字符串大小写转换C++

#include 
#include 
using namespace std;

int main()
{
   char ch[100];
   gets(ch);
   int len=strlen(ch);
   for(int i=0;i='A' && ch[i]<='Z')
   	{ch[i]=ch[i]+32;}
   	else if(ch[i]>='a' && ch[i]<='z')
   	{ch[i]=ch[i]-32;}
   	cout<