Wave print column wise

garbage value is being printed!

#include
using namespace std;

int main() {
int r,c;
cin>>r;
cin>>c;
int a[r][c];
for(int i=0;i<r;i++){
for(int j=0;j<c;j++){
cin>>a[r][c];
}
}

for(int i=0;i<c;i++){
	if(i%2==0){
		for(int j=0;j<r;j++){
			cout<<a[j][i]<<",";
		}
	}
	else
	for(int j=r;j>=0;j--){
		cout<<a[j][i]<<",";
	}
}
cout<<"END";
return 0;

}