CARIBBEAN EXAMINATIONS COUNCIL CARIBBEAN SECONDARY EDUCATION CERTIFICATE CSEC

CARIBBEAN EXAMINATIONS COUNCIL CARIBBEAN SECONDARY EDUCATION CERTIFICATE (CSEC) INFORMATION TECHNOLOGY SYLLABUS TEACHER TRAINING WORKSHOP JANUARY 20 th – 21 st, 2010 ST. LUCIA

PROBLEM–SOLVING CSEC INFORMATION TECHNOLOGY

INTRODUCTION How are instructions given to the computer? Computer programs – finite set of precise instructions written in a computer language Before program is written we must find a way to solve the problem After figuring out how to solve the problem we then translate the solution into a language meaningful to the computer

INTRODUCTION Giving precise, unambiguous instructions is not inherent in human nature Consider the following: Proceed a mile or so down the road until you reach the roundabout. Turn left at the roundabout and follow the road until you reach a green house on the right hand side. The Post office is about the 3 rd or 4 th building on the right after the green house. You can’t miss it!

INTRODUCTION Problem-solving statements/ instructions should be: Precise Unambiguous Must be in a logical sequence Finite, i. e. terminate after a definite number of steps Teachers must ensure that students have a good grasp of what these attributes mean Students should engage students in problem-solving exercises involving everyday problems they can relate to.

PROBLEM-SOLVING ON THE COMPUTER Examples of everyday problems: v Write a recipe for making a cheese sandwich v Write instructions to teach your mom how to retrieve voice message from a generic cell phone v Write instructions to give directions to a visitor to get to the nearest hospital, starting from the school premises v Write instructions to tell your grandparent how to download music from the Internet.

PROBLEM-SOLVING ON THE COMPUTER �The design of any computer program involves two phases: �The Problem-solving Phase �The Implementation Phase

PROBLEM-SOLVING ON THE COMPUTER � Problem-Solving Phase comprises the following steps: Define the problem Find a solution to the problem Evaluate alternate actions Represent the most efficient solution as an algorithm Test the algorithm for correctness

DEFINING THE PROBLEM �The problem must first be defined: Ensure that students understand the Problem �Defining the problem is the first step towards finding a solution �It helps the programmer to understand what is given and what is required � If the programmer does not fully understand what is required, he/she cannot produce the desired solution to the problem

DEFINING THE PROBLEM One of the biggest challenges faced in problem-solving is understanding the problem to be solved. Defining the program can be accomplished by decomposing the problem into three key components: 1. What is given (the inputs) 2. The tasks that must be performed (processing) 3. The expected results (the outputs)

THE DEFINING DIAGRAM Define the problem by constructing a defining diagram A table with three columns which represents the three components: input, processing and output. E. g. A program is required to read three numbers then calculate and print the sum. INPUT PROCESSING OUTPUT 3 numbers e. g. Num 1, num 2, num 3 1. Get 3 numbers 2. Add numbers together 3. Print Sum

THE DEFINING DIAGRAM Input Output Processing • Refers to the source data provided • Keywords: read, accept, input, given • Refers to the end result required • Keywords: display, print, output, produce • Refers to the actions performed to achieve the required output • Answers the question – What must I do with the input in order to produce the desired output

THE DEFINING DIAGRAM In the defining diagram, the actions must be listed in a logical sequential manner All necessary actions must be explicitly stated The processing section is not the solution to the problem. It’s the actions that must be done in order to solve the problem Note that in some cases the input, processing and output statements might not be as explicitly stated.

PROBLEM 2 �E. g. of problem not clearly stated to facilitate student identification of the problem: Given three integers representing the age of three boys respectively, write a solution to find their average and also determine the age of the oldest boy.

PROBLEM 2 �This problem statement could have been stated more precisely to read: Given three integers A, B, C, representing the age of three boys respectively, write a solution to find and display their average as well as the age of the eldest boy.

DEFINING DIAGRAM Two major tasks to be performed, each consisting of multiple actions: INPUT PROCESSING 3 integers 1. Read/accept/get 3 integers Say age 1, age 2, 2. Find the average of the three age 3 integers 3. Find the highest age 4. Print the average age, highest age OUTPUT Average-age Highest-age

PROBLEM 3 The cost of a new car is the sum of the wholesale cost, the sales tax and the dealer’s percentage mark-up. Assuming that the dealer’s mark-up is 10 percent of the wholesale cost and the sales tax is 6 percent, write an algorithm to read the wholesale cost of the car and print the cost to the consumer.

