Constructing Algorithms and Pseudocoding: A Step-by-Step Guide, Study notes of Construction

Download Constructing Algorithms and Pseudocoding: A Step-by-Step Guide and more Study notes Construction in PDF only on Docsity! 1Dept. of Computer and Information Sciences, IUSB Constructing Algorithms and Pseudocoding This document was originally developed by Professor John P. Russo Purpose: # Describe the method for constructing algorithms. # Describe an informal language for writing PSEUDOCODE. # Provide you with sample algorithms. Constructing Algorithms 1) Understand the problem A common mistake is to start writing an algorithm before the problem to be solved is fully understood. Among other things, understanding the problem involves knowing the answers to the following questions. a) What is the input list, i.e. what information will be required by the algorithm? b) What is the desired output or result? c) What are the initial conditions? 2) Devise a Plan The first step in devising a plan is to solve the problem yourself. Arm yourself with paper and pencil and try to determine precisely what steps are used when you solve the problem "by hand". Force yourself to perform the task slowly and methodically -- think about <<what>> you are doing. Do not use your own memory -- introduce variables when you need to remember information used by the algorithm. Using the information gleaned from solving the problem by hand, devise an explicit step-by-step method of solving the problem. These steps can be written down in an informal outline form -- it is not necessary at this point to use pseudo-code. 3) Refine the Plan and Translate into Pseudo-Code The plan devised in step 2) needs to be translated into pseudo-code. This refinement may be trivial or it may take several "passes". 4) Test the Design Using appropriate test data, test the algorithm by going through it step by step. Think about ordinary sets of input data as well as those which are "degenerate". If you find errors correct them. If serious errors are found, it may be necessary to scrap your pseudo-code and go back to step 2). 2Dept. of Computer and Information Sciences, IUSB An Informal Language for Writing Algorithms (Pseudocoding) # An algorithm is a finite series of unambiguous instructions that when given a set of input values produces a desired result. # A desirable property of an algorithm is that it be easy to translate into a programming language. # Algorithms are written during the programming process, which involves the steps below: Problem analysis Algorithm construction Translation of algorithm into programming language Testing and debugging of the program # The ability to write an algorithm is one of the most important skills to be learned by students taking this course! # It is a well known fact of programming life that if the algorithm construction is not done well, a programmer will probably spend excessive amounts of time debugging, i.e. finding and fixing errors in the program. # There are many ways of expressing algorithms. In this class we will adopt an informal algorithmic language which is powerful, concise, and easy to translate into the Turbo Pascal programming language. We will refer to this language as pseudocode. The textbook also discusses pseudocode, as well as flowcharts, which are another way of describing algorithms. # Before specifying our language formally, we'll look at some examples and discuss a few concepts and definitions that will be used below. Example 1: An algorithm that asks for a number, then prints its square. print "Enter a number" read N print "The square of " N " is " N * N Example 2: An algorithm that asks for a number, then tells whether it is odd or even. print "Enter a number" read Number if Number mod 2 = 0 then print N "is even" else print N "is odd" end if 5Dept. of Computer and Information Sciences, IUSB 3) Output statements are used to display information determined by an algorithm. There are two sorts of items which we will need to display. a) The value of a variable or expression. b) A string of characters. Several kinds of notation can be used to display the value of a variable X. These include output X display X write X print X Examples 1: print "Hello" Example 2: print "The value of X is " X 4) The if-then statement allows allow other statements to be executed conditionally, i.e. if a certain condition is met. The if-then statement is of the form if <condition> then <statements to be executed if condition is true> end if The "end if" serves a delimiter, to mark the end of the statements to be executed if the condition is true. Example 1: if Sum > 0 then print "The sum is positive" end if Example 2: if N < 0 then print "Please enter a positive number" input N end if 6Dept. of Computer and Information Sciences, IUSB 5) The if-then-else statement is a more powerful version of the if-then statement. It is of the form: if <condition> then <statements to be executed if condition is true> else <statements to be executed if condition is false> end if Example: if Y < 0 then print "Division by 0 is not allowed" else Quotient = X/Y print "The quotient is " Quotient end if 6) The while statement allows a group of statements to be repeated while a condition is true. It is of the form: while <condition is true> <statements to be executed while condition is true> end while Example: Algorithm to add numbers entered by user. print "Enter a series of number" print "Enter a non-positive number to end." Sum = 0 read N while N > 0 print "Adding " N "to " Sum Sum = Sum + N print "Enter next number " read N end while print "The sum of the numbers you entered is " Sum 7Dept. of Computer and Information Sciences, IUSB 7) The repeat-until statement allows a group of statements to be repeated until a condition is true. It is of the form repeat <statements to be executed until condition is true> until <condition is true> It is important to note that the statements in the repeat loop body are always executed at least once. Example: Algorithm to read integers entered by user and find the sum. Input ends when a non-positive number is entered. print "This algorithm adds numbers that you enter." print "Enter 0 or negative number to end." Sum = 0 repeat print "Enter next number" input N Sum = Sum + N until N <= 0 print "The sum of the numbers you entered is " Sum 8) The for statement allows a group of statements to be repeated an exact number of times. It is of the form for i = 1 to 10 <statements to be executed> end for The loop variable "i" is automatically incremented by 1, every time the loop is executed. Example: Algorithm to read 10 integers entered by user and find their sum. Input ends after the 10th. number is entered. print "This algorithm adds 10 numbers that you enter." Sum = 0 for i = 1 to 10 print "Enter a number" input N Sum = Sum + N end for print "The sum of the numbers you entered is " Sum