Lunar Arithmetic

January 22, 2019

Over at Numberphile, Neil Sloane (yes, that Neil Sloane), talks about lunar arithmetic, an “other-worldly” way of doing simple math:

Lunar addition and multiplication work digit by digit, the same as terrestrial addition and multiplication, except that the plus and times tables are unusual. Addition is done by taking the larger of the two digits, so 1 + 3 = 3 and 7 + 4 = 7. Multiplication is done by taking the smaller of the two digits, so 1 * 3 = 1 and 7 * 4 = 4. Thus:

      3 5 7        3 5 7
    +   6 4      *   6 4
    -------      -------
      3 6 7        3 4 4
                 3 5 6
                 -------
                 3 5 6 4

There are no carries. There is no subtraction or division, since the results would not be unique.

Your task is to write programs that perform lunar addition and multiplication. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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