pretty printing nested vectors - c++

I have the following code to pretty-print generic vectors -:
// 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 am getting a compiler error if I try to print a nested vector from it.
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 the -std=c++14 flag.
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>')

std::copy( object.begin(), --object.end(), std::ostream_iterator<T1>( out, ", " ) );
You are using copy to ostream iterator which is not defined for vector of std::vector<>. One work around is to implement operator << in terms of operator << of 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.
}
Live example here
This method will not work for the same reason for std::vector<my_type> if opeartor << is not defined for class my_type

Just using a normal forloop instead of std::copy would solve this issue. As #Mohit suggested, the ostream iterator is not defined 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;
}
Output would look like this:
[1, 2]
[3, 4]
[[1, 2], [3, 4]]

Related

map,vector in c++ [duplicate]

This question already has answers here:
How do I print out the contents of a vector?
(31 answers)
Closed 2 years ago.
error: no match for ‘operator<<’ (operand types are ‘std::basic_ostream’ and ‘std::pair<const std::__cxx11::basic_string, std::vector >’)
i want to same key and mutiple values, for example key is 10 values are 2,3,4
but "*iter" is wrong..
how to cout map,vector in c++?
In your code snippet the value of the expression *iter is an object of the type std::pair<std::string, std::vector<int>> for which the operator << is not defined.
And the error message
error: no match for ‘operator<<’ (operand types are ‘std::basic_ostream’ and
‘std::pair<const std::__cxx11::basic_string, std::vector >’)
says about this.
The simplest way is to use the range-based for loop.
Here is a demonstrative program.
#include <iostream>
#include <string>
#include <vector>
#include <map>
int main()
{
std::map<std::string, std::vector<int>> m;
m["10"].assign( { 2, 3, 4 } );
for ( const auto &p : m )
{
std::cout << p.first << ": ";
for ( const auto &item : p.second )
{
std::cout << item << ' ';
}
std::cout << '\n';
}
return 0;
}
The program output is
10: 2 3 4
If you want to write ordinary for-loops using iterators then the loops can look the following way.
#include <iostream>
#include <string>
#include <vector>
#include <map>
#include <iterator>
int main()
{
std::map<std::string, std::vector<int>> m;
m["10"].assign( { 2, 3, 4 } );
for ( auto outer_it = std::begin( m ); outer_it != std::end( m ); ++outer_it )
{
std::cout << outer_it->first << ": ";
for ( auto inner_it = std::begin( outer_it->second );
inner_it != std::end( outer_it->second );
++inner_it )
{
std::cout << *inner_it << ' ';
}
std::cout << '\n';
}
return 0;
}
Again the program output is
10: 2 3 4
I suggest using structured bindings and range-based for loops:
std::map<std::string,std::vector<int>> m;
for (auto&[str, vec] : m) { // bind str to "first" in the pair and vec to "second"
std::cout << str << ':';
for(auto lineno : vec) std::cout << ' ' << lineno;
std::cout << '\n';
}
You can define how to print things via std::ostream like this:
#include <iostream>
#include <map>
#include <vector>
#include <string>
// define how to print std::pair<std::string, std::vector<int>>
std::ostream& operator<<(std::ostream& stream, const std::pair<std::string, std::vector<int>>& pair) {
stream << "(" << pair.first << ", {";
bool first = true;
for (int e : pair.second) {
if (!first) stream << ", ";
stream << e;
first = false;
}
stream << "})";
return stream;
}
int main(void) {
std::string yytext = "hoge";
int lineno = 42;
// below is copied from the question
std::map<std::string,std::vector<int>> m;
m[yytext].push_back(lineno);
std::map<std::string,std::vector<int>>::iterator iter;
for (iter=m.begin(); iter!=m.end(); iter++){
std::cout<<iter->first<<":"<<*iter<<std::endl;}
}

