I have to print out the size of the vector and all the contents inside it. But my for loop doesnt iterate, it doesn't go up by one but instead stays at the value 0 for the whole loop.
#include "stdafx.h"
#include <string.h>
#include <string>
#include <iostream>
#include <cctype>
#include <vector>
#include <list>
using std::string;
using std::vector;
vector<int> v2(10);
for( auto i : v2)
{
if( i == 0 )
{
std::cout << v2.size() << std::endl;
}
std::cout << "Element value " << (i) << " is " << v2[i] << std::endl;
}
So I only want to print the size once, at the start. Then print out each element value which I know will be 0 by default. But it just prints out "Element value 0 is 0" 9 times.
If you want at first to print out the size of the vector then place this output statement before the range based for statement
std::vector<int> v2(10);
std::cout << v2.size() << std::endl;
size_t i = 0;
for ( auto x : v2 )
{
std::cout << "Element value " << i++ << " is " << x << std::endl;
}
But if you are using a count then it would be better to use the ordinary for statement
std::vector<int> v2(10);
std::cout << v2.size() << std::endl;
for ( std::vector<int>::size_type i = 0; i < v2.size(); i++ )
{
std::cout << "Element value " << i << " is " << v2[i] << std::endl;
}
Your question indicates that you want to treat the first element differently. This does not work with range-based for loops out of the box. You have two options:
Use an extra variable to remember whether the first iteration was made.
Use a traditional loop, using an index variable or an iterator.
Example for 1:
bool first_iteration = true;
for( auto i : v2)
{
if (first_iteration)
{
std::cout << v2.size() << std::endl;
first_iteration = false;
}
// ...
}
Example for 2:
for (auto iter = v2.begin(); iter != v2.end(); ++iter)
{
if (iter == v2.begin())
{
std::cout << v2.size() << std::endl;
}
// ...
}
Related
Ex:
vector<string> myVec = {apple, banana, grape}
How can I print these elements as an ordered list using a range-based loop
Output:
1 apple
2 banana
3 grape
A variation of Jeffrey's answer, but without additional variable:
for (const auto& s : myVec)
{
std::cout << &s - &myVec[0] << " " << s << "\n";
}
This, of course, prints a "correct" 0-based index. Feel free to add 1 to it :)
Using boost ranges really simplifies things Live Demo
using namespace boost::adaptors;
std::vector<std::string> myVec = {"apple", "banana", "grape"};
for (const auto& element : myVec | indexed(1))
{
std::cout << element.index() << " " << element.value() << "\n";
}
Produces
1 apple
2 banana
3 grape
You are looking for
size_t position = 1;
for(const auto& s: myVec)
{
std::cout << position << " " << s << "\n";
position++;
}
as in
#include <iostream>
#include <string>
#include <vector>
using std::vector;
using std::string;
vector<string> myVec = {"apple", "banana", "grape"};
int main()
{
size_t position = 1;
for(const auto& s: myVec)
{
std::cout << position << " " << s << "\n";
position++;
}
}
With range-v3, you could write:
for (auto [i, val] : myVec | ranges::views::enumerate)
{
std::cout << i << ' ' << val << "\n";
}
Here's a demo.
This would be a good issue for the original for loop:
const size_t quantity = myVec.size();
for (unsigned int i = 0; i < quantity; ++i)
{
cout << (i + 1) << " " << myVec[i] << "\n";
}
Simple, effective. Don't knock the old stuff. :-)
So i have a c++ vector which contains about 106 thousand words which are stored on vector<string>words i need to find the longest word in this vector and i also need to get the location at which the word is, for example (1,2,3) in my vector. I need this location because i have two other vectors which has the meaning and the type for the words. vector<string>definition, vector<string>type
Please help
My current code
this code is not working at all
copy_if(words.begin(), words.end(), back_inserter(length), [](const string& x) { return x.length() > 40; });// looks for words longer than 7 letters
for (const string& s : length)
{
cout << "found!!" << endl;
auto i = find(words.begin(), words.end(), s);//looks for the word in the words vector
if (i != words.end())
{
auto pos = i - words.begin();
//displays the word, type and the definition of the word that the user has entered
cout << "Word : " << words[pos] << '\n';
cout << "Type : " << definitions[pos] << '\n';
cout << "Definition: " << types[pos] << '\n';
cout << '\n';
}
else
cout << "word not found" << endl;
}
You could use the standard algorithm std::max_element to search through the vector<string>.