PROBLEM 3 Here, input and output data are clearly stated. To determine the processing steps we may ask “What should I do with wholesale cost in order to find the cost to the consumer? ” INPUT PROCESSING OUTPUT Wholesale-cost 1. Read/get wholesale-cost Consumer-Cost 2. Calculate dealer’s markup 3. Calculate the sales tax 4. Find the sum of the wholesale-cost, the dealer’s mark-up and the sales tax 5. Print the results

THE PROBLEM WITH PROBLEM SPECIFICATION Many real-world programming problems are not always precise Initial descriptions are often vague, and ambiguous Students should be encouraged to evaluate problem statements and ask questions if they are perceived as ambiguous Also, some information may be implicit in the problem statement and therefore should be taken into account when defining the problem

THE PROBLEM WITH PROBLEM SPECIFICATION Have students evaluate several problem statements, ranging from simple to complex, some precise, some ambiguous, lacking pertinent information. Here are some examples of imprecise problem statements: �#4: Write a program to print a list of all the employees who have been in the company for over five years �#5: Write a program that reads a file that contains information on the height, weight and age of 100 children. The program should print the names of all the children who are overweight.

FINDING A SOLUTION TO THE PROBLEM Introduction The Concept of Variables Choosing Variable Names Initialization of Variables

FINDING A SOLUTION TO THE PROBLEM INTRODUCTION Now that we have defined the problem, we know what we need to do. We must now figure out how to do it. A problem can have many different solutions Let students arrive at one that works, however longwinded Once a solution is found, it can then be reviewed to see how it can be optimized, or made more efficient The first approach in deriving a solution to a problem is to do the problem by hand, noting each step as you proceed.

FINDING A SOLUTION TO THE PROBLEM INTRODUCTION Problem #6: Find and print the average of three numbers Define the problem INPUT PROCESSING OUTPUT 3 Numbers Say num 1, num 2, num 3 Read/get 3 numbers Find the average Print average Create sample data: sample input data 5, 3, 25 Execute each processing step in the defining diagram manually

FINDING A SOLUTION TO THE PROBLEM INTRODUCTION �Having completed a manual solution to the problem, the next step is to write your solution as a sequence of instructions �Initial solution �Get the first number, call it num 1 �Get the second number, call it num 2 �Get the third number, call it num 3 �Add num 1 + num 2 + num 3 �Divide result by 3 �Print result �Stop

FINDING A SOLUTION TO THE PROBLEM THE CONCEPT OF VARIABLES In our solution to problem #6, we did not tell the computer where to put the result of the add operation. The result must be stored somewhere so it can be accessed later. Grandma’s fluffy pancakes: Method: Sift flour in a strainer. . . Melt the butter in a frying pan. . . Pour batter into a mixing bowl. . . Just as we need containers to hold or store ingredients, likewise, in computation, we need something to store or hold the values we manipulate

FINDING A SOLUTION TO THE PROBLEM THE CONCEPT OF VARIABLES In the computer, values are stored in memory locations. There are many memory locations, so in order to keep track of where our values are stored we need to place a label or identifier on a particular memory location. The label or identifier is called a variable. A variable is a symbolic name assigned to a memory location that stores a particular value.

FINDING A SOLUTION TO THE PROBLEM THE CONCEPT OF VARIABLES When we say num 1, num 2, etc we are actually defining a variable or an identifier for each number, so that we can refer to (access) it later. We need to tell the computer where to put the result of the addition of num 1, num 2, num 3 Add num 1 + num 2 + num 3 storing in Sum This would tell the computer that the result of the computation should be stored in a memory location called Sum.

FINDING A SOLUTION TO THE PROBLEM THE CONCEPT OF VARIABLES Likewise, we must address a similar issue in the last two statements where we stated, Divide result by 3 Print result Computations are performed by the ALU, part of the CPU and results are temporarily stored in registers within the cpu These values must be stored in memory locations if they are to be accessed later

THE CONCEPT OF VARIABLES The word variable is derived from the verb ‘to vary’. This means that the value stored in a particular location can change from time to time, although the label remains the same. When new values are placed into previously assigned memory locations, the old values are replaced by the new.

FINDING A SOLUTION TO THE PROBLEM THE CONCEPT OF VARIABLES �We can illustrate the concept of a variable by using diagrams to show the memory locations: Memory locations Num 1 Variables Num 2 Num 3 5 3 25

FINDING A SOLUTION TO THE PROBLEM THE CONCEPT OF VARIABLES We can now revise the initial solution to include the new variables called sum and average Get three numbers, say num 1, num 2, num 3 Add num 1, num 2, num 3, and store in Sum Divide Sum by 3, store in Average Print Average

THE CONCEPT OF VARIABLES CHOOSING VARIABLE NAMES It’s good practice to choose variable names that reflect the kind of data that is being stored It helps the programmer as well as the reader to understand the solution better It’s important that students grasp the concept of variables very early in the problem-solving phase.