C++ Displaying a vector

I'm learning C++ so my question might seem a bit stupid. I wanted to build a function that print every element in a vector. I come up with that but it seems to display the address of every element. I google it and find a nice solution but i wanted to do it this way so if anyone can explain me where i'm doing something wrong.
My code :
void display_vector(std::vector<int>& to_display);
int main()
{
std::vector<int> vector_to_sort = { 2,6,7,2,1,80,2,59,8,9 };
display_vector(vector_to_sort);
}
void display_vector(std::vector<int> &to_display)
{
for (int i = 0; i < to_display.size(); i++)
{
std::cout << to_display[i] << ', ';
}
std::cout << '\n';
}
The solution i found on internet :
#include <iterator>
void display_vector(const vector<int> &v)
{
std::copy(v.begin(), v.end(),
std::ostream_iterator<int>(std::cout, " "));
}
Output of my code :
21129661129671129621129611129680112962112965911296811296911296
You use ", " instead of ', '.
You can use any of the following print mechanism in the display() function:
void display_vector(std::vector<int> &to_display)
{
//by using Normal for loop
for (auto i = to_display.begin(); i != to_display.end(); ++i) {
cout << *i << " ";
}
cout << endl;
//by using Range based for loop
for (int & i : to_display) {
cout << i << " ";
}
std::cout << '\n';
}
In this statement
std::cout << to_display[i] << ', ';
^^^^^^
you are using a multybyte character literal that has an implementation defined value.
Substitute it for string literal ", ".
As for the function then for starters if the vector is not being changed in the function then the parameter should be a constant reference.
You can use the range-based for loop to outfput elements of the vector like for example
#include <iostream>
#include <vector>
std::ostream & display_vector( const std::vector<int> &to_display, std::ostream &os = std::cout );
int main()
{
std::vector<int> vector_to_sort = { 2,6,7,2,1,80,2,59,8,9 };
display_vector(vector_to_sort) << '\n';
}
std::ostream & display_vector( const std::vector<int> &to_display, std::ostream &os )
{
for ( const auto &item : to_display )
{
os << item << ", ";
}
return os;
}
Using such a function you can for example output the vector in a text file.
Just replace below line
std::cout << to_display[i] << ', ';
with
std::cout << to_display[i] << ", ";
Also note that if you just want to display vector in function then declare parameter as const reference as shown below
void display_vector(const std::vector<int> &to_display);
The debuggers make it easy to examine vectors but I include a simple template to print out vectors of standard types and often use it when debugging data that I wish to look at with other tools.
template<class T>
void print(const std::vector<T>& v){
for (auto x: v)
std::cout << x << std::endl;
}

C++ operator overloading << with vector

Hey guys I am new to C++ and I have a problem with this operator: (Also new in stackoverflow)
This is my class TestList:
class TestList{
public:
TestList() : listItems(10), position(0){};
TestList(int k) : listItems(k), position(0){};
int listItems;
int position;
std::vector<int> arr;
};
//my current operator is: What should be changed?
ostream& operator <<(ostream&, const TestList& tlist, int input){
os << tlist.arr.push_back(input);
return os;
}
//
int main() {
testList testlist(5);
testlist << 1 << 2 << 3; //how should I overload the operator to add these number to testlist.arr ?
return 0;
}
I hope someone could help me or can give me any tips? :)
The other answers are absolutely correct, I just want to say something general on operator<<. It always has the signature T operator<<(U, V), since it is always a binary operator, so it has to have exactly two arguments. Since the chain
a << b << c;
is evaluated as
(a << b) << c;
// That calls
operator<<(operator<<(a, b), c);
the types T and U should normally be the same, or at least compatible.
Furthermore, it is possible but very weird to assign the result of operator<< to something (like result = (a << b))). A good rule of thumb is "My code should not be weird". Therefore the type T should mostly be a reference (so X&) since otherwise it would only be a temporary copy that is unused. And that is pretty useless most of the time.
So in 90% of all cases, your operator<< should have the signature
T& operator<<(T&, V);
I think you mean the following
TestList & operator <<( TestList &tlist , int input )
{
tlist.arr.push_back( input );
return tlist;
}
Here is a demonstrative program
#include <iostream>
#include <vector>
class TestList{
public:
TestList() : listItems(10), position(0){};
TestList(int k) : listItems(k), position(0){};
int listItems;
int position;
std::vector<int> arr;
};
TestList & operator <<( TestList &tlist , int input )
{
tlist.arr.push_back( input );
return tlist;
}
std::ostream & operator <<( std::ostream &os, const TestList &tlist )
{
for ( const auto &item : tlist.arr )
{
std::cout << item << ' ';
}
return os;
}
int main()
{
TestList testlist(5);
testlist << 1 << 2 << 3;
std::cout << testlist << '\n';
return 0;
}
The program output is
1 2 3
You can even write instead of these two statements
testlist << 1 << 2 << 3;
std::cout << testlist << '\n';
only one statement
std::cout << ( testlist << 1 << 2 << 3 ) << '\n';
Pay attention to that there is a typo in your declaration
testList testlist(5);
There should be
TestList testlist(5);

