Delete last 3 elements in list c++ - c++

I have this c++ code which works fine in deleting the last 3 elements of a list, but i was wondering if it's the correct way to do such a thing since i am worried of deleting elements with iterators issues.
The code basically takes a list of 6 elements "Groups", divides it into 2 smaller lists "Group1" and "Group2", and then compares a different List "GroupToCompare" to "Group2" and if they're equal it removes the last 3 elements of "Groups".
#include "pch.h"
#include <iostream>
#include <iostream>
#include <string>
#include <list>
using namespace std;
int main()
{
std::list <string> Groups = {};
Groups = { "Spike", "Jet", "Faye", "Edward", "Vincent", "Elektra" };
std::list<string> Group1 = {};
std::list<string> Group2 = {};
std::list<string> GroupToCompare = {};
GroupToCompare = { "Edward", "Vincent", "Elektra" };
size_t half1 = Groups.size() / 2;
std::list<std::string>::iterator ig = Groups.begin();
advance(ig, half1);
Group1.insert(Group1.end(), Groups.begin(), ig);
Group2.insert(Group2.end(), ig, Groups.end());
std::list<std::string>::iterator removeIt = Groups.begin();
advance(removeIt, half1);
cout << "List Elements 1: " << endl;
std::list<string>::iterator itrofList = Group1.begin();
string firstvar;
for (itrofList = Group1.begin(); itrofList != Group1.end(); ++itrofList) {
firstvar = *itrofList;
cout << "Item: " << firstvar << endl;
}
cout << "List Elements 2: " << endl;
std::list<string>::iterator itrofList1 = Group2.begin();
string firstvar1;
for (itrofList1 = Group2.begin(); itrofList1 != Group2.end(); ++itrofList1) {
firstvar1 = *itrofList1;
cout << "Item: " << firstvar1 << endl;
}
if (Group2 == GroupToCompare) {
removeIt = Groups.erase(removeIt);
removeIt = Groups.erase(removeIt);
removeIt = Groups.erase(removeIt);
}
cout << "List Elements of Groups after removing the last 3 elements: " << endl;
std::list<string>::iterator itrofList2 = Groups.begin();
string firstvar2;
for (itrofList2 = Groups.begin(); itrofList2 != Groups.end(); ++itrofList2) {
firstvar2 = *itrofList2;
cout << "Item: " << firstvar2 << endl;
}
}
is there a more correct way in case i wanna delete more elements in the end of the list and avoid deleting issues with iterators?
Thanks in advance!

Even though calling std::list::erase() multiple times with the returned position works, calling the correct overload would help with readability and possibly performance:
iterator erase( iterator first, iterator last );
Example:
std::list<int> l{1, 2, 3, 4, 5};
l.erase(std::prev(l.end(), 3), l.end());
// l is {1, 2}
Beware: as-is, this code exhibit undefined behavior if l is not at least 3 in size.

Is there a more correct way to do it?
Not sure what that means, but im assuming you would be interested in learning std::list has a public member function pop_back which deletes the last element in a list. Using this seems ideal for your case.
Note: As stated in this refference
If the container is not empty, the function never throws exceptions
(no-throw guarantee). Otherwise, it causes undefined behavior.
So make sure the list is never empty when you call this function.

I would just...
std::list <string> Groups;
...
for (int i = 0; i < 3; ++i)
Groups.pop_back();

Related

accessing elements of dynamic array of lists

So, I don't know how can I print elements of such a list.
list<int>* a;
a = new list<int>(4);
a[0].push_back(1);
a[0].push_back(3);
a[2].push_back(5);
a[2].push_back(7);
cout << a[0].front() << '\n';
cout << a[1].back() << '\n';
Firstly, I tried to print it via range-based for loop, but it didn't work either.
for(auto element: a[0]) cout << element << '\n'; // doesn't work
Are you trying to store a list of integer lists? Because this implementation will not work since you only have a list of integers and no push_back() operation is available for the elements.
Remove the index operator for all those push_back() operations and take out the index operator for the front() and back() as those are not available to the elements either.
I would use a std::vector instead of new (which should technically be new[] in this case anyway).
#include <iostream>
#include <list>
#include <vector>
int main() {
std::vector<std::list<int>> a(4);
a[0].push_back(1);
a[0].push_back(3);
a[2].push_back(5);
a[2].push_back(7);
for (std::list<int> const& l : a) {
for (int i : l) {
std::cout << i << ' ';
}
std::cout << '\n';
}
}
Output
1 3
5 7

Duplicating std::list with std::copy and removal with std::list::erase

In the bellow example code after assigning example list with numbers I'm trying to duplicate container with std::copy but problem is at runtime it says "cannot dereference end list iterator".
my question is how do I duplicate the list so that duplicated range is inserted to the end of the list?
to the end because I later need to be able to remove duplicated range, that is why I save the beginning of the new range to iterator.
#include <iostream>
#include <list>
#include <algorithm>
void print(std::list<int>& ref)
{
for (auto& num : ref)
{
std::cout << num << std::endl;
}
}
int main()
{
std::list<int> mylist{ 1, 2, 3, 4 };
std::list<int>::iterator iter = mylist.end();
std::cout << "INITIAL LIST NUMBERS" << std::endl;
print(mylist);
// duplicate list, will cause runtime error
iter = std::copy(mylist.begin(), mylist.end(), --mylist.end());
std::cout << "COPIED LIST IS NOW CONTAINS DUPLICATE NUMBERS" << std::endl;
print(mylist);
// remove previsous duplication
mylist.erase(iter, mylist.end());
std::cout << "AFTER REMOVAL OF COPIED LIST SHOULD BE SAME AS INITIAL LIST" << std::endl;
print(mylist);
std::cin.get();
return 0;
}
You can use std::copy_n. This circumvents the issue with std::copy, which would execute an infinite loop of insertions when fed with a std::back_inserter(mylist) and an always valid mylist.end() iterator.
const std::size_t n = mylist.size();
std::copy_n(mylist.cbegin(), n, std::back_inserter(mylist));
De-duplication then works with
mylist.erase(std::next(mylist.begin(), n), mylist.end());
if (!mylist.empty()) --iter;
std::copy_n(mylist.begin(), mylist.size(), std::back_inserter(mylist));
if (!mylist.empty()) ++iter;
Unfortunately we can't use end iterator in copy(), since it might lead to an infinite loop, as new elements are added between the end and the current iterator all the time.

