字符串整數

#include <stdio.h>

main()
{
char a[20];
char b[20];
char c[20];
int a_len;
int b_len;
int i;

printf ("please input string a: ");
scanf ("%s", a);
printf ("please input string b: ");
scanf ("%s", b);

a_len = strlen (a);
b_len = strlen (b);

for (i = 0; i < a_len; i++)
{
if (a[i] < '0' || a[i] > '9')
{
printf ("輸入數字串a有錯誤%s\n");
return;
}
}

memset (c, 0, sizeof (c));
sprintf (c, "%d\n", atol (a)+atol(b));
printf ("相加後的數字=%s\n", c);
}