How to print vector<int> to terminal in c++ [duplicate]

This question already has answers here:
How do I print out the contents of a vector?
(31 answers)
Closed 4 years ago.
I am trying to print a vector of integers to the terminal using "cout", but I get an error message during compiling:
no match for 'operator<<' (operand types are 'std::basic_ostream' and 'std::vector')
cout << "Disparity at points: " << disparityVector << endl;
The snippet of code looks like this:
vector<int> disparityVector;
for ( int i=0; i<drawPixels.size(); i++) // Get disparity at each point of the drawn line
disparityVector.push_back((int)disparityMapOutput.at<int16_t>(pos[i].y, pos[i].x));
cout << "Disparity at points: " << disparityVector << endl;
There is no error with assigning the values to the vector, only the "cout" part of the code is making errors
For example, using ostream_iterator.
Sample from that page:
// ostream_iterator example
#include <iostream> // std::cout
#include <iterator> // std::ostream_iterator
#include <vector> // std::vector
#include <algorithm> // std::copy
int main () {
std::vector<int> myvector;
for (int i=1; i<10; ++i) myvector.push_back(i*10);
std::ostream_iterator<int> out_it (std::cout,", ");
std::copy ( myvector.begin(), myvector.end(), out_it );
return 0;
}
You'll need something like below, if you want to do it the way you've coded it.
template<class T>
inline std::ostream& operator<< (std::ostream& o, std::vector<T> const& v) {
for (auto const& i : v)
o << i << " ";
return o;
}
std::ostream& operator<<(std::ostream& os, const vector<int>& v)
{
std::ostream_iterator<int> _oit(cout, " ");
std::copy(v.begin(), v.end(), _oit);
return os;
}
This error means that vector<T> has no operator<< implementation.
You need to iterate over your vector and print each element:
for(int i = 0; i < disparityVector.size; i++) {
cout << disparityVector[i];
}
cout << endl;