EVALUATE ALTERNATIVE SOLUTIONS Having found a solution to a problem the programmer should proceed to explore alternative solutions The aim is to arrive at the most efficient solution The initial solution is usually the first that comes to mind, but is not always the best solution The programmer must evaluate all feasible alternatives and choose the optimal solution

EVALUATE ALTERNATIVE SOLUTIONS Points to consider when developing alternative solutions. Can the result be derived differently? Can the solution be made more general? Would it work if there were 100 integers, instead of 3? Can the solution be used for another problem? E. g. Average temperature or average grade? Can the number of steps be reduced and still maintain the logic? Can it be made more robust? What would happen if incorrect data is entered?

REDUCING THE NUMBER OF STATEMENTS We can reduce the number of statements by combining two arithmetic statements into one: 1. Add num 1, num 2, num 3 can be written as Sum = num 1 + num 2 + num 3 (the variable sum is assigned the value of num 1 plus num 2 plus num 3) 2. Divide sum by 3 storing in average can be written Average = sum/3 We can combine the two statements to produce: Average = sum/3

REVISED SOLUTION Option 2 Get num 1, num 2, num 3 Average = (num 1 + num 2 + num 3)/3 Print Average Stop Option 1 Get num 1, num 2, num 3 Sum = num 1 + num 2 + num 3 Average = sum/3 Print Average Stop The most efficient solution should have the following attributes: • Should be maintainable • Should be memory efficient • Should be robust

INITIALIZATION OF VARIABLES Variables that are used as counters or used to store totals should always be assigned an initial value of 0 before they are incremented. This is called initialization E. g. Count = 0 This ensures that the variable is cleared of any values that may have been assigned in a previous execution of the program.

WHAT IS AN ALGORITHM? At this time, we can now introduce the term ‘algorithm’. An algorithm is a sequence of precise instructions for solving a problem in a finite number of steps. Properties of an algorithm: �precise, �unambiguous, �logically sequenced, �give the correct solution an all cases, and �eventually end.

WHAT IS AN ALGORITHM? Algorithmic structure Header : Algorithm’s name or title Declaration : Brief description of algorithm and variables used. i. e. A statement of purpose as well as the initialization of variables Body : Sequence of steps Terminator : An end statement

ALGORITHMIC STRUCTURE Problem: Write an algorithm that prompts a student to enter his/her name and age, accepts the name and age and then display a welcoming message on the screen such as “hello Michael! You are 16 years old!” Write the algorithm identifying the header, declaration, body and terminator.

ALGORITHMIC STRUCTURE Algorithm Student data This algorithm displays a student’s name and age on the screen. Start Display “Enter your name: ” Accept Name Display “Enter your age: ” Accept Age Display “Hello”, Name Display “You are”, Age, “years old” Stop {Header} {Declaration} {Body} {Terminator}

Flowchart Pseudocode Gives view of program structure and logic flow Precise, resembles programming code Beginners can follow program flow much easier Better to represent longer, more complex algorithms

FLOWCHARTS VERSUS PSEUDOCODE Usage is a matter of preference for experienced programmers Students should use both flowcharts as well as pseudocode to represent algorithms Flowcharts must use special geometrical objects that designate the basic steps of a program: Input/Output Processing/ Assignment Decision Start/ Stop

PSEUDOCODE Start Get num 1, num 2, num 3 Average (num 1 + num 2 + num 3)/3 Print Average Stop FLOWCHART Start Read num 1, num 2, num 3 Average (num 1+num 2+num 3)/3 Print Average Stop

SELECTION STRUCTURES The IF…THEN selection statement Syntax: IF (expression) THEN {Statement (s)} Execute this statement if logical expression is TRUE This construct is used when no action needs to be performed if the expression returns false.

SELECTION STRUCTURES This requires only that the expression “Well done” be printed should the student score 50 marks or more. Get Mark IF Mark >= 50 THEN Print “Well done” Stop No action is intended should the mark be less than 50

SELECTION STRUCTURES IF … THEN … ELSE construct syntax: IF (expression) THEN {Statements} executed only if condition is TRUE ELSE {Statements} executed only if condition is FALSE ENDIF Only one group of statements could be executed each time the program is executed.

SELECTION STRUCTURES Get Mark IF Mark >= 50 THEN PRINT “ Well done” ELSE PRINT “Must do better” ENDIF Stop “Well done” will be printed should the student’s mark be 50 or greater. If the mark is less than 50 then the statement “Must do better” would be printed.

