Dc (computer Program) - Examples

Examples

Print primes number :

echo '2p3ps#s@s&ds.x'|dc

As an example of a relatively simple program in dc, this command (in 1 line):

dc -e 'psj]shsz PnP10Pdx]dx'

will convert distances from metres to feet and inches; the bulk of it is concerned with prompting for input, printing output in a suitable format and looping round to convert another number.

As an example, here is an implementation of the Euclidean algorithm to find the GCD:

dc -e '??dsax+p' # shortest dc -e 'P?P?dsax+Pp' # easier-to-read version

Computing the factorial of an input value,

dc -e '?sQdsFxp'

A more complex example performs Diffie-Hellman key exchange. This was popular as a signature block among cypherpunks during the ITAR debates:

#!/usr/bin/perl -- -export-a-crypto-system-sig Diffie-Hellman-2-lines ($g,$e,$m)=@ARGV,$m||die"$0 gen exp mod\n";print`echo "16dio1[d2%Sa2/d0A commented version is slightly easier to understand and shows how to use loops, conditionals, and the 'q' command to return from a macro. With a modern version of dc, the '|' command can be used to do arbitrary precision modular exponentiation without needing to write the X function.

#!/usr/bin/perl my ($g,$e,$m) = map { "\U$_" } @ARGV; die "$0 gen exp mod\n" unless $m; print `echo $g $e $m | dc -e ' # Hex input and output 16dio # Read m, e and g from stdin on one line ?SmSeSg # Function z: return g * top of stack sz # Function Q: remove the top of the stack and return 1 sQ # Function X(e): recursively compute g^e % m # It is the same as Sm^Lm%, but handles arbitrarily large exponents. # Stack at entry: e # Stack at exit: g^e % m # Since e may be very large, this uses the property that g^e % m == # if( e == 0 ) # return 1 # x = (g^(e/2)) ^ 2 # if( e % 2 == 1 ) # x *= g # return x % [ d 0=Q # return 1 if e==0 (otherwise, stack: e) d 2% Sa # Store e%2 in a (stack: e) 2/ # compute e/2 lXx # call X(e/2) d* # compute X(e/2)^2 La1=z # multiply by g if e%2==1 lm % # compute (g^e) % m ] SX le # Load e from the register lXx # compute g^e % m p # Print the result '`;

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