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# Lookup

Generate new columns by mapping existing data to common lookup tables.

## CPI deflation

The agate-lookup extension adds a `lookup` method to agate’s Table class.

Starting with a table that looks like this:

year | cost
--- | ---
1995 | 2.0
1997 | 2.2
1996 | 2.3
2003 | 4.0
2007 | 5.0
2005 | 6.0

We can map the `year` column to its annual CPI index in one lookup call.

    import agatelookup

    join_year_cpi = table.lookup('year', 'cpi')

The return table will have now have a new column:

year | cost | cpi
--- | --- | ---
1995 | 2.0 | 152.383
1997 | 2.2 | 160.525
1996 | 2.3 | 156.858
2003 | 4.0 | 184.000
2007 | 5.0 | 207.344
2005 | 6.0 | 195.267

A simple computation tacked on to this lookup can then get the 2015 equivalent values of each cost:

    cpi_2015 = Decimal(216.909)

    def cpi_adjust_2015(row):
        return (row['cost'] * (cpi_2015 / row['cpi'])).quantize(Decimal('0.01'))

    cost_2015 = join_year_cpi.compute([
        ('cost_2015', agate.Formula(agate.Number(), cpi_adjust_2015))
    ])

And the final table will look like this:

year | cost | cpi | cost_2015
--- | --- | --- | ---
1995 | 2.0 | 152.383 | 2.85
1997 | 2.2 | 160.525 | 2.97
1996 | 2.3 | 156.858 | 3.18
2003 | 4.0 | 184.000 | 4.72
2007 | 5.0 | 207.344 | 5.23
2005 | 6.0 | 195.267 | 6.66