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The Lesson The Program
In this lesson you learn how to use while and for loops:
Declaring multiple variables at the same time:
As long as you are declaring the same type of variables, you can declare multiple variables on the same line.
int numberOne, numberTwo, numberThree;
String stringOne, stringTwo, stringThree;
Doing this improves the readability of your code but is completely optional.
I only do it on occasion if I feel it works well or if the variables are not just of the same type but are also related.
You can also initialize multiple variables of the same type on the same line but this is slightly sloppier and I have never seen it.
int numberOne = 12, numberTwo = 13, numberThree = 15;
String stringOne = "", stringTwo = "hi", stringThree = "out of ideas";
Just because I have not seen it doesn't mean it's never been done as it is a feature which means someone must use it.
You should try to use standards that other programmers use though as it increases readability so use this technique sparingly.
While loops:
While loops allow you to do something until what is being tested returns false.
You can think of it as similar to an if statement if that helps.
if (...) {do this and then check the if statement again} is the same as while (...)
Just remember that you can not do something like I did in the if statement above so you must use a while loop.
While loops are usually used in this manner to repeat something a certain number of times:
int i = 0; while (i < SOMENUMBER) {DO SOMETHING AND THEN INCREMENT i}
Keep in mind the while loop is usually not all on one line I am just trying to save space.
Look at the program to see examples.
You can use anything that returns or is a boolean in the parentheses of a while loop just like you can with an if statement.
Do while loops:
Do while loops are special loops that you use to do whatever is within the body of your loop at least one time before checking your condition.
They are rather basic loops and you will learn when it is best to use them over the normal while loop through practice.
The syntax for a Do while loop is:
do {DOSTUFF HERE} while (CHECK);
For loops:
For loops are similar to while loops except that they compress what you are writing into one usually neat line.
int i = 0; while (i < SOMENUMBER) {DO SOMETHING AND THEN INCREMENT i}
for (int i = 0; i < SOMENUMBER; INCREMENT i) {DO SOMETHING}
When looping through something a select number of times you should usually use a for loop rather than a while loop.
I know I said while loops are usually used for that but that is true until you learn for loops and even then you still can.
For loops just make sure you are always incrementing your i (which can be named anything by the way)
Keep in mind ALL programmers not just beginners will sometimes forget to increment their i in a while loop.
This leads to an infinite loop and your computer freezing up.
For loops can be used for other things though so another way to look at the relationship between the two loops is this:
STEP 1; while (STEP 2) {DO SOMETHING AND THEN STEP 3}
for (STEP 1; STEP 2; STEP 3) {DO SOMETHING}
Keep in mind that:
STEP 1 is usually declaring a variable.
STEP 2 is usually a logical operation with that variable.
STEP 3 is usually incrementing that variable.
More Operators:
I hope you were studying your operators from the if statement lesson and if not you should go review them.
There are two more types of operators we are going to learn here and then that only leaves two more which we won't learn for a while.
Unary Operators:
Unary operators are always attached to a variable or literal and can only be used on specific kinds.
and - are the operators used on numbers such as int, long, float, and double variables and literals.
+6, -15
int x = 15, y = -x;
++ and -- are the increment and decrement operators and they allow you to increase or decrease a number by 1.
The definition of number for ++ and -- is similar to the one above except that it can only be variables and not literals.
This is because you can change the value of a variable but not of a literal (hence its name).
++ and -- can be used before or after a number and it changes the order that things are done in your program.
While the order may be different, the number will end up the same whether ++ or -- are before or after a number.
If we have an int variable called x which equals 5 (int x = 5) then:
System.out.println(++x) will print 6 as we add before printing the variable.
System.out.println(x++) will print 5 as we add after printing the variable.
System.out.println(x) after either of these lines will always print 6 though since we incremented no matter the order.
The same logic applies to the -- operator.
! is the logical complement operator.
The name of this operator is just a fancy way to say that it reverses the value of a boolean.
That means if we have a boolean variable names x that equals true (boolean x = true) then:
if (x) {System.out.println("Hi"} will print "Hi" but...
if (!x) {System.out.println("Hi"} will not print "Hi" as the if statement only sees false.
You cannot do x! like you can with ++ and --.
You are also not changing the value of x like with ++ and --.
Conditional Operators:
Conditional operators are useful in if statements as well as within loops and can be seen along with the logical complement operator.
&& is the "conditional and" which checks if the two things on both sides of it are true.
System.out.println(true && true) will print true.
System.out.println(true && false) will print false no matter the order.
System.out.println(false && false) will print false.
|| is the "conditional or" which checks if either of the two things on either side of it are true.
System.out.println(true || true) will print true.
System.out.println(true || false) will print true no matter the order.
System.out.println(false || false) will print false.
There is a third conditional operator but we will be ignoring it for now as it slightly complicates things.
It also causes code to be less readable but only if used incorrectly so for now we should focus on easier things.
In this program we have the user input an integer as many times as they want and add it to the total variable.
We only stop looking for input when the user enters -1.
At the end of the program you can also see both a while and a for loop being used for incrementing.
Unfortunately I do not use any of the new conditional operators we learned but its better that you know them now.
Make sure to study them so when we start using them you are a pro (which will be helpful as we only keep learning from here).
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