REPRESENTING ALGORITHMS. REPRESENTING ALGORITHMS IB DP Computer science Standard Level ICS3U

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Rolf Robertson
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1 C A N A D I A N I N T E R N A T I O N A L S C H O O L O F H O N G K O N G 2.1 Introduction 2.2 Representing Algorithms algorithm should be clear, precise, and unambiguous one possibility is to use the natural language the familiar written and spoken language the algorithm is easy to read, write, and understand can be extremely verbose, causing the resulting algorithms to be rambling, unstructured, and hard to follow unstructured, free-flowing writing style natural language without any clues to guide the reader, such as indentation, line numbering, locating the beginning of a loop or a condition can be a time-consuming task too rich in interpretation and meaning relies on either context or a reader s experiences to give precise meaning to a word or phrase desirable when writing poetry or fiction but not for writing algorithms which must always execute in the same way and produce identical results 1

2 natural language punctuations, grammar, and syntax are not interpreted age vs Age vs AGE age = 14 as in an assignment or a comparison a notation to design and represent algorithms a set of English language constructs designed to resemble statements in a programming language but that do not actually run on a computer a compromise between the two extremes of natural and formal languages simple, highly readable, and has virtually no grammatical rules it is not a formal language with rigidly standardized syntactic and semantic rules and regulations an informal design notation used solely to express algorithms 2

3 SEQUENTIAL OPERATIONS three basic sequential operations computation set the value of age to 14 set the value of c to ( a + b ) input get a value for age output print the value for age print the message for Hello World! Tasks 1. Write pseudocode versions of an algorithm that gets three data values x, y, and z as input and outputs the average of those three values. 2. Write pseudocode versions of an algorithm that inputs your current credit card balance, the total dollar amount of new purchases, and the total dollar amount of all payments. The algorithm computes the new balance, which includes a 12% interest charge on any unpaid balance. CONDITIONAL and ITERATIVE OPERATIONS for non-sequential operations such as branching and repetition to alter the normal sequential flow of control in an algorithm 3

4 CONDITIONAL OPERATIONS conditional statements are the question-asking operations if time is before 12noon then print the message Good Morning else if time is before 6pm then print the message Good Afternoon else print the message Good Evening CONDITIONAL OPERATIONS conditions are checked from the top to the bottom subsequent alternatives are considered only when the previous ones have not been satisfied ITERATIVE OPERATIONS iterative statements are repetitions and loops two types while iteration for iteration 4

5 ITERATIVE OPERATIONS: while instructions are repeated indefinitely until the terminating condition is reached the instructions may not even begin repeating if the terminating condition is reached before it the iteration begins while speed is less than 100km/h press on the gas pedal ITERATIVE OPERATIONS: do while a variation of the while iteration the instructions are executed once before the terminating condition is checked do press on the gas pedal while speed is less than 100km/h ITERATIVE OPERATIONS: for instructions are repeated within a definite number of iterations for total is less than 10 increment total by 1 5

6 Task Write an if/then/else statement that sets the variable y to the value 1 if x 0. If x < 0, then the statement should set y to the value 2. (Assume x already has a value.) Pg. 50 # 2 Write pseudocode versions of an algorithm that gets the radius r of a circle as input. Its output is both the circumference and the area of a circle of radius r. Pg. 50 # 5 Write pseudocode versions of an algorithm that is given the length and width, in feet, of a rectangular carpet and determines its total cost given that the material cost is $23/square yard. 6

7 Pg. 61 # 2 Write an algorithm that gets as input three data values x, y, and z and outputs the average of these values if the value of x is positive. If the value of x is either zero or negative, your algorithm should not compute the average but should print the error message Bad Data instead. Pg. 61 # 5 Write an algorithm that inputs the length and width, in feet, of a rectangular carpet and the price of the carpet in $/square yard. It then determines if we can afford to purchase this carpet, given that our total budget for carpeting is $500. Pg. 85 # 1 Write pseudocode instructions to carry out each of the following computational operations. 1. Determine the area of a triangle given values for the base b and the height h. 2. Compute the interest earned in 1 year given the starting account balance B and the annual interest rate I and assuming simple interest, that is, no compounding. Also determine the final balance at the end of the year. 3. Determine the flying time between two cities given the mileage M between them and the average speed of the airplane. 7

8 Pg. 85 # 11 Develop an algorithm to compute gross pay. The inputs to your algorithm are the hours worked per week and the hourly pay rate. The rule for determining gross pay is to pay the regular pay rate for all hours worked up to 40, time-and-a-half for all hours over 40 up to 54, and double time for all hours over 54. Compute and display the value for gross pay using this rule. After displaying one value, ask the user whether he or she wants to do another computation. Repeat the entire set of operations until the user says no. 8

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