Example:
#include <algorithm> // max_element
#include <iostream>
#include <iterator> // distance
#include <string>
#include <vector>
int main() {
std::vector<std::string> words{"a", "bb", "ccc"};
auto it = std::max_element(words.begin(), words.end(),
[](const auto& a, const auto& b) {
return a.size() < b.size();
});
std::cout << "The longest word is " << *it << " at (zero-based) pos "
<< std::distance(words.begin(), it) << '\n';
}
Output:
The longest word is ccc at (zero-based) pos 2
I would prefer thinking simply: just check length of elements according to each indice and update information according to that.
std::vector<std::string> length;
// initialize the vector length
size_t max_length = 0; // the length of longest word(s)
std::vector<size_t> max_indice; // the indice of longest word(s)
for (size_t i = 0; i < length.size(); i++) {
size_t this_len = length[i].length();
if (this_len > max_length) {
// new record
max_length = this_len;
max_indice.clear();
max_indice.push_back(i);
} else if (this_len == max_length) {
// tie
max_indice.push_back(i);
}
}
for (size_t pos : max_indice) {
cout << "Word : " << words[pos] << '\n';
cout << "Type : " << definitions[pos] << '\n';
cout << "Definition: " << types[pos] << '\n';
cout << '\n';
}
Assume that you have an std::vector<T> of some type T and a selection of indices std::vector<int> of this vector. Now I'm looking for a function permute(const std::vector<T>& vector, const std::vector<int>& indices), that returns the permuted vector with respect to the given indices.
The problem is easily solved by writing a short function like depicted below:
template<typename T>
std::vector<T> permute(const std::vector<T>& matrix, const std::vector<int>& indices) {
std::vector<T> ret;
for (auto p : indices) {
ret.push_back(matrix[p]);
}
return ret;
}
int main(int, char**) {
std::vector<int> perm{ 1,2,0 };
std::vector<std::vector<double>> matrix = { {1.,2.,3.},{4.,5.,6.},{7.,8.,9.} };
auto matrixPerm=permute(matrix, perm);
std::cout << matrixPerm[0][0] << " == " << matrix[1][0] << std::endl;
std::cout << matrixPerm[1][0] << " == " << matrix[2][0] << std::endl;
std::cout << matrixPerm[2][0] << " == " << matrix[0][0] << std::endl;
}
I'm now wondering what might be most elegant version of this program, if we can use STL or even the Boost libraries. In STL for example we have shuffle(), but we cannot say in what way to shuffle.
Does anyone now, how to shorten the function?
Solution using std::transform()
#include <vector>
#include <iostream>
#include <iterator>
#include <algorithm>
int main(int, char**) {
std::vector<int> perm{ 1,2,0 };
std::vector<std::vector<double>> matrix = { {1.,2.,3.},{4.,5.,6.},{7.,8.,9.} };
std::vector<std::vector<double>> output;
std::transform(perm.begin(), perm.end(), std::back_inserter(output), [&](int i) { return matrix[i]; });
std::cout << output[0][0] << " == " << matrix[1][0] << std::endl;
std::cout << output[1][0] << " == " << matrix[2][0] << std::endl;
std::cout << output[2][0] << " == " << matrix[0][0] << std::endl;
}
You can transform the indices into iterators and then create an indirect range with Boost.Range.
#include <iostream>
#include <iterator>
#include <algorithm>
#include <boost/range/adaptor/indirected.hpp>
#include <boost/range/adaptor/transformed.hpp>
#include <boost/range/algorithm/copy.hpp>
int main(int, char**) {
using namespace boost::adaptors;
std::vector<int> perm{ 1,2,0 };
std::vector<std::vector<double>> matrix = { {1.,2.,3.},{4.,5.,6.},{7.,8.,9.} };
std::vector<std::vector<double>> output;
auto permutation = perm | transformed( [&matrix](int x) { return matrix.begin() + x; }) | indirected;
boost::copy(
permutation,
std::back_inserter(output));
std::cout << output[0][0] << " == " << matrix[1][0] << std::endl;
std::cout << output[1][0] << " == " << matrix[2][0] << std::endl;
std::cout << output[2][0] << " == " << matrix[0][0] << std::endl;
}
You could skip copying the elements and just process the range if you don't need a real vector.
The range adaptor uses the permutation iterator from the Boost.Iterator library. You can also use this directly, but you have to manually define begin and end:
auto begin = make_permutation_iterator( matrix.begin(), perm.begin() );
auto end = make_permutation_iterator( matrix.end(), perm.end() );
std::copy(begin, end, std::back_inserter(output) );
In my C++ script, I want to insert some elements of a list into a vector (from the beginning of list to a specific position "it"), and then try to add the vector at the top of the list and keeping the same order of the vector but I get unwanted additional elements in the vector.
