Read, transform, print

Problem

Read a hex digit from stdin, double it, print the result. A minimal “compute and print” program.

Code

stl.startup_and_init_all

hex.input_as_hex input_buf, hex_err

// double
hex.add input_buf, input_buf

// print and exit
hex.print 1, input_buf
stl.output '\n'
stl.loop

hex_err:
    stl.output "not a hex digit\n"
    stl.loop

input_buf: hex.hex

Walkthrough

  • stl.startup_and_init_all — needed because we use the hex.* family, which depends on lookup tables initialised by hex.init.

  • hex.input_as_hex dst, err_label — read one ASCII character from stdin, parse it as a hex digit (0-9, a-f, A-F), store the 4-bit value in dst. Branches to err_label on bad input.

  • hex.add input_buf, input_buf — adds input_buf to itself, in place. Doubles it.

  • hex.print 1, input_buf — print as ASCII hex (one nibble = one digit).

Variations

Wider value (use a 4-nibble buffer):

input_buf: hex.vec 4      // 4 nibbles instead of 1
hex.print 4, input_buf

Decimal input instead of a single hex digit — read a whole decimal number until newline/EOF:

hex.input_dec_uint 4, value, hex_err     // read into a 4-nibble value
// ...
value: hex.vec 4                          // declared below stl.loop

hex.input_dec_uint n, dst, error reads ASCII '0'–'9' into the n-nibble dst, stopping at '\n' or '\0' (EOF); it jumps to error only on a byte that is neither a digit nor a terminator. hex.input_dec_int additionally accepts a leading -. Unlike hex.print_dec_uint, these readers use the hex lookup tables, so the program must run stl.startup_and_init_all (as the main example above does), not just stl.startup.

Several numbers on one line — since flipjump 1.4.0 the readers above are thin wrappers around hex.input_dec_uint_until / hex.input_dec_int_until, which stop at the first non-digit byte (after the signed reader’s optional leading -) and hand it back to you instead of treating it as an error:

hex.input_dec_uint_until 4, a, stop_byte  // reads "12 34" up to the space
hex.input_dec_uint_until 4, b, stop_byte  // reads the second number
// ...

// declared below stl.loop:
a:         hex.vec 4
b:         hex.vec 4
stop_byte: hex.vec 2

stop_byte[:2] receives the byte that ended the number (the space, comma, '\n', …), so you decide what separators mean — ideal for parsing space-separated input.

See also