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Lookup table

A lookup table is an array that can replace an expensive computation with a linear interpolation. A good way to understand the usage of this module is to run:


After which you can inspect the file usage_example.f90 to see how you can incorporate the module in your own programs.


A modern Fortran compiler, such as gfortran 4.8 or newer.


Creating a lookup table

Lookup tables are created with a call to the function LT_create:

lt = LT_create(x_min, x_max, n_rows, n_cols, xspacing)

Here n_rows controls how many x-values are stored. The x-spacing of the lookup table can be set to:

  • LT_xspacing_linear (the default, in which the spacing is given by dx = (x_max - x_min) / (n_rows - 1))
  • LT_xspacing_quadratic for quadratic spacing
  • LT_xspacing_cubic for cubic spacing, which is a bit slower

The argument n_cols specifies how many different columns you want to store, but it also possible to add extra columns later with LT_add_col.

The columns of a table are set using LT_set_col:

call LT_set_col(lt, col_ix, x, y)

where x and y are vectors of the same length, with the only requirement that the x array is sorted from low to high. These y values will be linearly interpolated and stored in the table. Alternatively, it is possible to directly specify the y-values in a column:

call LT_set_col_data(lt, col_ix, y)

Getting values from a table

There are multiple ways to get values from a lookup table:

  • You can directly look up a single column at some x-coordinate, using LT_get_col
  • You can get all the columns at some x-coordinate, using LT_get_mcol
  • You can also get a location in the table with a LT_loc_t object, and then do lookups using that object. This can be used to speed up multiple lookups of different columns

The type lt_loc_t is defined as:

type LT_loc_t
   integer  :: low_ix   !< The x-value lies between low_ix and low_ix+1
   real(dp) :: low_frac !< The distance from low_ix (up to low_ix+1), given
                        !< as a real number between 0 and 1.
end type LT_loc_t



Linear lookup table implemented in modern Fortran







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