1. A device for computing a function value of a function F, comprising: a memory storing a look-up table comprising a first set of entries storing numbers in a first fixed-point format and a second set of entries storing numbers in a second fixed-point format, each entry of the first and second set of entries of which entries having associated with a corresponding domain and a corresponding approximation function for approximating the function F on the associated domain; a truncator unit arranged to truncate or round a first value X1 to generate a second value X2; a selector unit, coupled to the truncator unit and the memory, and arranged to select an entry of the look-up table according to the second value X2, thus selecting the approximation function that is associated with the selected entry; a first fixed-point to floating-point converter comprising a first register, a second register, and a first multiplexer, the first register and the second register coupled to the first multiplexer and the first multiplexer configured to select contents of the first or second register, wherein the first register stores a first representation of a first set number of the first set of entries, the second register stores a second representation of the first set number of the first set of entries, and the first multiplexer is configured to select one of the first or second registers to output a floating-point representation of the first set number based on one or more of an index of the first set of entries or a characteristic of the first set number; a second fixed-point to floating-point converter comprising a third register, a fourth register, and a second multiplexer, the third register and the fourth register coupled to the second multiplexer and the second multiplexer configured to select contents of the third or fourth register, wherein the third register stores a first representation of a second set number of the second set of entries, the fourth register stores a second representation of the second set number of the second set of entries, and the second multiplexer is configured to select one of the third or fourth registers to output a floating-point representation of the second set number based on one or more of an index of the second set of entries or a characteristic of the second set number; and an evaluator unit, coupled to the selector unit, and arranged to determine the function value of the selected approximation function at the first value X1.