Structured Problem Solving 2009-2010

www.hope.ac.uk Faculty of Sciences and Social Sciences

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Structured Problem Solving2009-2010

Week 8: Java: Selection and Repetition

Stewart Blakeway

FML 213

[email protected]


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Java: Selection & Repetition

Section 7: Pages 88 to 102

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What we have done already• Seen what an algorithm is

– a set of instructions that, if carried out, will lead to a successful conclusion

• Learned how to represent algorithms in– Structured English– Flow charts

• Used variables to remember

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What we have done already• Applied the top down, stepwise refinement

approach to creating algorithms• Looked at problems more oriented towards

being solved on a computer – stacks, queues, functions, procedures

• Seen how high level languages like Java are used to create programs with the aid of compilers

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What we shall do today

• The problem solving constructs in Java

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What we shall do today

• The problem solving constructs in Java– Sequence– Selection– Repetition

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Conditions from Problem Solving

• Conditions in constructs– if today is Tuesday– while there are objects on conveyor belt

• Too vague for computers – need to be more specific

• Computers deal only in comparing numbers– JEZ in Threebit

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Conditions in programming

• algorithm conditions .....

...... must reduce to .....

............. testing the value of numbers

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Simple numerical conditions

a < b a is less than b

a > b a is greater than b

a == b a equals b

a != b a does not equal b

a >= b a is greater than or equal to b

a <= b a is less than or equal to b

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Example

int age;

System.in.read(age);if (age<18) { System.out.println("Too young! "); }else { System.out.println("Pint sir? "); }

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Exampleint age;

System.in.read(age);if (age!=18) { System.out.println("Too young! "); }else { System.out.println("Pint sir? "); }

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Variations on this


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Other kinds of variables

• Character variables – store single characters• Boolean variables – store true or false• String variables

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Character variables

char c;

c = 'a';

System.out.println("c contains character " + c);

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Character variableschar type;

System.in.read(type);if (type=='x') { System.out.println("Cross!"); }else { System.out.println("Not a cross!"); }

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Boolean variablesboolean raining;

raining = true;if (raining == true) { System.out.println("Get an umbrella!"); }else { System.out.println("Get the sun tan lotion!"); }

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Boolean variables

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boolean raining;

raining = true;if (raining) { System.out.println("Get an umbrella!"); }else { System.out.println("Get the sun tan lotion!"); }

Since a boolean can only have the values true and false we can use it on its

own as a condition


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String variablesString studentname;System.in.read(studentname);if (studentname < "N") { System.out.println("1st half of alphabet!"); }else { System.out.println("2nd half of alphabet!"); }

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Comparing char, Boolean, Strings

• Internally, all data types such as char, boolean and Strings are stored as numbers

• Comparisons are carried out on those numbers– E.g. ‘A’ is stored as ASCII code 65– ‘B’ is stored as ASCII code 66– Thus it is true to say ‘A’ < ‘B’ because of their ASCII

codes make it so

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ASCII Codes

• ASCII = American Standard Code for Information Interchange

• Uses 7 bits – hence numbers 0 to 127• Hence 128 different characters

– 94 printable• English alphabet upper and lower case• Digits• Punctuation

– 33 non-printing to control printers etc• E.g. carriage return

– Space

• Fits into a byte – spare bit used as parity check

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ASCII Codes

• Parity check can be even parity or odd parity• Even parity

– Set eighth bit to 1 if there are an odd number of 1s in the other seven bits otherwise set it to 0

• Odd parity– Set eighth bit to 1 if there are an even number of 1s in

the other seven bits otherwise set it to 0

• Parity bit used to check data as it is received down a transmission line. – If parity bit is wrong then error must have occurred in

transmission

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Compound conditions

• We often want to say things like– While a is more than 4 but less then 8

• A is to be simultaneously more than 4 and less than 8 – both conditions must be true for the overall condition to be true

• We write this as– While a is more than 4 AND a is less than 8

• In Java we get:– while ((a > 4) && (a < 8))

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Compound conditions

• while ((a > 4) && (a < 8))

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Compound conditions

• while ((a > 4) && (a < 8))

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&& is the Java (and C) symbol for AND


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SAQ 7.3 – try this now

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int count;

int sum;

count = 10;

sum = 50;

if ((count < 10) && (sum <= 100))

{

System.out.println("True");

}

else

{

System.out.println("False");

}

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Compound conditions

• While a is less than 4 or greater than 8• Here either is enough for the whole condition to

be true• We write this as

– While a is less than 4 OR a is more than 8

• In Java we get:– while ((a < 4) || (a > 8))

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Compound conditions

• while ((a < 4) || (a > 8))

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Compound conditions

• while ((a < 4) || (a > 8))

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|| is the Java (and C) symbol for OR

| is on the same key as \ (to the left of the Z key)


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int count;

int sum;

count = 10;

sum = 100;

if ((count < 10) || (sum <= 100))

{

System.out.println("True");

}

else

{

System.out.println("False");

}

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! - NOT

boolean a;

boolean b;

a = false;

b = !a;

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If a is true then !a is false

If a is false then !a is true


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Truth tables

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• ((month >= 1) && (month <= 12))

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• ((answer == ‘y’) || (answer == ‘Y’))

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Programming Repetition

• the while loop• the for loop

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The while loop in Java

while (condition)

{

statements to be executed

}

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Design

Sum := 0

while Sum is less than 1000 begin

display ‘Enter Number’read(Number)Sum := Sum + Number

end

display ‘Sum is ’ , Sum

Java

sum = 0;

while (sum < 1000){System.out.print("Enter Number ");System.in.read(number);sum = sum + number;}

System.out.println("Sum is " + sum);


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Something to Discuss page 97int choice;choice = 6;System.out.println("1. Display the Time");System.out.println("2. Display the Date");System.out.println("3. Display the Weather Forecast");System.out.println("4. Display the Traffic Report");System.out.println("5. Exit");

while ((choice < 1) || (choice > 5)) { System.out.println("Please Enter Your Choice 1 to 5"); System.in.read(choice); }

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Design

for 100 times

begindisplay ‘I am sorry’

end

Java

for (i = 1; i <= 100; i++) { System.out.println(“I am sorry”); }

The for Loop in Java


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for loop variations

• Java for loops do exactly what they appear to say.

• How many times do each of the following repeat?– for (i = 5; i <=14; i++)– for (i = 12; i >=8; i--)– for (i = -3; i <=2; i++)

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Programming Selection

• Depends on the number of alternatives

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One alternativeif (condition)

{

statements to be performed when condition is true

}

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if (age < 18) { System.out.println(“Too young”); }

System.out.println(“Done”);

Java


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Two alternatives

if (condition) {

statements to be performed when condition is true }else {

statements to be performed when condition is false }

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Two alternatives

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if (age < 18) { System.out.println(“Too young”); }else { System.out.println(“Pint Sir ?”); }

System.out.println(“Done”);

Java


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More than two alternatives

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if (size == 1) {

price = 50; }if (size == 2) { price = 70; }if (size == 3) { price = 80; }System.out.println(“Done”);


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So far we have seen that ..

• Conditions must be reduced to comparing numbers or chars or booleans or Strings

• We can make compound conditions with && (AND) and || (OR) symbols in Java

• Repetition is done with– while { }– for { }

• Selection is done with– if .{ } ... else { }

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Structured Problem Solving 2009-2010