I am a new student in the compilers world ^_^ and I want to know is legal represent negative number in the stack.
For example:
infix: 1-5=-4 postfix: 15-
The statements are:
push(1)
push(5)
x=pop()
y=pop()
t=sub(y,x)
push(t)
The final result in the stack will be (-4)
How can i represent this if it is legal??
Thank you ^_^
Yes. Negative numbers are stored in Two s complement form in memory, so you don t need an additional cell on the stack for the sign.
In case you are referring to representing the expression textually (perhaps in a file that is read in by your program), then you would probably define some syntactic rules for your expression - say separation of tokens by whitespace
For example, is 444/ in postfix the same as (4 / 44) or (44 / 4) or (4 / (4 / 4)) in infix? You would need some way of seperating multi-digit numbers.
Now, assuming you decide on whitespace, you could make a rule that a negative integer would be a minus sign followed by a series of digits without any separating whitespace
So the infix expression -1 * (3 ^ (-4) - 7 could become -1 3 -4 ^ * 7 -
Is this what you were looking for?
PS - With a proper parser, you could actually do it without whitespace for operators, but you still need to separate operands from each other.
If you talk about a stack you talk about abstract data types. As long as you have a push/pop functionality it makes no difference what you put in the stack
First note that there is a difference between the dash, - , used as the subtraction operator and as the negative sign. Though we use the same character, they have a different meaning.
Both positive and negative integers, like -4, take only a single slot in the stack.
If your postifx language can only take single-digit integers and the arithmetic operators, you can represent negative numbers by subtracting from zero:
04-2+
This is equivalent in infix notation to
0-4+2
Here is some terminology: the subtraction operation is a "binary operator," that is, it takes two operands; the negative sign is a "unary operator," that is, it takes one operand. Infix and postfix are notations for binary operators and their operands.
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