@@ -43,14 +51,14 @@ they appear. These operators act on the two values on the stack preceding them (to the left). Read these two values on the stack from left to right inserting the operator in the middle. If the resulting statement is true, then replace the three values from the -stack with ``1''. If the statement if false, replace the three values -with ``0''.
-For example, think about ``2,1,GT''. This RPN expression could be -read as ``is two greater than one?'' The answer to that question is -``true''. So the three values should be replaced with ``1''. Thus the +stack with "1". If the statement if false, replace the three values +with "0".
+For example, think about "2,1,GT". This RPN expression could be +read as "is two greater than one?" The answer to that question is +"true". So the three values should be replaced with "1". Thus the RPN expression 2,1,GT evaluates to 1.
-Now consider ``2,1,LE''. This RPN expression could be read as ``is -two less than or equal to one?''. The natural response is ``no'' +
Now consider "2,1,LE". This RPN expression could be read as "is +two less than or equal to one?". The natural response is "no" and thus the RPN expression 2,1,LE evaluates to 0.
@@ -58,21 +66,21 @@ and thus the RPN expression 2,1,LE evaluates to 0.
The IF RPN logic operator can be straightforward also. The key to reading IF operators is to understand that the condition part -of the traditional ``if X than Y else Z'' notation has *already* +of the traditional "if X than Y else Z" notation has *already* been evaluated. So the IF operator acts on only one value on the stack: the third value to the left of the IF value. The second -value to the left of the IF corresponds to the true (``Y'') branch. +value to the left of the IF corresponds to the true ("Y") branch. And the first value to the left of the IF corresponds to the false -(``Z'') branch. Read the RPN expression ``X,Y,Z,IF'' from left to -right like so: ``if X then Y else Z''.
-For example, consider ``1,10,100,IF''. It looks bizarre to me. -But when I read ``if 1 then 10 else 100'' it's crystal clear: 1 is true +("Z") branch. Read the RPN expression "X,Y,Z,IF" from left to +right like so: "if X then Y else Z".
+For example, consider "1,10,100,IF". It looks bizarre to me. +But when I read "if 1 then 10 else 100" it's crystal clear: 1 is true so the answer is 10. Note that only zero is false; all other values -are true. ``2,20,200,IF'' (``if 2 then 20 else 200'') evaluates to 20. -And ``0,1,2,IF'' (``if 0 then 1 else 2) evaluates to 2.
+are true. "2,20,200,IF" ("if 2 then 20 else 200") evaluates to 20. +And "0,1,2,IF" ("if 0 then 1 else 2) evaluates to 2.Notice that none of the above examples really simulate the whole -``if X then Y else Z'' statement. This is because computer programmers -read this statement as ``if Some Condition then Y else Z''. So it's +"if X then Y else Z" statement. This is because computer programmers +read this statement as "if Some Condition then Y else Z". So it's important to be able to read IF operators along with the LT, LE, GT, GE and EQ operators.
@@ -85,9 +93,9 @@ you must know the algorithm for evaluating RPN expressions: iterate searches from the left to the right looking for an operator. When it's found, apply that operator by popping the operator and some number of values (and by definition, not operators) off the stack.
-For example, the stack ``1,2,3,+,+'' gets ``2,3,+'' evaluated (as ``2+3'') +
For example, the stack "1,2,3,+,+" gets "2,3,+" evaluated (as "2+3") during the first iteration and is replaced by 5. This results in -the stack ``1,5,+''. Finally, ``1,5,+'' is evaluated resulting in the +the stack "1,5,+". Finally, "1,5,+" is evaluated resulting in the answer 6. For convenience, it's useful to write this set of operations as:
@@ -98,10 +106,10 @@ operations as: with multiple logic operators:1) 20,10,GT,10,20,IF eval is 20,10,GT = 1 result is 1,10,20,IF-read the eval as pop ``20 is greater than 10'' so push 1
+read the eval as pop "20 is greater than 10" so push 1
2) 1,10,20,IF eval is 1,10,20,IF = 10 result is 10-read pop ``if 1 then 10 else 20'' so push 10. Only 10 is left so +
read pop "if 1 then 10 else 20" so push 10. Only 10 is left so 10 is the answer.
Let's read a complex RPN expression that also has the traditional multiplication operator:
@@ -111,29 +119,29 @@ multiplication operator: 3) 0,128,8,*,IF eval 128,8,* result is 1024 4) 0,7000,1024,IF result is 1024
Now let's go back to the first example of multiple logic operators, -but replace the value 20 with the variable ``input'':
+but replace the value 20 with the variable "input":1) input,10,GT,10,input,IF eval is input,10,GT ( lets call this A )-
Read eval as ``if input > 10 then true'' and replace ``input,10,GT'' -with ``A'':
+Read eval as "if input > 10 then true" and replace "input,10,GT" +with "A":
2) A,10,input,IF eval is A,10,input,IF-
read ``if A then 10 else input''. Now replace A with it's verbose +
read "if A then 10 else input". Now replace A with it's verbose description again and--voila!--you have a easily readable description of the expression:
if input > 10 then 10 else input
Finally, let's go back to the first most complex example and replace -the value 128 with ``input'':
+the value 128 with "input":1) input,8,*,7000,GT,7000,input,8,*,IF eval input,8,* result is A-
where A is ``input * 8''
+where A is "input * 8"
2) A,7000,GT,7000,input,8,*,IF eval is A,7000,GT result is B-
where B is ``if ((input * 8) > 7000) then true''
+where B is "if ((input * 8) > 7000) then true"
3) B,7000,input,8,*,IF eval is input,8,* result is C-
where C is ``input * 8''
+where C is "input * 8"
4) B,7000,C,IF
At last we have a readable decoding of the complex RPN expression with @@ -145,7 +153,7 @@ a variable:
Exercise 1:
-Compute ``3,2,*,1,+ and ''3,2,1,+,*`` by hand. Rewrite them in +
Compute "3,2,*,1,+ and "3,2,1,+,*" by hand. Rewrite them in traditional notation. Explain why they have different answers.
Answer 1:
@@ -156,7 +164,7 @@ traditional notation. Explain why they have different answers.One may be tempted to shorten the expression
input,8,*,56000,GT,56000,input,*,8,IF-by removing the redundant use of ``input,8,*'' like so:
+by removing the redundant use of "input,8,*" like so:
input,56000,GT,56000,input,IF,8,*Use traditional notation to show these expressions are not the same.