How to Program

Ozymandias I met a traveler from an antique land Who said: Two vast and trunkless legs of stone Stand in the desert . . . Near them, on the sand, Half sunk, a shattered visage lies, whose frown, And wrinkled lip, and sneer of cold command Tell that its sculptor well those passions read Which yet survive, stamped on these lifeless things, The hand that mocked them, and the heart that fed. And on the pedestal these words appear: "My name is Ozymandias, king of kings: Look on my works, ye Mighty, and despair!" Nothing beside remains. Round the decay Of that colossal wreck, boundless and bare The lone and level sands stretch far away.

Characteristics of a System Input Output Proc1 Proc2 Proc3

Characteristics of a Computer System User Defined Programs CPU Peripherals Networks Operating System (O/S) Applications Software (S/W) Hardware (H/W) Computer System

Algorithms An Algorithm is a series of instructions Examples of algorithms include Musical scores Knitting patterns Row 1: SL 1, K8, K2 tog, K1, turn Recipes

Algorithms In computers we typically design an algorithm before we convert it into a computer program, and execute it.

Algorithms Problem Solving Analyse Problem (Analysis) Develop Algorithm (Design) Convert into programming language (Development)

Example Making a Cup of Tea Utilities Cup, tea bag, kettle, water, power supply, sugar, user, milk, spoon/fork.

Example Steps Organise everything together Plug in kettle Put teabag in cup Put water into kettle Turn on kettle Wait for kettle to boil Add water to cup Remove teabag with spoon/fork Add milk and/or sugar Serve

Example : Step-wise Refinement Step-wise refinement of step 4 (Put water into kettle) 4.1 Bring kettle to tap 4.2 Put kettle under water 4.3 Turn on tap 4.4 Wait for kettle to be full 4.5 Turn off tap

Example : Conditional Statement Conditional Statement so If Sugar required then add sugar to tea endif If <Condition> { <Statements> }

or Example : Conditional Statement If <Condition> { <Statements1> } else { <Statements2> }

Nested IF Statements If <Condition> then if <Condition> then <Statements> endif else if <Condition> <Statements> endif endif

Program Correctness I Syntax : Rules which define a correct structure The cat sat on the mat (legal) Sat the on the cat mat (illegal)

Program Correctness II Semantics : Rules which attach meaning to symbols The mat sat on the cat (syntactically correct, but semantically incorrect) Choose a number 1 to 14, call this number N, write down the Nth month of the year (syntactically correct, but semantically incorrect)

Program Correctness III Logical : Correct & complete description of an operation Circumference = PI * radius Syntax = correct Semantics = correct Logically = incorrect Circumference = PI * radius * radius

Iteration : WHILE Loop Consider filling the kettle WHILE kettle not full DO keep filling kettle ENDWHILE; So this will keep looping while the kettle is not full, as soon as it’s full, the loop finishes.

Iteration : WHILE Loop Consider the problem of searching for an entry in a phone book with only condition : Get first entry If this is the required entry Then write down phone number Else get next entry If this is the correct entry then write done entry else get next entry if this is the correct entry …………… .

Iteration : WHILE Loop We may rewrite this as follows: get first entry; call this entry N; WHILE N is NOT the required entry DO Get next entry; call this entry N; ENDWHILE;

Iteration : WHILE Loop General Form of WHILE While <Condition> { <Statements> }

Variables We can create a container to store values, we call these variables. A number variable can be declared as follows; int x; This means there a container created that is called ‘x’ and is of type ‘int’ (integer) If we wanted to declare a container for a single character it would be char y;

Variables To insert a value into a variable, we called this “assignment”, and we use the “=“ symbol, so x = 5; Which reads as “the integer variable x is assigned the value 5” For the character y y = “H” Which reads as “the character variable y is assigned the value H”

Increment If we do int x; x=5; To add one onto X, we can do the following; x = x + 1; Which means x gets a new value, which is the old value of x but with one added to it. We can also just do; int x = 5;

Check if number is prime How do we check if a number is prime ? If the number is X then it is prime if only 1 and X divide evenly into it. Or to restate, if the numbers 2 to X-1 do not divide evenly into X, then it is prime. Thus our program has to count from 2 to X-1 and divide that value by X, if the answer is a whole number X is not prime.

Check if number is prime N = 2; WHILE Prime > (N – 1) { IF (Prime % N eq 0){ Print “The Number is Not Prime” } N = N +1; }

Features of Algorithms Sequence Statement 1 Statement 2 Statement 3 Selection if (condition) { seq-of-stmts; } Iteration while (condition) { seq-of-stmts; }

Double Number Program DoubleNumber: { get number; print number * 2; }

Double Number (Alternative) Program DoubleNumber: { get number; answer = number * 2; print answer; }

Triple Number Program TripleNumber: { get number; answer = number * 3; print answer; }

Check for Odd/Even Program IsOddOrEven: { get number; IF(number /2 gives no remainder) THEN print “It’s Even”; ELSE print “It’s Odd”; EndIf; }

Count Next Ten Program CountToTen: { get number; StopNumber = number + 10; WHILE (StopNumber > Number) DO print number; number = number + 1; ENDWHILE; print “I’m Finished”; }

Sum Numbers 1 to 10 Program SumNos1to10: { Total = 0; Number = 1; WHILE (10 > Number) DO Total = Total + Number; Number = Number + 1; ENDWHILE; print Total; }

Check if number is prime N = 2; WHILE Prime > (N – 1) { IF (Prime % N eq 0) { Print “The Number is Not Prime”; } N = N +1; }

JAVA Keywords abstract double int strictfp boolean else interface super break extends long switch byte final native synchronized case finally new this catch float package throw char for private throws class goto protected transient const if public try continue implements return void default import short volatile do instanceof static while

Variables According to the syntax rules of Java, a name is a sequences of one or more characters. It must begin with a letter and must consist entirely of letters, digits, and the underscore character '_'. For example, here are some legal names: N n rate x15 quite_a_long_name HelloWorld Uppercase and lowercase letters are considered to be different, so that HelloWorld, helloworld, HELLOWORLD, and hElloWorLD are all distinct names. Certain names are reserved for special uses in Java, and cannot be used by the programmer for other purposes. These are the reserved words .

