string - Error with strcat in C -

i'm trying use strcat function manipulate strings in linux throws me error. have no idea why.. here code

int yx = 1234; char yx2[100]; sprintf(yx2, "%d", yx);  //converting int str char str[5] = {1,2,3,4,5}; strcat(yx2, str[4]);  //snprintf(yx2, sizeof(yx2), "%s%s", str[4])  <- 1 won't work neither 

the system crash after this...

both arguments of strcat should pointer null terminated char strings (char * type). str[4] of char type.