本人新手，刚开始学编程，求大神帮忙看看这个程序编译没有错误，只是最后不输出结果

#include <iostream>
using namespace std;
int main()
{
    char str[100];
    int i,a=0,b=0,c=0,d=0;
    cout<<"input a string:  ";
    cin>>str;
    while(str[i]!='\0');
    {
        if(str[i]==' ')
        a++;
        else if(str[i]>='a'&&str[i]<='z'||str[i]>='A'&&str[i]<='Z')
        b++;
        else if(str[i]>='0'&&str[i]<='9')
        c++;
        else
        d++;
        i++;
    }
    cout<<"the number of space is"<<a;
    cout<<"the number of letter is"<<b;
    cout<<"the number of figure is"<<c;
    cout<<"the number of else character"<<d;
    return 0;
}