Assignment (I) In Java, the only way to get data into a variable -- that is, into the box that the variable names -- is with an assignment statement . An assignment statement takes the form: variable = expression ; where expression represents anything that refers to or computes a data value. When the computer comes to an assignment statement in the course of executing a program, it evaluates the expression and puts the resulting data value into the variable. For example, consider the simple assignment statement rate = 0.07;

Assignment (II) The variable in this assignment statement is rate, and the expression is the number 0.07. The computer executes this assignment statement by putting the number 0.07 in the variable rate, replacing whatever was there before. Now, consider the following more complicated assignment statement, which might come later in the same program: interest = rate * principal;

Declaring Variables (I) A variable can be used in a program only if it has first been declared . A variable declaration statement is used to declare one or more variables and to give them names. When the computer executes a variable declaration, it sets aside memory for the variable and associates the variable's name with that memory. A simple variable declaration takes the form: type-name variable-name-or-names ; The variable-name-or-names can be a single variable name or a list of variable names separated by commas. (We'll see later that variable declaration statements can actually be somewhat more complicated than this.)

Declaring Variables (II) Good programming style is to declare only one variable in a declaration statement, unless the variables are closely related in some way. For example: int numberOfStudents; String name; double x, y; boolean isFinished; char firstInitial, middleInitial, lastInitial;

Hello World Program public class HelloWorld { // A program to display the message // "Hello World!" on standard output public static void main(String[] args) { System.out.println("Hello World!"); } } // end of class HelloWorld

General Class Declaration public class program-name { optional-variable-declarations-and-subroutines public static void main(String[] args) { statements } optional-variable-declarations-and-subroutines }

Bubblesort Sorting lists of data is an important thing to be able to do

Array A list of variables is called an array. Instead of; int x1=0; int x2=0; int x3=0; int x4=0; We can do; int x[4]; And then we can access the elements as follows; x[0]=0; x[1]=0; x[2]=0; x[3]=0;

Comments To add in a few text comments into a program, to help the reader understand it, enclose the comment in “/*” and “*/”

Swapping values If x =5 and y=8, and we want to swap the two values, this will NOT work; x = y; y = x; Because if we look at these statements, what we are saying one step at a time Before : x =5 and y=8 x = y Now: x = 8 and y = 8 y = x; So same thing; x = 8 and y = 8

Swapping values So what we need to do is add another variable Z; Now if x =5 and y=8, z = x; x = y; y = z; Because if we look at these statements, what we are saying one step at a time Before : x =5 and y=8 and z =0 z = x Now: x = 5 and y = 8 and z=5 x = y; Now: x = 8 and y = 8 and z=5 y = z; Now: x = 8 and y = 5 and z=5

Modularisation If we review the Bubblesort program, it is clear that there are two main processes occurring in the program ; Sort Array Print out solution

Bubblesort - Modularised main( ) { sort_array; print_out_solution; }

Which variables are owned by which module? myarray[] is shared by everyone; outercount , innercount and swap_variable are used by sort_array a new count variable should be used to print out variable values.

Find the index where a value X is stored

Find the index where a value X is stored Assume vector is called A and is of length 10. FIND-X-IN-A i = 0 While A(i) not = X and i not =10 i = i + 1 End-While If i not = 10 Print X, " resides in vector element number ", i else Print X, " is not resident in vector A" End-if END

Searching a sorted vector Search sorted vector A of length n for value x. Process Find the middle of the vector. If the value x is greater than the middle element, search the top half of the vector. If the value x is less than the middle element, search the bottom half of the vector. If the value x is equal to the middle element, the value x has been found. If the bottom of the vector = the top of the vector the element is not there.

Constructs Subroutine which searches the vector, given the vector name, the vector top and bottom boundaries and the value being sought. In the subroutine: Test for empty vector If vector is not empty: when middle value = sought value, value is found when middle value < sought value, search upper half when middle value > sought value, search lower half Call the subroutine. Position = BINARY-SEARCH(P, Q, A, X)

BINARY-SEARCH (P, Q, A, X) if P > Q position = 0 else middle = (P+Q)/2 Case TRUE of X < A(middle): position =BINARY-SEARCH(P, middle - 1, A, X) X > A(middle): position =BINARY-SEARCH(middle + 1, Q, A, X) X = A(middle): position = middle end-case end-if END

/* program to take in an array and an element value and find the element in the array */ #define N 10 #include <stdio.h> main () { int binary_search(int,int,int [],int); int a[N]= {2, 4, 6, 8, 10, 12, 14, 16, 18, 20}; int pos, x, start, end; printf ("\nEnter the number you are looking for:"); scanf ("%d",&x); start = 0; end = N-1; pos = binary_search(start, end, a, x); if (pos >= 0) printf ("\nThe value %d is in position %d",x,pos); else printf ("\nThe value %d is not in the array",x); return 0; }

How to Program

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How to Program