SELECTION STRUCTURES A worker in a clothing factory is paid an additional incentive according to the amount of shirts she folds. The basic salary is $300. 00 per week. Should the worker fold 500 or more shirts per week, an additional 15%, is added to her basic salary. If the worker folds less than 500 shirts however, she is paid just the basic salary. Write an algorithm to determine and print the salary of the worker.

SELECTION STRUCTURES Write an algorithm to accept any two numbers, the first a numerator and the second the denominator. If the second is equal to zero print “Cannot divide by 0”. Otherwise divide the first number by the second and print the answer.

SELECTION STRUCTURES A teacher has realized an error in his marking of students’ scripts. He has therefore decided to credit all students who got 55 marks and under with an additional 10 marks. Those who got more than 55 would be credited with an additional 15 marks. Write an algorithm to accept the mark from a student in a class. Calculate and print the student’s adjusted mark.

SELECTION STRUCTURES A store offers customers a discount of 15% if the cost of any item purchased is equal to or exceeds $200. Should the cost be less than $200. 00 however, then a 5% discount is applied. Write an algorithm to accept the cost of an item. Apply the relevant discount and calculate and print the net cost (cost – discount).

REPETITION STRUCTURES Repetition or Loop or Iteration structures allow statements to be repeated a fixed number of times or until a stated condition evaluates to false. There are three repetition constructs FOR Loop • (counted) WHILE Loop REPEAT Loop • (conditional)

REPETITION STRUCTURES The FOR loop construct syntax: FOR <counter> = <start value> TO <end value> DO {Statements} ENDFOR e. g. FOR X = 1 to 10 DO {Statements to be executed} ENDFOR

REPETITION STRUCTURES The FOR loop is used when the required number of iterations (loops) is known beforehand The statements are repeated as long as the value of the identifier specified by the <counter> element does not exceed the value specified by <end>. A comparison is made each time the loop iterates to check whether the <end> is exceeded. If it is not then the counter value is incremented by one (the default step value)

REPETITION STRUCTURES The 12 workers employed in the folding section of a clothing factory are each paid a basic salary of $500. 00 per week. An incentive is offered according to the amount of shirts folded. Should a worker fold 600 or more shirts per week a bonus of 20%, is added to his/her basic salary. If a worker does not fold as much as 600 shirts however, he/she is paid just the basic salary. Write an algorithm to determine and print the salary of each worker.

REPETITION STRUCTURES A teacher has realized an error in his marking after having marked all his students’ scripts. He has therefore decided to credit all students who got less than 60 with an additional 5 marks. Those who got 60 and over would be credited with an additional 15 marks. Write an algorithm to accept the initial marks obtained by the 20 students in the class. Calculate and print each student’s adjusted mark. The teacher would like to determine the class average after the marks are credited. Should the class average exceed 60, “Instructions well received” should be printed. Otherwise, “Reconsider Teaching Methods” should be printed.

REPETITION STRUCTURES For Loop A store offers customers a discount of 15% if the total cost of items purchased is equals or exceeds $400. Should the total be less than $400. 00 then 5% discount is given. Write an algorithm to accept the costs of five items. Calculate and print the total cost. Apply the relevant discount and calculate and print the net cost (total cost – discount).

REPETITION STRUCTURES For Loop Write an algorithm to calculate and print the total amount of money earned by a family doctor at the end of a six-day work week. The amount earned each day is dependant on the number patients he sees each day. To see the doctor a patient must pay a fee of $120. 00

REPETITION STRUCTURES For Loop Write an algorithm to print the seven-times table for numbers 1 to 15 Write an algorithm to calculate and print the square of all even numbers between 1 and 20. The algorithm must also calculate and print the sum of all the squares.

REPETITION STRUCTURES For Loop Write an algorithm to accept the scores made by each player on a cricket team. The algorithm should determine and print the highest score made as well as the team average score.

REPETITION STRUCTURES WHILE Loop The WHILE Loop construct syntax: INPUT Value WHILE (expression) Do {statements} executed if Boolean argument in logical expression is true INPUT value to be tested ENDWHILE

REPETITION STRUCTURES WHILE Loop In processing, the argument (condition) between WHILE and DO is tested to see if it is true. If the condition returns true, the statement(s) that follow DO will be executed. If the condition returns false, execution of the WHILE statement is stopped. It is possible for the loop never to be executed if the initial input renders the condition to be proven false.

REPETITION STRUCTURES WHILE Loop Write an algorithm that reads a number of arbitrary integers and find the average of all numbers read. The program should be terminated if the sentinel value 999 is entered.

