Pretty printable nested vectors
I have the following code to print generic vectors correctly -:
// print a vector
template<typename T1>
std::ostream& operator <<( std::ostream& out, const std::vector<T1>& object )
{
out << "[";
if ( !object.empty() )
{
std::copy( object.begin(), --object.end(), std::ostream_iterator<T1>( out, ", " ) );
out << *--object.end(); // print the last element separately to avoid the extra characters following it.
}
out << "]";
return out;
}
I get a compiler error if I try to print a nested vector.
int main()
{
vector<vector<int> > a;
vector<int> b;
// cout << b ; // Works fine for this
cout << a; // Compiler error
}
I am using GCC 4.9.2 with a flag -std=c++14
.
The error message given by the compiler is - -
no match for 'operator<<' (operand types are
'std::ostream_iterator<std::vector<int>, char, std::char_traits<char>::ostream_type {aka std::basic_ostream<char>}' and 'const std::vector<int>')
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std::copy( object.begin(), --object.end(), std::ostream_iterator<T1>( out, ", " ) );
You are using copy for an ostream iterator that is not defined for a vector std::vector<>
. One job is to implement operator <<
in terms of operator <<
children.
if ( !object.empty() )
{
//std::copy( object.begin(), --object.end(), std::ostream_iterator<T1>( out, ", " ) );
for(typename std::vector<T1>::const_iterator t = object.begin(); t != object.end() - 1; ++t) {
out << *t << ", ";
}
out << *--object.end(); // print the last element separately to avoid the extra characters following it.
}
This method will not work for the same reason for std::vector<my_type>
if opeartor <<
not defined forclass my_type
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Just using regular forloop instead of std :: copy will solve this problem. As @Mohit suggested, the ostream iterator is undefined for nested vectors.
#include <iostream>
#include <vector>
#include <iterator>
#include <algorithm>
#include <functional>
using namespace std;
// print a vector
template<typename T1>
std::ostream& operator <<( std::ostream& out, const std::vector<T1>& object )
{
out << "[";
if ( !object.empty() )
{
for(typename std::vector<T1>::const_iterator
iter = object.begin();
iter != --object.end();
++iter) {
out << *iter << ", ";
}
out << *--object.end();
}
out << "]";
return out;
}
int main() {
std::vector<std::vector<int> > a;
std::vector<int> b;
b.push_back(1);
b.push_back(2);
std::vector<int> c;
c.push_back(3);
c.push_back(4);
std::cout << b << std::endl;
std::cout << c << std::endl;
a.push_back(b);
a.push_back(c);
cout << a; // Compiler error
return 0;
}
The result will look like this:
[1, 2]
[3, 4]
[[1, 2], [3, 4]]
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