readme.md

Lesson Four

Table of Contents

The Lesson
The Program

The Lesson

• 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}
      • is the same as
    • 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}
      • which is the same as
    • 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.

The Program

• 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).