一个简单的复制过来的字符串到数字的转换

#include <stdio.h>
#include <stdlib.h>

int main(int argc, const char *argv[])
{
char a[] = "123a45";
char *p = a;
int sum = 0;

printf("%s\n",a);
//  printf("%d\n",atoi(a));

while(*p != '\0')
{
if('0' <= *p && *p <= '9')
{
sum = (sum * 10) + (*p - '0');
p++;
}
else
{
break;
}
}

printf("sum = %d\n",sum);

return 0;
}