# Infix, Postfix, Prefix and Reverse Polish Notation

## Before, after or in.

A simple application for a `Stack` in a computer application, but also a very important one in Computer Science too!

Difficulty: Beginner | Easy | Normal | Challenging

# Prerequisites:

• Some understanding of a stack (guide HERE)

# Terminology

Infix: The notation commonly used in mathematical formulae

Operand: The value on which an operator is performed

Operator: A symbol like minus that shows an operation

Postfix: A mathematical notation in which operators follow operands

Prefix: A mathematical notation in which operands follow operators

Reverse Polish Notation (RPN): A mathematical notation in which operators follow operands

# Infix, Postfix and Prefix?

We are going to work through the basics here before we touch on `Reverse Polish Notation` in detail.

## Infix

`Infix` notation is seen a the “normal” way to create mathematical formulae. We can take a simple addition sum:

2 + 3

and name the parts of the formulae as follows:

We can see that the `Operator` is sandwiched in between the `Operands` for the `Infix` notation.

This is true for any mathematical formulae which uses `Infix` notation:

2 + 3

5 — 2

and

2 + 3 + 4

2 + 5 / 2

1 + 2 / 3

Of course, when we mix `Operands` we have a precedence (that is, order of operations)issue. Which `Operator` should be used first?

Yes, you can use brackets for this, but people typically use BODMAS as a system (guide HERE). This order of precedence is usually coded in computer language compilers (or at least one would hope).

## Prefix

Prefix notation puts the operator before all of the operands. This means that the precedence is always clear. We are going to compare the `Infix` notation to this `Prefix` notation.

So in an example of 2 + 3 as `Infix` notation we can convert this to `prefix` notation

The Prefix Example of 2+3

`Infix: 2 + 3`

this can be written with a circle where we put the `operator` in a blue section:

Which is then followed by the `operands`

so to derive the `prefix` equivalent we simply take the `operator` then move from left to right reading off the `operands` in left-to-right order.

`Prefix: + 2 3`

This gets a little bit tricky when we have multiple `operators` in our formuale.

The Prefix Example of 2+3+4

`Infix: 2 + 3 + 4`

this can be represented as two of the RPN circles

Once again the same rules apply. However we move from the outer circle to the inner circle.

`Prefix: + 2 + 3 4 `

The secret here being that each operator takes the two expressions that immediately follow it.

this is really the thing — each `operand` needs to receive two fully formed `prefix` expressions.

## Postfix OR Reverse Polish Notation

Postfix notation puts the operator before all of the operands.

This is just like the `prefix` notation, but the `operand` comes at the end of the expression, and similarly removes the need for brackets within any expressions.

It is called Polish due to the nationality of Jan Łukasiewicz who invented the notation.

The same ideas as above applies. We still have the `operand` at the top of the circle, but this time that goes at the end of the operators

`Infix: 2 + 3Postfix: 2 3 +`

So we take the bottom two `operand` first from left-to-right then append the `operator` at the end.

Similarly when we have two expressions we have to remember the `operators` to append to the end.

`Infix: 2 + 3 + 4Postfix: 2 3 4 + +`

# Conclusion:

`infix` is not the only possible way to develop expressions, although it is quite easy for people to understand. Computers are likely to use a stack to evaluate expressions — and therefore `Reverse Polish Notation` is often used as it is a great way to simply evaluate an expression using a stack (read more about this in the extend your knowledge section below).

Just remember — Polish here isn’t anything to do with cleaning. But it is much to do with coding.

• You can (if you choose) read Apple’s documents HERE

Any questions? You can get in touch with me here

Written by