Sed substitution possible with arithmetic involved?

Question

File I need to modify contains the following:

block: 16, size: 16, start: 8, length: 4

I\'d like the file so that values for block<

Answer 1

(The right tool to do this is awk, but for the fun of a sed exercise...)

It is possible in sed. After all, a multiplication by 2 is set of substitution of the last digit according to some simple rules:

• 0 --> 0
• 1 --> 2
• 2 --> 4
• 3 --> 6
• ...
• 8 --> 16
• 9 --> 18

To take care of the carry digit, each rules should be written twice.

This sed script, that can be run with sed -f script, do the multiplication by 2 of all the numbers on the input lines:

s/$/\n\n/
:loop
s/0\n1\n/\n\n1/;t loop
s/0\n\n/\n\n0/;t loop
s/1\n1\n/\n\n3/;t loop
s/1\n\n/\n\n2/;t loop
s/2\n1\n/\n\n5/;t loop
s/2\n\n/\n\n4/;t loop
s/3\n1\n/\n\n7/;t loop
s/3\n\n/\n\n6/;t loop
s/4\n1\n/\n\n9/;t loop
s/4\n\n/\n\n8/;t loop
s/5\n1\n/\n1\n1/;t loop
s/5\n\n/\n1\n0/;t loop
s/6\n1\n/\n1\n3/;t loop
s/6\n\n/\n1\n2/;t loop
s/7\n1\n/\n1\n5/;t loop
s/7\n\n/\n1\n4/;t loop
s/8\n1\n/\n1\n7/;t loop
s/8\n\n/\n1\n6/;t loop
s/9\n1\n/\n1\n9/;t loop
s/9\n\n/\n1\n8/;t loop
s/\n1\n/\n\n1/;t loop
s/\(.\)\n\n/\n\n\1/;t loop
s/^\n\n//

Dividing an even number by 2, is the same logic, but from left to right instead of right to left:

s/^/\n\n/
:loop
s/\n1\n0/5\n\n/;t loop
s/\n\n0/0\n\n/;t loop
s/\n1\n1/5\n1\n/;t loop
s/\n\n1/\n1\n/;t loop
s/\n1\n2/6\n\n/;t loop
s/\n\n2/1\n\n/;t loop
s/\n1\n3/6\n1\n/;t loop
s/\n\n3/2\n1\n/;t loop
s/\n1\n4/7\n\n/;t loop
s/\n\n4/2\n\n/;t loop
s/\n1\n5/7\n1\n/;t loop
s/\n\n5/2\n1\n/;t loop
s/\n1\n6/8\n\n/;t loop
s/\n\n6/3\n\n/;t loop
s/\n1\n7/8\n\n/;t loop
s/\n\n7/3\n1\n/;t loop
s/\n1\n8/9\n\n/;t loop
s/\n\n8/4\n\n/;t loop
s/\n1\n9/9\n1\n/;t loop
s/\n\n9/4\n1\n/;t loop
s/\n1\n/5\n\n/;t loop
s/\n\n\(.\)/\1\n\n/;t loop
s/\n\n$//

Combining those, this script do the job:

h
s/, start.*//
s/^/\n\n/
t loopa
:loopa
s/\n1\n0/5\n\n/;t loopa
s/\n\n0/0\n\n/;t loopa
s/\n1\n1/5\n1\n/;t loopa
s/\n\n1/\n1\n/;t loopa
s/\n1\n2/6\n\n/;t loopa
s/\n\n2/1\n\n/;t loopa
s/\n1\n3/6\n1\n/;t loopa
s/\n\n3/2\n1\n/;t loopa
s/\n1\n4/7\n\n/;t loopa
s/\n\n4/2\n\n/;t loopa
s/\n1\n5/7\n1\n/;t loopa
s/\n\n5/2\n1\n/;t loopa
s/\n1\n6/8\n\n/;t loopa
s/\n\n6/3\n\n/;t loopa
s/\n1\n7/8\n\n/;t loopa
s/\n\n7/3\n1\n/;t loopa
s/\n1\n8/9\n\n/;t loopa
s/\n\n8/4\n\n/;t loopa
s/\n1\n9/9\n1\n/;t loopa
s/\n\n9/4\n1\n/;t loopa
s/\n1\n/5\n\n/;t loopa
s/\n\n\(.\)/\1\n\n/;t loopa
s/\n\n$//
H
g
s/.*, start/, start/
s/\n.*//
s/$/\n\n/
t loopb
:loopb
s/0\n1\n/\n\n1/;t loopb
s/0\n\n/\n\n0/;t loopb
s/1\n1\n/\n\n3/;t loopb
s/1\n\n/\n\n2/;t loopb
s/2\n1\n/\n\n5/;t loopb
s/2\n\n/\n\n4/;t loopb
s/3\n1\n/\n\n7/;t loopb
s/3\n\n/\n\n6/;t loopb
s/4\n1\n/\n\n9/;t loopb
s/4\n\n/\n\n8/;t loopb
s/5\n1\n/\n1\n1/;t loopb
s/5\n\n/\n1\n0/;t loopb
s/6\n1\n/\n1\n3/;t loopb
s/6\n\n/\n1\n2/;t loopb
s/7\n1\n/\n1\n5/;t loopb
s/7\n\n/\n1\n4/;t loopb
s/8\n1\n/\n1\n7/;t loopb
s/8\n\n/\n1\n6/;t loopb
s/9\n1\n/\n1\n9/;t loopb
s/9\n\n/\n1\n8/;t loopb
s/\n1\n/\n\n1/;t loopb
s/\(.\)\n\n/\n\n\1/;t loopb
s/^\n\n//
H
g
s/[^\n]*\n//
s/\n//

(Much easier in awk thought.)

Note: I once saw a Turing Machine implementation is sed, so I try to remember that anything that can be done with a programming language can be done in sed. That of course does not mean that sed is the good tool in all situations.