Short Expressions & Formulas

Doing arithmetic in SEL

Let's dig into a few concrete examples of how we can use events to perform math.

Formulas

When you create an event on a board, an event box appears, along with an input field to enter code, similar to the way you can write directly into a spreadsheet cell.

Like spreadsheet formulas, SEL is a declarative language, which means we will use these formulas and expressions to declare the behavior of the event.

When we declare an event's behavior, we should think of it at two points in time:

• First, how should the event behave the first time it runs.
• Second, how should it behave the next time it runs (if applicable).

Simple formulas may cause an event to behave the same way over and over, for example:

=2+2

An event, we'll call it Event A, with this formula will always evaluate to 4. If the event points to any other events, this value will be passed to those events (like cell references!).

A lot of events begin with an equals sign = because the event simply needs to set a value and pass it along. However, events can also operate on values they receive from other events. So if you point Event A at an Event B, you could write an expression like so:

/ 4

This will divide whatever Event B receives by, you guessed it, 4. So when A sends 4 to B, B will divide it by 4, and store 1 as its own internal state. This state can then be passed along to another event.

We have a choice, then, when we create a model using SEL, as to whether we want to split a complex formula apart into its separate pieces to take advantage of labeling and a visual structure on the canvas, or if we want to collapse the formula into a single statement like:

=(2+2) / 4

Either way, you will get 1, but which path is "more correct" is up to you, and whether it's more clear to be able to label the / 4 with its own title and whether you'd like to re-use the / 4. So you could also have an Event C that points to Event B and divides the output of C by four.

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Code Re-use

Turning an expression as simple as / 4 into its own event may sound strange, but if / 4 represents a parameter or policy of your business, there's a strong chance you'll re-use it, and labeling it can create clarity for others that may use or extend your model later.

Numbers

SEL formulas and expressions can contain integers, decimals, positive or negative. And though many have resisted it throughout history -- we're also okay with zero.

Numbers in SEL should not contain commas, but you may use underscores _ to separate place values. In practice it is encouraged to write 100000 as 100_000.

Because it's more natural, Summit also supports numerical abbreviations:

K, k, M -> thousand
MM, mm, m -> million
B, b -> billion
T, t -> trillion

For example: 1.2mm is shorthand for 1_200_000 or 1200000.

You can also use decimals with these abbreviations, so 123.5k works.

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Currency formatting

We love financial modeling, but you should not put currency symbols directly into your SEL expressions.

To format an event as currency, use the $ option in the output table at the bottom of the screen. You'll need to have an event or row in focus for those formatting options to appear.

Uncertainty

SEL has built-in symbols for declaring estimates and incorporating uncertainty. This is a valid formula:

=2 + ~5

This means two plus five, plus or minus 10%. So the answer of this formula will be somewhere between 6.5 (at a minimum) and 7.5 (maximum).

Any number in a SEL expression can be preceded by a ~ to inject this behavior.

You may also deepen this uncertainty by adding ?'s. For example:

=2 + ~5?

This means two plus five, plus or minus 25%. Each ? represents an additional 25% variation. For example:

=~100????

Will translate into a random number somewhere between zero (100% less than 100) and two hundred (100% more than 100).

If you use ?, you must still begin the expression with ~.