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Introduction to Computer Systems

Introduction to Computer Systems. Lecturer: Steve Maybank Department of Computer Science and Information Systems sjmaybank@dcs.bbk.ac.uk Autumn 2014 Week 8b: Pseudo Code and Algorithms. Algorithms.

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Introduction to Computer Systems

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  1. Introduction to Computer Systems Lecturer: Steve Maybank Department of Computer Science and Information Systems sjmaybank@dcs.bbk.ac.uk Autumn 2014 Week 8b: Pseudo Code and Algorithms Birkbeck College, U. London

  2. Algorithms • Definition: an algorithm is an ordered set of unambiguous, executable steps that defines a terminating process. • The steps are often called instructions. • Termination is by a special instruction: halt. Brookshear section 5.1

  3. Terminology • Programming language: system for representing algorithms in a human readable form suitable for conversion to machine code. • Program: representation of an algorithm in a programming language. • Pseudo code: system for the informal representation of algorithms. • Hardware: device to carry out the steps or instructions in a program. Brookshear sections 5.1 and 5.2

  4. Representation of an Algorithm The same algorithm can be represented in many different ways: • F = (9/5)C+32 • F <- (9*C)/5+32 • To obtain F multiply C by (9/5) and then add 32. • Lookup table for Fahrenheit and Centigrade Brookshear Section 5.1

  5. Program 1 for XOR Input: binary digits a,b Output: a XOR b • If a==0 and b==0 Return 0 • If a==0 and b==1 Return 1 • If a==1 and b==0 Return 1 • If a==1 and b==1 Return 0 • Halt Birkbeck College, U. London

  6. Program 2 for XOR Input: binary digits a,b Output: a XOR b • If a==b Return 0 • Else Return 1 • Halt Birkbeck College, U. London

  7. Program 3 for XOR Input: binary digits a,b Output: a XOR b • Convert a to the integer ia • Convert b to the integer ib • ic=remainder on dividing ia+ib by 2 • Return ic • Halt Birkbeck College, U. London

  8. Pseudo Code for Algorithm Representation • Very useful informal representation of algorithms. • Preferable to using program code when designing an algorithm. • Basic structures (assignment, loops, arrays …) are similar across many programming languages. Brookshear section 5.2

  9. Pseudo Code: assignment • General form name <- expression • Examples funds <- current + savings x <- 10 Brookshear, Section 5.2

  10. Pseudo Code: conditional selection • General form if(condition)then(activity) else(activity) • Example if (year is leap year) then d <- total divided by 366 else d <- total divided by 365 • Key or reserved words in bold Brookshear section 5.2

  11. Pseudo Code: repeated execution • General form while(condition)do(activity) • Example while (tickets remain) do (sell a ticket) Brookshear section 5.2

  12. Pseudo Code: indentation • if (not raining) then (if (temperature = hot) then (go swimming) else (play golf) ) else (watch television) • if (not raining) then (if (temperature=hot) then (go swimming) else (play golf)) else (watch television) Brookshear section 5.2

  13. Procedure • Definition: set of instructions which can be used as a single unit in different contexts. • A procedure call contains the name of the procedure and any data supplied to the procedure • A result calculated by the procedure can be obtained using a return statement. Brookshear, Section 5.2

  14. Example of a Procedure procedure temp(c) f=(9/5)*c+32; return f; endProcedure f1=temp(34) Definition of a procedure Procedure call Birkbeck College

  15. Pseudo Code: procedures • General type procedurename • Examples procedure helloWorld //no parameters procedure sort (List) // parameter List Brookshear Section 5.2

  16. Pseudo Code: alternatives to brackets • procedurename (activity) endProcedure • while (condition) do (activity) endDo endWhile Brookshear Section 5.2

  17. Exercise 1 • Write an algorithm in pseudo code to carry out the following task: input: a 1-dimensional array A of integers output: the integer -1 if A[i]≥ A[i+1] for 0≤i≤Length[A]-2, otherwise the least integer j such that A[j]<A[j+1]. Birkbeck College

  18. Exercise 2 • Design an algorithm for finding all the factors of a positive integer. For example, in the case of the integer 12, your algorithm should report the values 1,2,3,4,6 and 12. Brookshear Ch 5 review problems 11

  19. Exercise 3 • The following is an addition problem in decimal notation. Each letter represents a different digit. Which letter does each digit represent? XYZ + YWY ==== ZYZW Brookshear Ch 5 review problems 15

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