2. The Basics

O revolves around one stack in which you can push and pop values to and from. Here are some basic rules for writing O code.

2.1. I/O

There are a couple ways to get a user’s input. i will push the input to the stack as a string, while j will push the input to the stack as an integer. o pops the top value off the stack and outputs it. p outputs with a new line. Here’s an example:

>>> io

This is the smallest cat program possible. i gets the input and pushes it, o pops it and outputs it to stdout.

Another cool feature of O is that the stack contents will be outputted when the code finishes execution. Meaning 1234 will output 1234, while 1234oooo will output 4321. This feature does not work in the REPL.

2.1.1. Input as a number

j will parse the input as a number, and throw an error if otherwise.

>>> j5+o

q will push the input as a number if it is one, and push it as a string if otherwise.

>>> q5+o
>>> q5+o

Q will do the same except assign it to a variable called Q. You will learn about variables more later.

2.2. Number Literals are pushes individually

1234 doesn’t push the number one thousand twenty-four, it pushes 1, then 2, 3, and finally 4. Each individual digit is pushed to the stack:

>>> 1234oooo

Each o pops one number off and outputs it, meaning the number that was pushed last will pop off first.

2.2.1. Hexadecimal works too

Hexadecimal numbers work for capital notation. Lowercase notation is saved for other functions.


1 was pushed to the stack, then A pushed 10 to the stack, and then they were outputted.

2.3. Strings are enclosed in quotes

To make a string, you just need to enclose it within quotes.

>>> "Hello, World!"o
Hello, World!

What about printing something different?

>>> "Hello'World!"oo

2.3.1 Strings don’t need quotes at all

Remember when I said you need to put them in quotes? I lied. ' has some interesting properties with strings. When used by itself, it will push the next character to the stack as a string.

>>> 'ao

If you are in the middle of making a string, it will push the current string buffer to the stack and start making a new string (like a macro for "").

>>> "a'a"oo
>>> "hello'madam"oo

The next section will cover basic arithmetic.

2.3.2 Strings don’t need quotes all the time

If a string is at the end of a file, you don’t need to put a quote at the end.

"Hello, World!

Put that in a file and it will print:

Hello, World!