How a sheet is worked out

Top to bottom, then left to right

A sheet is worked out like reading a page: from the top down, and across each line from the left. Each statement sees the definitions above it (and to its left), and nothing below. A math region counts from the baseline of its first line, so a tall fraction beside a definition still comes after it.

The same name can be defined again further down; from there on it has its new value. This is how a sheet tries a second case without renaming everything.

Recalculation

The sheet is worked out again after every change, so results are always current. F9 works out the whole sheet again, for example after a data file it reads has changed.

Globals

A global definition (≡, typed ~, written ::=) is worked out before everything else, so it can be used anywhere on the page, even above it:

As text

y := G*2 = ?
G ::= 10

Open this example · Download .bbm

Globals are worked out among themselves top to bottom first. A global is defined once: defining its name again, with := or another ≡, is an error, so a global means the same thing everywhere in the sheet.

Use globals sparingly: for constants and settings that the whole sheet shares, such as material properties at the bottom of a page.

What a function sees

A function’s body is worked out when it is called, but its other names are looked up where the function was defined:

As text

a := 2
f(x) := a*x
a := 3
f(1) = ?

Open this example · Download .bbm

f was defined while a was 2, so changing a below doesn’t change f. Define the function again below to use the new value, or pass a as an argument.

A program’s local variables (←) belong to the program alone: assigning to a sheet’s name inside a program makes a local copy, and never changes the sheet’s value.

Errors carry forward

A definition that fails leaves its name broken, and each statement that uses it says so: c has an error: a has an error: b is not defined. The first error in the chain is the one to fix.