference In python, arguments are passed by assignment (object reference) Calling program and UDF share objects by reference Different names can be used as seen already Some arguments such as integers are immutable and so cannot be changed so it is like sending a copy to the UDF Other arguments such as lists are mutable so can be changed in UDF (like traditional C pass by reference) Dr. Hugh Melvin, Dept. of IT, NUI,G 45 15

16 return statement in UDF Used to pass information back to the calling program Eg. Send 3 numbers to UDF and use return to send average back to calling program In main program Average= CalAverage2(num1, num2, num3) In UDF def CalAverage2( x, y, z): avg=(x+y+z)/3 return avg Dr. Hugh Melvin, Dept. of IT, NUI,G 46 Dr. Hugh Melvin, Dept. of IT, NUI,G 47 Note flow of execution return sends result back Result assigned to variable in Calling Program Dr. Hugh Melvin, Dept. of IT, NUI,G 48 16

17 Class Exercise Rewrite the UDF that calculates the net income of an employee given number of hrs worked, rate per hour and tax rate. Net salary should now be returned to calling program.. In main code Read in data and call UDF NetSal= CalNetSal(Hours,Rate,Taxrate) Dr. Hugh Melvin, Dept. of IT, NUI,G 49 UDF Key concept in structured programming Top-down design Use logical divisions within the program Use hierarchy chart Aids code creation, debugging, maintainability, user-friendliness and reusability Facilitates team work Dr. Hugh Melvin, Dept. of IT, NUI,G 50 17