A Ruby program to find the complete ISBN number along with check digit. For example,
Take each digit, from left to right and multiply them alternatively by 1 and 3
Sum those numbers
Take mod 10 of the summed figure
Subtract 10 and if the end number is 10, make it 0
Example for 978014300723:
(9×1) + (7×3) + (8×1) + (0×3) + (1×1) + (4×3) + (3×1) + (0×3) + (0×1) + (7×3) + (2×1) + (3×3)
86
86 mod 10 = 6
10 – 6 = 4
Therefore the complete ISBN is: 9780143007234