So dc is a great tool for converting between bases - handy for those bit twiddling coding jobs. e.g to convert 1078 into binary I can do this:
bash> echo "2o1078p" | dc
10000110110
However I can't get it to print fractions between 0 and 1 correctly. Trying to convert 0.3 into binary:
bash> echo "2o10k 0.3p" | dc
.0100
But 0.0100(bin) = 0.25 not 0.3.
However if I construct the value manually I get the right answer
bash> echo "2o10k 3 10 / p" | dc
.0100110011001100110011001100110011
Well it looks like its giving me more than the 10 significant figures I ask for but thats OK
Am I doing something wrong? Or am I trying to make dc do something that its not able to do?
bash> dc --version
dc (GNU bc 1.06) 1.3
...
It seems that dc is getting the number of significant figures from the input.
Now 1/log10(2)=3.32 so each decimal significant digit is 3.3 binary digits.
Looking at the output of dc for varying input SF lengths shows:
`dc -e "2o10k 0.3 p"` => .0100
`dc -e "2o10k 0.30 p"` => .0100110
`dc -e "2o10k 0.300 p"` => .0100110011
`dc -e "2o10k 0.3000 p"` => .01001100110011
A table of these values and expected value, ceil(log10(2)*SFinput) is as follows:
input : output : expected output
1 : 4 : 4
2 : 7 : 7
3 : 10 : 10
4 : 14 : 14
And dc is behaving exactly as expected.
So the solution is to either use the right number of significant figures in the input, or the division form dc -e "2o10k 3 10 / p"