关于使用if函数来输出出错提示的有关问题

关于使用if函数来输出出错提示的问题
比如正确的条件是x>=0，用户输入x<0时自然输出错误提示，如何保证用户输入其他信息时也输出出错提示(比如输入一个字母神马的)
# include <stdio.h>
# include <math.h>
int main ()
{
int n,i;
double sum=0;
printf ("Enter n: ");
scanf ("%d",&n);
if (n<=0){
printf ("Error!\n");
}
else {
for (i=1;i<=n;i++){
sum=sum+pow(-1,i+1)*(double)i/(double)(2*i-1);
}
printf ("sum=%g\n",sum);
}
return 0;
}

[解决办法]

C/C++ code

#include <stdio.h>#include <math.h>int main (){    int n, i;    double sum = 0;    printf("Enter n: ");    if (scanf("%d", &n) != 1)        printf ("Error!\n");    else    {        if (n <= 0)            printf ("Error!\n");        else        {            for (i = 1; i <= n; ++i)                sum += pow(-1, i + 1) * (double)i / (double)(2 * i - 1);            printf ("sum = %g\n", sum);        }    }    return 0;}
[解决办法]
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C/C++ code

#include <stdio.h>
#include <math.h>

int main ()
{

    int n, i;
    double sum = 0;

    printf("Enter n: ");
    if (scanf("%d", &amp;n) != 1)
        printf ("Error!\n");
    else
 ……