请问这个输出菱形图案的怎么错了？

#include <iostream>
#include <cmath>
using namespace std;
int main()
{
int n,m;
cin>>n;
char a[n-1][n-1];
int i,j;
m=(n-1)/2;
for (i=0;i<n;i++)
for (j=0;j<n;j++)
{if (abs((i-m)+(j-m))<(m+1)&&abs((i-m)-(j-m))<(m+1))
a[i][j]='*';
else a[i][j]=' ';}
for (i=0;i<n;i++)
{
for (j=0;j<n;j++)
cout<<a[i][j];
cout<<endl;
}
return 0;
}