The functions in this section calculate **depreciation**, which is
the amount of value that a possession loses over time. These functions
are characterized by three parameters: `cost`, the original cost
of the asset; `salvage`, the value the asset will have at the end
of its expected "useful life"; and `life`, the number of years
(or other periods) of the expected useful life.

There are several methods for calculating depreciation that differ in the way they spread the depreciation over the lifetime of the asset.

The `b S` (`calc-fin-sln`

) [`sln`

] command computes the
"straight-line" depreciation. In this method, the asset depreciates
by the same amount every year (or period). For example,
``sln(12000, 2000, 5)'` returns 2000. The asset costs $12000
initially and will be worth $2000 after five years; it loses $2000
per year.

The `b Y` (`calc-fin-syd`

) [`syd`

] command computes the
accelerated "sum-of-years'-digits" depreciation. Here the depreciation
is higher during the early years of the asset's life. Since the
depreciation is different each year, `b Y` takes a fourth `period`
parameter which specifies which year is requested, from 1 to `life`.
If `period` is outside this range, the `syd`

function will
return zero.

The `b D` (`calc-fin-ddb`

) [`ddb`

] command computes an
accelerated depreciation using the double-declining balance method.
It also takes a fourth `period` parameter.

For symmetry, the `sln`

function will accept a `period`
parameter as well, although it will ignore its value except that the
return value will as usual be zero if `period` is out of range.

For example, pushing the vector [1,2,3,4,5] (perhaps with `v x 5`)
and then mapping `V M ' [sln(12000,2000,5,$), syd(12000,2000,5,$),
ddb(12000,2000,5,$)] RET` produces a matrix that allows us to compare
the three depreciation methods:

[ [ 2000, 3333, 4800 ] [ 2000, 2667, 2880 ] [ 2000, 2000, 1728 ] [ 2000, 1333, 592 ] [ 2000, 667, 0 ] ]

(Values have been rounded to nearest integers in this figure.)
We see that `sln`

depreciates by the same amount each year,
`syd` depreciates more at the beginning and less at the end,
and `ddb` weights the depreciation even more toward the beginning.

Summing columns with `V R : +` yields [10000, 10000, 10000];
the total depreciation in any method is (by definition) the
difference between the cost and the salvage value.

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