Goal seek

Package: GOALSEEK_PACKAGE. Registered by createEngine(); for a slimmer engine, register it explicitly (see choosing packages).

The engine computes forwards, so answering “what input gives me this result” usually means editing a number and re-reading the answer until it looks right. Goal seek does that search for you, against a line reference.

solve line 4 for rate = 900 reads as “find the value of rate that makes line four equal 900”. The variable named after for must be one the target line uses, since changing it is how the target moves. The result is that value.

Expression	Meaning
solve line 4 for rate = 900	the rate that makes line four equal 900
solve line 2 for deposit = 1,200	the deposit that makes line two equal 1,200

A worked document. Line three works the repayment forward at the starting deposit; the last line solves backward for the deposit that makes it 900:

:deposit = 100000
:rate = 4%
monthly repayment on deposit over 25 years at rate   // 527.84
solve line 3 for deposit = 900                        // 170,507.23

There are two mechanisms, chosen automatically. When the target line is closed form in the variable, the answer is inverted exactly, the same algebra the solve(...) verb uses. Otherwise (a finance formula, say) a bounded numeric search narrows in on it, assuming the relationship rises or falls steadily across the search and crosses the target once.

That search is deliberately fenced in, so a document can never make it spin. It looks for a positive input up to a billion, and stops after a fixed number of steps (vm.maxGoalSeekIterations, a hundred by default). A target no input in range can reach, a relationship that jumps across the target rather than passing through it, or the step limit, each ends in an error rather than a guess or a hang. Solutions outside that range, or relationships with several crossings, are out of scope for now.

Like line references, goal seek only works inside a document, since it re-runs another line. The single-expression entry point has no document to solve against.