Lesson 3

Stack Notation

Lesson 3

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Before we go further, you should become acquainted with a special notation that tells someone reading your program listing what's happening on the stack before and after a command. The format is:

( before -- after )

The arrangement of values on the stack is shown both before and after the operation (note the space between the opening parenthesis and the start of the description). The actual operation is implied by the double-hyphen (--). Therefore, in an addition operation (just the + operation, not the extra stuff to display it and move the Mops prompt) you have two numbers on the stack before the operation, and you end up with a single number, the sum of those numbers, on the stack after the operation. That is, you start with your first and second numbers on the stack and end with the sum on the stack. The stack notation looks like this:

( n1 n2 -- sum )

This, therefore, is the description for the addition operation.

For the dot command, the description is:

( n -- )

because this command takes the topmost value from the stack and displays it on the screen. The value is removed from the stack in the process, leaving no trace of it on the stack after the operation.

In the CR command there is nothing happening to values in the stack, it simply moves the prompt to the left margin of the next line. Because no stack operations are involved, the CR command's notation is:

( -- )

The definition of every Mops word you define in a program should be accompanied by its stack notation. Our convention is that we may omit the stack notation if it is ( -- ), but only in this situation.

Peruse the [../Classes/index.html Classes section of this manual] to see how we have noted the stack actions of all the words in the Mops dictionary. While the notation will at first help you learn how Mops words work, it will also help you later when you start writing programs in an editor. The words and numbers in parentheses (with at least one space after the opening parenthesis) are not compiled into the program, so they won't add even one byte to the size of your final program. The notations are there to aid you in tracing your program if you run into a snafu during program development.

All in all, the stack notation is a handy shortcut to documenting your programs.

Note: Since anything in parentheses (i.e., starting with an open parenthesis followed by one or more spaces) is ignored by Mops, you don't have to type stack notation for words you define at the Mops prompt. Stack notation is strictly an aid for reading source code. In this tutorial, we often show the stack notation for words you define. The notation is presented to help you better understand the definition and show you how your definitions should look once you begin writing your own programs in an editor.

[#ArithmeticOperators Arithmetic Operators]

Here are Mops stack descriptions of the four basic arithmetic operations:

+	( n1 n2 -- sum )	Adds ˜n1' and ˜n2' and leaves the sum on the stack.
-	( n1 n2 -- diff )	Subtracts ˜n1' and ˜n2' and leaves the difference on the stack.
*	( n1 n2 -- prod )	Multiplies ˜n1' and ˜n2' and leaves the product on the stack.
/	( n1 n2 -- quot )	Divides ˜n1' and ˜n2' and leaves the quotient on the stack.

To newcomers, the stack order (the way in which numbers come out in the reverse order) may be confusing when it comes to subtraction and division, because the order of the numbers is critical to those operations. If you want to subtract 4 from 10, you want to make sure that those numbers come out of the stack in the correct order for the subtraction operation to work on them. Fortunately, Mops saves you from performing all kinds of mental gymnastics in the process.

In the kind of arithmetic notation you probably learned in school, you write the problem like this:

10 - 4

and get the desired answer, 6. In Mops arithmetic, the order of the numbers going on the stack is the same. All you do is move the operation sign to the right.

In this case, the problem becomes:

10 4 -

The same goes for division. The formula for dividing 200 by 25 changes from

200 / 25

into

200 25 /

The four basic arithmetic operations are usable only on integers, that is, whole numbers like -2, , 3, -453, and 1002. Numbers with digits to the right of the decimal don't count. Don't worry, however, because Mops has plenty of ways to handle all kinds of numbers, as you'll see later on.

Experiment using the four simple arithmetic operations. Place one, two, three, and four integers (or more if you like) in the stack to understand how the operations make use of the numbers in the stack. Try them out now, and pay special attention to answers to division problems.

If you tried it, everything should have worked well, except when you divided numbers that were not even multiples of each other. For example, if you divide 10 by 3, the Mops answer is 3.

10 3 / . CR
3

When you use the divide operation (/) in Mops, the remainder is lost forever. But Mops has two other operations that take care of the remainder for you:

/MOD	( n1 n2 -- rem quot )	Divides ˜n1' by ˜n2' and then places the quotient and remainder on the stack.
MOD	( n1 n2 -- rem )	Divides ˜n1' by ˜n2' and then places only the remainder on the stack.

Let's try dividing 10 by 3 again, but this time using the /MOD operation instead of straight division (/). (Remember! Mops is case-insensitive.)

10 3 /mod . . cr
3 1

Notice now that both the quotient (3) and remainder (1) were returned to the stack (and subsequently printed out by two dot commands). Notice also the order of the two answers as they came out of the stack and how the order compares with the order of the /MOD stack notation above. The rightmost value in the stack definition, the quotient, was on the top of the stack and was therefore the first one to be printed out on the display.

Division involving negative numbers can be done in two different ways. In Mops we use the convention used by the Apple hardware, namely "towards zero" division (often referred to as symmetric division). If the exact quotient isn't an integer, the quotient that the division operation gives will be the next integer towards zero. For example, -10 divided by 7 will give a quotient of -1, with a remainder of -3. (The remainder will always have the same sign as the first operand, the dividend, unless it is zero.)

[#Postfix Mastering Postfix Notation]

If you're not particularly well versed in this reverse notation, called postfix notation (also known as reverse Polish notation), then it is important to recognize that complex math formulas need to be analyzed before they can be entered into Mops's postfix, integer arithmetic environment. For example, you may find yourself confronted with having to include the following formula in a Mops program:

  1.25 * 12 * 50
 ----------------
       10
 

If so, then Mops' integer arithmetic might seem like a stumbling block, and its postfix notation may seem worthless. But call upon simple algebra to convert everything to integers, and break up the complex formula into the same steps you would use to solve it with a pencil and paper.

The Mops equivalent of this formula is:

5 12 50 * * 40 /

It's worth following what happens to the stack during a complex formula like this. First of all, to make the 1.25 an integer, multiply it and the denominator (10) by four. Then put all three numbers to be multiplied into the stack. The first multiplication operation multiplies the topmost two numbers (50 times 12) leaving the result (600) on the stack. That leaves 600 on the top of the stack, and 5 just below it. The second multiplication operation multiplies the two numbers remaining on the stack (600 times 5) and leaves the result (3000) on the stack. This result is the dividend (numerator) of the division about to take place. Now it's time to put the divisor (denominator), 40, on top of the stack. Then the final operation, the division, divides the two numbers in the stack.

Don't be discouraged by all this concern over the stack! You'll learn in Lesson 10 that Mops provides you with two powerful tools, [Chapter10.html#NamedInput named input parameters] and [Chapter10.html#LocalVariables local variables], that let you substitute readily identifiable names for the values on the stack and use them at will. The stack will become almost invisible to you. It is important, however, to understand the stack fundamentals just the same.