c - Variable (short) changes values when copied/assigned -

i have following code:

  short num_short = 1;   int possible_new_short = 1;   valid = 1;    while (valid) {     possible_new_short = num_short * 10;     printf("----\n");     printf("num short: %d\n", num_short);     printf("possible new short: %d\n", possible_new_short);     if (possible_new_short % 10 == 0) {       num_short = possible_new_short;       printf("new! %d\n", num_short);     } else {       valid = 0;     }     if (num_short == 0) {       valid = 0;     }   }   printf("num_short: %d\n", num_short); 

the output follows:

---- num short: 1 possible new short: 10 new! 10 ---- num short: 10 possible new short: 100 new! 100 ---- num short: 100 possible new short: 1000 new! 1000 ---- num short: 1000 possible new short: 10000 new! 10000 ---- num short: 10000 possible new short: 100000 new! -31072 ---- 

as can see, value of possible_new_short 100000, when reassigned num_short goes -31072. why happening?

i'm new c , guess num_short variable overflowed. because num_short , possible_new_short stored in different slots of memory, 1 can overflow , 1 can't?

what best practices guard against this?

your guess correct, num_short of type short, 16-bit in machines today.

a 16-bit signed integer can hold values @ 2¹⁵ − 1, 32767. last value 100000 overflows.

possible_new_short doesn't overflow because type int. in general, think maximum possible values variable holds, , define type accordingly. example, can use long or long long instead.