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4.9 Counting Characters

This script shows another way to do arithmetic with sed. In this case we have to add possibly large numbers, so implementing this by successive increments would not be feasible (and possibly even more complicated to contrive than this script).

The approach is to map numbers to letters, kind of an abacus implemented with sed. ‘a’s are units, ‘b’s are tens and so on: we simply add the number of characters on the current line as units, and then propagate the carry to tens, hundreds, and so on.

As usual, running totals are kept in hold space.

On the last line, we convert the abacus form back to decimal. For the sake of variety, this is done with a loop rather than with some 80 s commands(9): first we convert units, removing ‘a’s from the number; then we rotate letters so that tens become ‘a’s, and so on until no more letters remain.

#!/usr/bin/sed -nf

# Add n+1 a's to hold space (+1 is for the newline)
s/./a/g
H
x
s/\n/a/
# Do the carry.  The t's and b's are not necessary,
# but they do speed up the thing
t a
: a;  s/aaaaaaaaaa/b/g; t b; b done
: b;  s/bbbbbbbbbb/c/g; t c; b done
: c;  s/cccccccccc/d/g; t d; b done
: d;  s/dddddddddd/e/g; t e; b done
: e;  s/eeeeeeeeee/f/g; t f; b done
: f;  s/ffffffffff/g/g; t g; b done
: g;  s/gggggggggg/h/g; t h; b done
: h;  s/hhhhhhhhhh//g
: done
$! {
  h
  b
}
# On the last line, convert back to decimal

: loop
/a/! s/[b-h]*/&0/
s/aaaaaaaaa/9/
s/aaaaaaaa/8/
s/aaaaaaa/7/
s/aaaaaa/6/
s/aaaaa/5/
s/aaaa/4/
s/aaa/3/
s/aa/2/
s/a/1/
: next
y/bcdefgh/abcdefg/
/[a-h]/ b loop
p

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