Sort std::vector<myclass> in one line using sort function from STL

Question is about sorting std::vector<myclass> using function sort from STL's algorithms class.
Standard way is : sort(v.begin(), v.end(), &myfunct)
where myfunct is:
bool myfunct( myclass first, myclass second ) {
if (first.value < second.value)
return true;
else return false;
}
Approach above takes more than one line. I am curious how to do it in one line. Is it possible define function that compares myclass objects inside sort function? May be somehow use this (a < b) ? a : b. I remember that there is something like this in C#, but I forgot how is it called. Is it possible to do in C++.
First, you can just return first.value < second.value but this doesn't get rid of the function. In C++2011 you can use a lambda function:
std::sort(begin, end, [](myclass const& f, myclass const& s){ return f.value < s.value; });
Without C++2011 I think you'll need a function object because there isn't anything which projects your class to the value you actually want to compare.
BTW, you definitely want to pass everything but the most trivial objects by reference to your comparison function.
You could use boost::lambda and boost::lambda::bind (with boost lambda placeholders)
std::sort(vec.begin(), vec.end(),
boost::lambda::bind(&A::a, boost::lambda::_1)
<
boost::lambda::bind(&A::a, boost::lambda::_2));
sort passes 2 values to the comparison function so you need to compare those 2 values. The bind part of code just selects variable a from the struct A from each structures being compared (referenced by _1 and _2).
Example code:
#include <iostream>
#include <algorithm>
#include <boost/lambda/lambda.hpp>
#include <boost/lambda/bind.hpp>
#include <boost/array.hpp>
struct A
{
A() : a(0), b(0) {}
int a;
int b;
};
std::ostream & operator<<(std::ostream & os, A & a)
{ return os << a.a << ":" << a.b; }
int main()
{
boost::array<A,5> vec;
std::fill(vec.begin(),vec.end(),A());
vec[0].a = 1;
vec[1].a = 3;
vec[2].a = 4;
vec[3].a = 0;
vec[4].a = 2;
std::for_each(vec.begin(),vec.end(), std::cout << boost::lambda::_1 << ' ');
std::cout << std::endl;
std::sort(vec.begin(), vec.end(),
boost::lambda::bind(&A::a, boost::lambda::_1)
<
boost::lambda::bind(&A::a, boost::lambda::_2));
std::for_each(vec.begin(),vec.end(), std::cout << boost::lambda::_1 << ' ');
std::cout << std::endl;
}
Output:
1:0 3:0 4:0 0:0 2:0
0:0 1:0 2:0 3:0 4:0
why not copy the vector into a set:
std::copy(v.begin(),v.end(),std::inserter(s,s.end()));
Now the elements in the set are sorted in ascending order and use set now.
A one liner call to sort() : sort(my_vector_of_class_object.begin(),my_vector_of_class_object.end(),compare);
Code of a working demo of a "sort vector of class objects" is provided below:
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
using namespace std;
class my_Class
{
public:
my_Class(int r,int n, int s):rollno(r),name(n),status(s) { }
int getRollno() const { return rollno;}
int getName() const { return name;}
int getStatus() const { return status;}
private:
int rollno;
int name;
int status;
};
bool compare(const my_Class& x, const my_Class& y) {
return x.getRollno() < y.getRollno();
}
int main()
{
vector<my_Class> my_vector_of_class_object;
vector<my_Class>::const_iterator iter;
my_Class s1(10,20,30);
my_Class s2(40,50,60);
my_Class s3(25,85,9);
my_Class s4(1,50,2);
my_Class s5(90,70,90);
my_Class s6(85,85,3);
my_Class s7(20,6,89);
my_Class s8(70,54,22);
my_Class s9(65,22,77);
my_vector_of_class_object.push_back(s1);
my_vector_of_class_object.push_back(s2);
my_vector_of_class_object.push_back(s3);
my_vector_of_class_object.push_back(s4);
my_vector_of_class_object.push_back(s5);
my_vector_of_class_object.push_back(s6);
my_vector_of_class_object.push_back(s7);
my_vector_of_class_object.push_back(s8);
my_vector_of_class_object.push_back(s9);
cout <<"Before vector sort \n";
for(iter=my_vector_of_class_object.begin(); iter!=my_vector_of_class_object.end();++iter)
std::cout << (*iter).getRollno() << '\t' << (*iter).getName() << '\t' << (*iter).getStatus() << '\n';
cout <<" \n\n";
sort(my_vector_of_class_object.begin(),my_vector_of_class_object.end(),compare);
cout <<"After vector sort \n";
for(iter=my_vector_of_class_object.begin(); iter!=my_vector_of_class_object.end();++iter)
std::cout << (*iter).getRollno() << '\t' << (*iter).getName() << '\t' << (*iter).getStatus() << '\n';
cout <<" \n\n";
return 0;
}