REPETITION STRUCTURES WHILE Loop Write an algorithm to accept an unspecified amount of integers. Calculate and print the sum of the numbers entered. The algorithm should also determine and print the highest number entered. The process should be terminated if the number entered exceeds 1000.

REPETITION STRUCTURES WHILE Loop Write an algorithm to accept the price of items purchased at a supermarket. The entry of items should continue as long as the price entered is not $101. 00. Calculate and print the total cost of items purchased.

REPETITION STRUCTURES REPEAT Loop Repeat Loop construct syntax: REPEAT {statements to be executed} UNTIL (Boolean expression) The repeat loop is used to iterate a sequence of events until a specific condition becomes true. The statements in the body of a Repeat loop are executed at least once, because the test of the condition takes place at the end of the loop.

TRACE TABLES State what is printed when the following algorithm is executed. FOR X = 1 to 3 DO FOR Y = 1 to 3 DO Product = Y * X Print Product ENDFOR

TRACE TABLES What’s printed by the following Algorithm? AMOUNT = 2 G = 4 WHILE AMOUNT < 50 Do G = G*2 AMOUNT = AMOUNT + G Print AMOUNT, G ENDWHILE

TRACE TABLES INPUT A, B, C A = B + C B = A – C C = A + B IF A > B THEN C = A – B ELSE C = B – A ENDIF Print A, B, C State what is printed by the algorithm above when the following data is input: (i) 5, 7, 9 (ii) 9, -7, 5

TRACE TABLES Complete the trace table based on the following algorithm. B = 2 C = 4 Total = 3 WHILE B <= 45 Do B = B + C C = C + B Total = Total + C ENDWHILE Print Total B C TOTAL 2 4 3

LIST PROCESSING USING ARRAYS What is an Array? Accessing the Elements in an Array Initializing Arrays Reading Values into an Array Displaying Array Values Traversing Arrays Manipulating Arrays

WHAT IS AN ARRAY? A group of data items that are all of the same type Contains any number of storage locations to store variables of the same type A single name (identifier) is used to identify an array The storage locations in an array are uniquely identified by using a subscript

WHAT IS AN ARRAY? Elements in an array are organized in sequence Elements can be accessed directly by specifying their position in the sequence, using the subscript or index If only one index (subscript) is used the array is called a one-dimensional array

WHAT IS AN ARRAY? �We can illustrate the one-dimensional array as follows. Array Temp[1] 45 Temp[2] 32 Temp[3] 100 Temp[4] 98 Temp[5] Data values in adjacent memory locations

ACCESSING ELEMENTS IN AN ARRAY �Elements can be accessed individually by specifying the name of the array followed by the index or subscript. �A special variable must be declared as the index of the array (i, j, k, are commonly used), e. g. Temp[i] �Using the index, the elements in an array can be manipulated the same way that we manipulate ordinary variables.

INITIALIZING ARRAYS �Set i to 1 �Repeat 10 times � Temp[i] 0 � Increment i End-repeat First iteration i = 1 and Temp[1] would be assigned 0 Second iteration i = 2 and Temp[2] would be assigned 0

READING VALUES INTO AN ARRAY �Read 10 values into the array Temp �Set i to 1 �For i = 1 to 10 Do � Read Temp[i] End-For 1 2 3 4 5 6 7 8 9 10 45 32 100 98 60 37 30 28 18 75

DISPLAY ARRAY VALUES �Writing or printing values stored in arrays is similar to reading values into an array. �We would already know the number of items stored �Set i to 1 �Repeat 10 times � Display Temp[i] � Increment i End-repeat

TRAVERSING ARRAYS �Moving through the array in a sequential manner in order to manipulate the elements in some way. �Array is traversed when we print the elements, search the elements for a particular item, sort the list of items in a specific order, etc. �Very often we traverse an array to search for an item

LINEAR SEARCH � Set Size to 20 � Set Target to 45 � Set index to 1 � Set Found to false � While ((Found is false) and (Index <=Size)) � If (Temp[index] = Target) then � {if the value is found set the flag to true} Set Found to true Else increment index � End-while � If (found = true then {otherwise move on to the next element in the array} � display “Target found at location”, index Else display “Target not found” Stop.

TOP-DOWN DESIGN METHODOLOGY �What is Top-Down Design? �Hierarchy Charts �Sub-dividing a Problem into Modules �Steps in Modularization �Representing Modules in Pseudocode and Flowcharts �Communication between Modules �Advantages of Top-Down Design Method

FROM ALGORITHMS TO PASCAL PROGRAMS Translating Algorithms to Specific Programming Language Structure of a Pascal Program Pascal in a Nutshell Translating Pseudocode into Pascal Code Summary