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README.gnm_float

For floating point operations, Gnumeric uses the C type gnm_float. Currently that is "double" or "long double", depending on whether Gnumeric was configured with --with-long-double. This has some coding style consequences... 1. Don't use (say) "exp"; use "expgnum". If you simply use "exp", the argument will be cast from gnm_float to double, thus potentially losing precision and the answer will also have reduced range and precision. 2. For exact, fractional constants, use GNM_const(273.18). That will expand to either "273.18" or "273.18L", i.e., a constant with the right precision. Don't bother for fractions that are a power of 2, i.e., for 232.25. Also, don't bother for constants that aren't exact to begin with. Note: make very sure you have a decimal point or an "e" in the argument to GNM_const. Use GNM_const also for huge numbers, e.g., GNM_const (1e100). 3. Various <float.h> constants have "gnum" counterparts. See numbers.h. For example, GNUM_DIG. 4. Various <math.h> constants have "gnum" counterparts. See mathfunc.h. For example, M_PIgnum and M_LN2gnum. 5. To print gnm_float use one of the formats GNUM_FORMAT_f, GNUM_FORMAT_g, ... defined in numbers.h ----------------------------------------------------------------------------- Don't use... Use... --------------------------------------------------- loggnum (1+x) log1pgnum (x) loggnum (1-x) log1pgnum (-x) expgnum (x * loggnum (y)) powgnum (y, x) expgnum (x) - 1 expm1gnum (x) 1 - expgnum (x) -expm1gnum (x) powgnum (1+x,y) pow1p (x,y) powgnum (1+x,y) - 1 pow1pm1 (x,y)