Here is my code:
#include <iostream>
#include <iterator>
#include <vector>
#include <list>
int main() {
std::list<int> mylist;
for (int i = 0; i < 10; i++)
mylist.push_back(i * 10);
for (std::list<int>::iterator i = mylist.begin(); i <= mylist.end(); ++i) {
std::cout << *i << ", ";
}
std::cout << std::endl;
std::advance(it, 6);
std::cout << "The 6th element in mylist is: " << *it << std::endl;
// The vector that will contain mylist elements
std::vector<int> intAdeplacer;
intAdeplacer.insert(intAdeplacer.end(), mylist.begin(), it);
std::cout << std::endl;
std::cout << "Nombre d'éléments dans le vecteur : " << intAdeplacer.size() << std::endl;
std::cout << std::endl;
// see the content of the vector
std::cout << "Le vecteur de deplacement contient : " << std::endl;
for (std::vector<int>::const_iterator i = intAdeplacer.begin();
i <= intAdeplacer.end(); ++i) {
std::cout << *i << ", ";
}
I get this output:
Le vecteur de deplacement contient :
0, 10, 20, 30, 40, 134985,
134985 is not wanted..
// Insert in front of the list the values of the vector and keeping the same order of elements in the vector
for (std::vector<int>::const_iterator i = intAdeplacer.end();
i >= intAdeplacer.begin(); --i) {
mylist.push_front(*i);
}
std::cout << std::endl;
std::cout << std::endl;
std::cout << "nouvelle composition de mylist : " << std::endl;
for (std::list<int>::const_iterator j = mylist.begin(); j != mylist.end();
++j) {
std::cout << *j << ", ";
}
std::cout << std::endl;
std::cout << std::endl;
// erasing the added elements from mylist
std::list<int>::iterator debut = mylist.begin();
std::list<int>::iterator fin = mylist.end();
std::advance(fin, 6);
mylist.erase(debut, fin);
for (std::list<int>::iterator j = mylist.begin(); j <= mylist.end(); ++j) {
std::cout << *j << ", ";
}
return 0;
}
If it is an iterator pointing at the 6th element of your list, the following insert() will insert from begin() (included) to it (excluded) into the vector:
intAdeplacer.insert(intAdeplacer.end(), mylist.begin(), it);
So you'll have only 5 elements, from 0 to 40. Unfortunately your printing loop does include the end() of the vector, which is out of range. This is why you get this strange trailing number.
Just correct your loop into:
for (auto i = intAdeplacer.begin(); i != intAdeplacer.end(); ++i) {
std::cout << *i << ", ";
}
Or consider this alternate range-for syntax:
for (auto &element: intAdeplacer) {
std::cout << element << ", ";
}
Is it possible to replace the BOOST_FOREACH in this example with a "pure" C++11 equivalent?
#include <map>
#include <functional>
#include <boost/foreach.hpp>
#include <iostream>
int main() {
std::map<int, std::string> map = {std::make_pair(1,"one"), std::make_pair(2,"two")};
int k;
std::string v;
BOOST_FOREACH(std::tie(k, v), map) {
std::cout << "k=" << k << " - " << v << std::endl;
}
}
The key feature being keeping the key/value pair in the references to k and v.
I tried:
for(std::tie(k,v) : map)
{
std::cout << "k=" << k << " - " << v << std::endl;
}
and
auto i = std::tie(k,v);
for(i : map)
{
std::cout << "k=" << k << " - " << v << std::endl;
}
But none of the ranged based for loop ideas seemed to work. Presumably the ranged based for loop is required to have a declaration before the :, since even:
std::vector<int> test;
int i;
for (i : test);
Isn't valid.
The closest equivalent I can find is:
for (auto it = map.begin(); it!=map.end() && (std::tie(k,v)=*it,1); ++it)
{
std::cout << "k=" << k << " - " << v << std::endl;
}
which isn't quite as succinct as the BOOST_FOREACH version!
Is there a way to express the same thing succinctly without boost in C++11?
for (auto & i : map)
{
std::tie(k,v) = i;
// your code here
}
This produces the same output as the Boost macro
for( auto const& k : map ) {
std::cout << "k = " << k.first << " - " << k.second << std::endl;
}
With C++17 this can now be done using structured bindings, for instance:
#include <map>
#include <string>
#include <iostream>
int main() {
const std::map<std::string, std::string> map = {std::make_pair("hello", "world")};
for (auto& [k,v]: map) {
std::cout << "k=" << k << ", v=" << v << "\n";
}
}
This is certainly what I'd choose to do in newer projects.