请问这个关于结构体的C++小程序哪里出错了呢?
程序的用意是将数组中的元素首位倒置

#include<iostream>
using namespace std;

typedef struct DATA{
char num[10];
int len;
}DATA;

void turnstr(DATA str)
{
int i=0;
int len_mid=str.len/2;
char t;
while( i==len_mid){
t=str.num[i];
str.num[i]=str.num[str.len];
str.num[str.len]=t;
i++;
str.len--;
}
}
int main()
{
DATA data={"abcdefg" ,7};
for(int i=0;i<7;i++){
cout<<data_num[i];
}
cout<<endl;
turnstr(data);
for(int i=0;i<7;i++){
cout<<data_num[i];
}
cout<<endl;
return 0;
}

错误提示是这样的
--------------------Configuration: test - Win32 Debug--------------------
Compiling...
test.cpp
C:\Documents and Settings\Administrator\桌面\program\test\test.cpp(28) : error C2065: 'data_num' : undeclared identifier
C:\Documents and Settings\Administrator\桌面\program\test\test.cpp(28) : error C2109: subscript requires array or pointer type
C:\Documents and Settings\Administrator\桌面\program\test\test.cpp(35) : error C2109: subscript requires array or pointer type
执行 cl.exe 时出错.
test.exe - 1 error(s), 0 warning(s) C++?C?结构体 C++ C 结构体 struct
[解决办法]
C:\Documents and Settings\Administrator\桌面\program\test\test.cpp(28) : error C2065: 'data_num' : undeclared identifier
应该是cout<<data_num[i];你这手误吧