6.24.2 Floating-point output

Floating-point output is always displayed using decimal base (even if BASE is different). The words starting with e. output the FP number in a format that the text interpreter accepts as input.

f. ( r –  ) floating-ext “f-dot”

Display (the floating-point number) r without exponent, followed by a space.

fe. ( r –  ) floating-ext “f-e-dot”

Display r using engineering notation (with exponent dividable by 3), followed by a space.

fs. ( r –  ) floating-ext “f-s-dot”

Display r using scientific notation (with exponent), followed by a space.

fp. ( r –  ) floating-ext “f-p-dot”

Display r using SI prefix notation (with exponent dividable by 3, converted into SI prefixes if available), followed by a space.

e. ( f: r –  ) gforth “e-dot”

Print r with the least number of mantissa digits such that the result, when converted with >float, produces r (for finite r). Caveat: e. only works as specified if represent (which calls the C library’s ecvt_r()) produces the closest mantissa for the given buffer length; that is the case on not-too-old versions of glibc.

e.p ( r u –  ) gforth-experimental “e-dot-p”

Print r with up to u mantissa digits, leaving away trailing mantissa digits. One additional mantissa digit is displayed if this can avoid showing an exponent.

e.exact ( r –  ) gforth-experimental “e-dot-exact”

Print r at full length, with possibly more than 700 mantissa digits. Caveat: e.exact only provides the exact output if represent does (which calls the C library’s ecvt_r()); that is the case on not-too-old versions of glibc.

Examples of printing the number 1234.5678E23 in the different floating-point output formats are shown below.

1234.5678E23
fdup f.      \ 123456780000000000000000000.
fdup fe.     \ 123.456780000000E24
fdup fs.     \ 1.23456780000000E26
fdup fp.     \ 123.456780000000Y
fdup e.      \ 1.2345678e26 ok
fdup 5 e.p   \ 1.2346e26 ok
fdup e.exact \ 123456780000000004618977280e ok
fdrop

Note that 1234.5678E23 cannot be represented exactly as IEEE double-precision (binary64) FP number (Gforth’s float format), so there is a rounding error when converting that string to a float, and the output of e.exact shows the exact value of the resulting number. All the other output words produce a result that rounds the mantissa to fit in a certain number of digits (and possibly extends that with trailing zeros).

The length of the output of f. is influenced by:

precision ( – u  ) floating-ext

u is the number of significant digits currently used by F. FE. and FS.

set-precision ( u –  ) floating-ext

Set the number of significant digits currently used by F. FE. and FS. to u.

You can control the output in more detail with:

f.rdp ( rf +nr +nd +np –  ) gforth-0.6 “f-dot-rdp”

Print float rf formatted. The total width of the output is nr. For fixed-point notation, the number of digits after the decimal point is +nd and the minimum number of significant digits is np. Set-precision has no effect on f.rdp. Fixed-point notation is used if the number of siginicant digits would be at least np and if the number of digits before the decimal point would fit. If fixed-point notation is not used, exponential notation is used, and if that does not fit, asterisks are printed. We recommend using nr>=7 to avoid the risk of numbers not fitting at all. We recommend nr>=np+5 to avoid cases where f.rdp switches to exponential notation because fixed-point notation would have too few significant digits, yet exponential notation offers fewer significant digits. We recommend nr>=nd+2, if you want to have fixed-point notation for some numbers; the smaller the value of np, the more cases are shown in fixed-point notation (cases where few or no significant digits remain in fixed-point notation). We recommend np>nr, if you want to have exponential notation for all numbers.

To give you a better intuition of how they influence the output, here are some examples of parameter combinations; in each line the same number is printed, in each column the same parameter combination is used for printing:

    12 13 0    7 3 4   7 3 0   7 3 1   7 5 1   7 7 1   7 0 2  4 2 1
|-1.234568E-6|-1.2E-6| -0.000|-1.2E-6|-1.2E-6|-1.2E-6|-1.2E-6|****|
|-1.234568E-5|-1.2E-5| -0.000|-1.2E-5|-.00001|-1.2E-5|-1.2E-5|****|
|-1.234568E-4|-1.2E-4| -0.000|-1.2E-4|-.00012|-1.2E-4|-1.2E-4|****|
|-1.234568E-3|-1.2E-3| -0.001| -0.001|-.00123|-1.2E-3|-1.2E-3|****|
|-1.234568E-2|-1.2E-2| -0.012| -0.012|-.01235|-1.2E-2|-1.2E-2|-.01|
|-1.234568E-1|-1.2E-1| -0.123| -0.123|-.12346|-1.2E-1|-1.2E-1|-.12|
|-1.2345679E0| -1.235| -1.235| -1.235|-1.23E0|-1.23E0|-1.23E0|-1E0|
|-1.2345679E1|-12.346|-12.346|-12.346|-1.23E1|-1.23E1|   -12.|-1E1|
|-1.2345679E2|-1.23E2|-1.23E2|-1.23E2|-1.23E2|-1.23E2|  -123.|-1E2|
|-1.2345679E3|-1.23E3|-1.23E3|-1.23E3|-1.23E3|-1.23E3| -1235.|-1E3|
|-1.2345679E4|-1.23E4|-1.23E4|-1.23E4|-1.23E4|-1.23E4|-12346.|-1E4|
|-1.2345679E5|-1.23E5|-1.23E5|-1.23E5|-1.23E5|-1.23E5|-1.23E5|-1E5|