IntVariablesAndOperators.md

Numeric Variables and Operators

Video - Int variables and Operators

One of the most important concepts in programming is that of a variable, which is a named location in memory. Variables in Java have a name, a type and a value. We can change the value of a variable as our program runs.

Although you'd normally use variables in the context of full programs, you can play with variables and operators using JavaRepl, a web site that allows you to instantly evaluate simple java expressions.

Declaring variables

We declare a variable by specifying its type and its name. Let's start with int variables, which can contain whole numbers (integers). Java supports integers as a primitive type, and names the type int (we will later see ints are related to the java.lang.Integer class). For example:

int a;

Declares a variable a of type int. Notice the statement ends in a semicolon, like most statements in Java.

In Java, we have to declare a variable before we can use it, and we have to specify its type. The type of a variable can never change, but we can change its value as many times as we need or want.

Assigning

We can assign a value to a variable with the assignment operator, = . For example, to assign the value 10 to the variable a we would do:

a=10;

We can use more complicated expressions on the right-hand side of the assignment; for ints, the usual math operators work:

a=3+4;

Notice we can change the value of a variable as many times as we want, although we must always store values of the same type.

Also, although it looks like we're doing algebra, the assignment operator here means 'put the value into the variable'; in algebra, once we declare a variable has a certain value or property, it has to keep it forever.

In Java we can do things like this, which would be atrocious in math:

int a;
int b;
a=10;
b=a+1; // b is now 11
a=b+9 // since b is 11, a is 11+9=20

Declaring and initializing

We can declare a variable and give it an initial value (initialize it) in one statement, like this:

int c=10;

This line both declares a variable c, of type int, and assigns to it the value 10.

Although JavaRepl does not support this syntax, in Java we can also declare several variables of the same type and initialize them at the same time, like this:

int x=10, y, z=x+1;

Variable names

We should always name our variables to denote their meaning (although i sometimes use short names like a or x in these notes :).

Variables in java are composed of letters, numbers (digits), dollar sign ($) and underscore (_). Variable names cannot start with a digit. Java distinguishes between lowecase and uppercase letters, so a is a different variable than A.

There's no specific limit to the lenght of variable names (you'll get tired before the compiler does :).

There's a strong convention to start variables with a lowecase letter, and use capitalization to separate words, so we have variable names like: gearRatio, numberOfSides etc.

Basic Integer operators

In Java we have the standard mathematical operators, with slight variations:

• +, - (and unary minus) behave as in elementary school, with the operators being evaluated from left to right (in the order in which we read them). So, for example, 3+4-1-2 yields 4 (3+4=7, 7-1=6, 6-2=4)
• * is the multiplication operator (instead of the x symbol, since it would be confused with the letter x)
• / is the integer division operator (it calculates the integer value, like in elementary school :), so 3/4 yields 0, and 4/3 yields 1, NOT 1.333)
• % is the integer remainder or modulo operator, so 5%2 is 1, since 5 divided by 2 is 2 with a remainder of 1.

We can mix and match this, with the usual order (PEMDAS, except there's no operator for exponents :), and use parenthesis to modify the order of operations. If you can't remember the apropriate order, it is perfectly ok to use extra parenthesis.

Shorthand operators

To save you some typing, Java supports several shorthand operators; we write the operator followed by the = sign, to avoid repeating a variable; so instead of:

a=a+5;

We can write

a+=5;

We can also use:

b*=3; // instead of b=b*3
b++; // instead of b+=1, meaning b=b+1
b--; // instead of b-=1, meaning b=b-1

Notice you do not need to use the shorthand operators, but since many people use them, you want to make sure you can read them.

Exercises

• hint: feel free to use JavaRepl to verify these. Make sure you understand why they work; try other combinations

1. What would be the value of x at the end of this code fragment ?

   int a=10;
   int b=3*a;
   int x=b-5;

2. What would be the value of ans at the end of this code fragment ?

   int a=5;
   int b=a+1;
   int ans=a*b;
   ans++;

Floating point numbers

Computers also allow us to represent 'decimal' numbers; we call them floating point numbers (and they are represented in something similar to scientific notation). In Java, we have double and float variables. Doubles use more memory but have a bigger range and precission. For now, use double when you want to represent numbers that are not integers.

Another thing to keep in mind is that the computer represent the number internally in a slightly imprecise manner; even though it prints 0.3, internally it may represent it as 0.2999998 ; the main thing to remember is that you cannot use == to coompare two floats, but need to use >= and <= and a small precission value.

All the arithmetic and relational operators work as for ints, with the exception of == and division, where / does regular division and % doesn't make sense.

Example

double a=1.22;
a+=1;
a/=2; // what's the value of a ?