variables, types, conditionals, loops and arrays. · conditionals excersize 1 loops arrays...

Variables and TypesConditionalsExcersize 1

LoopsArrays

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Variables, Types, Conditionals, Loops and Arrays.

Jemar Jones

Department of Computing and SoftwareMcMaster University

Tutorial 2

Jemar Jones Variables, Types, Conditionals, Loops and Arrays.


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Outline1 Variables and Types

Variable DeclarationVariable Assignment

2 ConditionalsThe if statementThe conditionOthers

3 Excersize 14 Loops

The While LoopThe For Loop

5 ArraysArray DeclarationAccesing an arrayIteration

6 Excersize 2Jemar Jones Variables, Types, Conditionals, Loops and Arrays.


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Variable Declaration

int︸︷︷︸type

answer︸︷︷︸name

= 42︸︷︷︸value

;



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Primitive Types

Type Description Operators

byte Whole numbers from -128 to 127 + - * /

short Whole numbers from -32,768 to 32,767 + - * /

int Whole numbers from −231 to 231 - 1 + - * /

long Whole numbers from −263 to 263 - 1 + - * /

float Floating point numbers (decimals) + - * /

double Floating point numbers with greater range + - * /

boolean true or false value and, or, not

char Single characters compare

*String Sequences of characters concatenation

*Technically String is not a Java primitive.



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Variable names

Case sensitive

Must start with a letter, dollar sign, or underscore.Subsequent characters may be letters, digits, dollar signs, orunderscores.

Can’t be certain reserved words.

Should be meaningful and clear.

Convention is to use camel casing (ie. myVar).



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Variable Assignment

Quick Example:

int myNum = 10; //Declaration and initialization

myNum = 3; //Overwriting via assignment

int myNum2; //Declaration with no initialization

myNum2 = (myNum * 5) - (4/2) + 10; //initialization after

Declaration



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The if statementThe conditionOthers

The if statement

The if statement is one way of controlling the flow of your program.

if (condition){

//Some statements

}else if (condition2){

//Some alternative statements

//As many else if’s as you want...

}else{

//Some other alternative statements

}



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The condition

The condition must evaluate to a boolean value.

This means the condition must ultimately be either true orfalse

Typically this means the condition will be made up ofrelational and boolean operators.



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Relational Operators

Syntax Description

== equal to

!= not equal to

> greater than

>= greater than or equal to

< less than

<= less than or equal to

*Note that the equality operator ”==” is different from theassignment operator ”=”



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Boolean Operators

Syntax Description

&& Logical AND

|| Logical OR

! Logical NOT



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Some possible conditions

true

1 == 1

!{1 == 1}

a > 5

(a > 5) !! (b != a)

(answer == 7*6 && name == "Arthur") || p >= 9001



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...

if ((answer == 7*6 && name == "Arthur") || p >= 9001){

System.out.println("Some sort of cultural reference");

}else{

System.out.println("Something uninteresting");

}



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Others

Shorter form:

if (condition)

statement;

Ternary operator for very common case:

a = condition? firstOption : secondOption;

//This is the same as:

if (condition){

a = firstOption;

}else{

a = secondOption;

}Jemar Jones Variables, Types, Conditionals, Loops and Arrays.


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Try it!

Write a program that takes in a usercode and a passcode (*asint’s) and checks them against some hard coded values. If thecredentials are correct, indicate that the user now has access. Ifonly the usercode is correct, indicate that the user is an imposter.Otherwise, simply indicate that access to the system is declined.

*Hint: To turn a String into an int use: Integer.parseInt(myStr)



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public static void main(String[] args) {

int usercode = Integer.parseInt(args[0]);

int passcode = Integer.parseInt(args[1]);

if (usercode == 123 && passcode == 456){

System.out.println("Welcome.");

}else if (usercode == 123){

System.out.println("You’re an imposter..");

}else{

System.out.println("You don’t have access, sorry.");

}

}



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Loops

A loop is used to repeat some sequence of statement(s) accordingto some condition.



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The While Loop

while (condition){

//Some statement(s)

}



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What’s going on here:

1 Evaluate boolean condition

2 If the condition is true, run some statements.

3 Jump back to 1.



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Quick Example:

int n = 0;

while (n < 6){

System.out.println(n);

n = n + 1;

}

System.out.println("Final n value: " + n);

Output:

0

1

2

3

4

5

Final n value: 6Jemar Jones Variables, Types, Conditionals, Loops and Arrays.


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The infinite loop

while (true){

System.out.println("Still running");

}

This loop doesnt terminate!It will run either forever, or more likely until it causes a crash.



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The For Loop

for (statement; condition; statement){

//Some statement(s)

}

Almost universally you will see the following structure:

for (init; condition; increment){

//Some statement(s)

}



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What’s going on here:

1 Run initialization statement

2 Check boolean condition. If it’s false jump out of for loop.

3 Run increment statement.

4 Run statement(s) in the loops body.

5 Jump back up to step 2



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For example:

for (int i = 0; i < 10; i++){

System.out.println(i);

}

Or with a different increment:

for (int i = 0; i < 10; i += 3){


}

Or maybe in reverse:

for (int i = 10; i >= 0; i--){


}



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Array DeclarationAccesing an arrayIteration

Arrays

An array is used to store an indexed collection of same type values.



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Array Declaration

If we wanted an array of int’s for example:

int[] nums = new int[5];

The type of this variable is ”int[]”, or an array of ints.

The ”new int[5]” creates and initializes an array of 5 ints.

The indices of arrays will always start at 0.

These arrays are fixed length after initialization.



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Accesing an array

int[] myNums = new int[3];

System.out.println(myNums[0]);




Output:

0

0

0

Exception in thread "main"

java.lang.ArrayIndexOutOfBoundsException: 3



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Exception in thread "main"

java.lang.ArrayIndexOutOfBoundsException: 3

This is a very common error.Remember, indices start at 0, size != (upper) index.



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Iteration

Accesing each index manually is tedius.. is there a better way?

Yes, loops and arrays go together very well..


for (int i = 0; i < myNums.length; i++){

System.out.println(myNums[i]);

}



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This is extremely useful to remember at scale..


for (int i = 0; i < myNums.length; i++){

System.out.println(myNums[i]);

}



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Try it!

Write a program that takes in a number n and creates an arraycorresponding to the Fibonacci series from the 0th term up to thenth term.



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import java.util.Arrays;//This library is useful for

dealing with arrays.

public class Fib{

public static void main(String[] args) {

int n = Integer.parseInt(args[0]);

int[] fibs = new int[n + 1];//We specifically need

to include the nth term

if (n > 0)

fibs[1] = 1;

for (int i = 2; i < n + 1; i++){

fibs[i] = fibs[i-2] + fibs[i-1];

}

System.out.println(Arrays.toString(fibs));

}

}



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Is our solution correct though?



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Is our solution correct though?

java Foo 50

[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377,

610, 987, 1597, 2584, 4181, 6765, 10946, 17711,

28657, 46368, 75025, 121393, 196418, 317811, 514229,

832040, 1346269, 2178309, 3524578, 5702887, 9227465,

14930352, 24157817, 39088169, 63245986, 102334155,

165580141, 267914296, 433494437, 701408733,

1134903170, 1836311903, -1323752223, 512559680,

-811192543, -298632863]



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Discussion



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The End

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variables, types, conditionals, loops and arrays. · conditionals excersize 1 loops arrays...

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