in reply to Re: A mod2 Machine.
in thread A mod2 Machine.

Even in C, we can do this without any conversions (i.e. convert a number to string) we can directly read a string and do this

#include <stdio.h> #include <string.h> int main(int argc, char **argv) { char *str = malloc(strlen(argv[1]); strcpy(argv[1], str); char last = str[strlen(str) - 1]; switch (last){ case '0': case '2': case '4': case '6': case '8': case 'A': case 'a': case 'C': case 'c': case 'E': case 'e': printf("even\n"); break; default: printf("odd\n"); break; }

A caveat, in search of a better word, would be the string allocation and copying routine. I used the standard string library functions but it will still take time even though the actual logic (the switch statement) is really fast.

As for the generalization of the idea, yes, this logic is applicable only for mod2 and mod5 machines where only 2 states are sufficient. And yeah, this is applicable for all bases that are of the form 2^n and base 10 (example, convert 112358 base 10 to base 32 and 11235813 base 10 to base 32).

For bases >16, we need to go beyond `F' for example, in base 32, 15 base 10 will be represented as F and 16 as G. Therefore, in base 32, if the number ends with a G, it is even.


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Re^3: A mod2 Machine.
by hdb (Parson) on Jul 04, 2013 at 07:26 UTC

    Correct, if you have the number as a string already, you do not need to convert the number to a string.

    You do not need to copy the string though, you could get the last character directly from argv[1] in your example.