CODEWITHSUNDEEP
javaalgorithm
Priya
Priya

Reputation: 69

Use of postfix/prefix expressions

I understand in prefix/postfix, there is no need to bother about precedence of operators. But where do we use prefix/postfix expressions? Are they internally converted by the compiler? Is there a way I could write expression in prefix/postfix and the compiler evaluates it for me?

Upvotes: 1

Views: 891

Answers (3)

mcdowella
mcdowella

Reputation: 19601

I suspect that prefix and postfix operations and notations are mostly useful for theories these days, but they have been used "for real". See e.g. https://en.wikipedia.org/wiki/Stack_machine, https://en.wikipedia.org/wiki/Forth_%28programming_language%29, http://linux.about.com/library/cmd/blcmdl1_dc.htm, and https://en.wikipedia.org/wiki/Reverse_Polish_notation#Hewlett-Packard

Upvotes: 1

Holt
Holt

Reputation: 37616

Few languages use postfix notation (the only one I know of would be PostScript), but the resulting CPU instructions would be the same as for infix notation:

a = (3 + 4) * 5

or 

a = * + 3 4 5

Would be converted (in an imaginary, very simple, instruction sets):

add r0 3 4   # r0 =  3 + 4
mul r1 r0 5  # r1 = r0 * 5

Your CPU does not care about the initial notation, all it accepts is a set of « basic » instructions, so the compiler has to convert the notation you use to instructions understandable by your CPU.

Upvotes: 0

SamTebbs33
SamTebbs33

Reputation: 5647

EDIT: Ff you're actually talking about postfix (RPN) notation, then no, the compiler uses infix notation for expressions. You could write an algorithm that converts an RPN expression into an infix expression, try this

Postfix and prefix operators are just like a binary operator, but they work on only one operand.

Postfix increment (the increment occurs after the variable is evaluated):

int x = 1;
int y = x++; // y = 1 but after the assignment occurs x = 2
int z = x; // z = 2

Prefix increment:

int x = 1;
int y = ++x; // y = 2 and before the assignment occurs x = 2
int z = x; // z = 2

The rules that apply to the increment operator also apply to the decrement operator.

Prefix NOT (inverts a boolean expression):

boolean a = true;
boolean b = !a; // b = false, a = true

I think I have understood the question correctly, but I'm not sure what you mean about the compiler "evaluating" them "for you".

Upvotes: 1

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