vector element compare c++

This program takes a word from text and puts it in a vector; after this it compares every element with the next one.
So I'm trying to compare element of a vector like this:
sort(words.begin(), words.end());
int cc = 1;
int compte = 1;
int i;
//browse the vector
for (i = 0; i <= words.size(); i++) { // comparison
if (words[i] == words[cc]) {
compte = compte + 1;
}
else { // displaying the word with comparison
cout << words[i] << " Repeated : " << compte; printf("\n");
compte = 1; cc = i;
}
}
My problem in the bounds: i+1 may exceed the vector borders. How to I handle this case?
You need to pay more attention on the initial conditions and bounds when you do iteration and comparing at the same time. It is usually a good idea to execute your code using pen and paper at first.
sort(words.begin(), words.end()); // make sure !words.empty()
int cc = 0; // index of the word we need to compare.
int compte = 1; // counting of the number of occurrence.
for( size_t i = 1; i < words.size(); ++i ){
// since you already count the first word, now we are at i=1
if( words[i] == words[cc] ){
compte += 1;
}else{
// words[i] is going to be different from words[cc].
cout << words[cc] << " Repeated : " << compte << '\n';
compte = 1;
cc = i;
}
}
// to output the last word with its repeat
cout << words[cc] << " Repeated : " << compte << '\n';
Just for some additional information.
There are better ways to count the number of word appearances.
For example, one can use unordered_map<string,int>.
Hope this help.
C++ uses zero-based indexing, e.g., an array of length 5 has indices: {0, 1, 2, 3, 4}. This means that index 5 is outside of the range.
Similarly, given an array arr of characters:
char arr[] = {'a', 'b', 'c', 'd', 'e'};
The loop for (int i = 0; i <= std::size(arr); ++i) { arr[i]; } will cause a read from outside of the range when i is equal to the length of arr, which causes undefined behaviour. To avoid this the loop must stop before i is equal to the length of the array.
for (std::size_t i = 0; i < std::size(arr); ++i) { arr[i]; }
Also note the use of std::size_t as type of the index counter. This is common practice in C++.
Now, let's finish with an example of how much easier this can be done using the standard library.
std::sort(std::begin(words), std::end(words));
std::map<std::string, std::size_t> counts;
std::for_each(std::begin(words), std::end(words), [&] (const auto& w) { ++counts[w]; });
Output using:
for (auto&& [word, count] : counts) {
std::cout << word << ": " << count << std::endl;
}
My problem in the bounds: i+1 may exceed the vector borders. How to I
handle this case?
In modern C++ coding, the problem of an index going past vector bounds can be avoided. Use the STL containers and avoid using indices. With a little effort devoted to learning how to use containers this way, you should never see these kind of 'off-by-one' errors again! As a benefit, the code becomes more easily understood and maintained.
#include <iostream>
#include <vector>
#include <map>
using namespace std;
int main() {
// a test vector of words
vector< string > words { "alpha", "gamma", "beta", "gamma" };
// map unique words to their appearance count
map< string, int > mapwordcount;
// loop over words
for( auto& w : words )
{
// insert word into map
auto ret = mapwordcount.insert( pair<string,int>( w, 1 ) );
if( ! ret.second )
{
// word already present
// so increment count
ret.first->second++;
}
}
// loop over map
for( auto& m : mapwordcount )
{
cout << "word '" << m.first << "' appears " << m.second << " times\n";
}
return 0;
}
Produces
word 'alpha' appears 1 times
word 'beta' appears 1 times
word 'gamma' appears 2 times
https://ideone.com/L9VZt6
If some book or person is teaching you to write code full of
for (i = 0; i < ...
then you should run away quickly and learn modern coding elsewhere.
Same repeated words counting using some C++ STL goodies via multiset and upper_bound:
#include <iostream>
#include <vector>
#include <string>
#include <set>
int main()
{
std::vector<std::string> words{ "one", "two", "three", "two", "one" };
std::multiset<std::string> ms(words.begin(), words.end());
for (auto it = ms.begin(), end = ms.end(); it != end; it = ms.upper_bound(*it))
std::cout << *it << " is repeated: " << ms.count(*it) << " times" << std::endl;
return 0;
}
https://ideone.com/tPYw4a

C++ Usage of set, iterator, find line where duplicate was found

The program adds different strings to a set. The iterator checks the set for a certain string, what i want to achieve is to get the line where the iterator finds this certain string. Is it possible to get this with a set or do i have to create a vector? The reason i use sets is because i also want not to have duplicates in the end. It is a bit confusing i know, i hope you'll understand.
Edit: i want to get the line number of the original element already existing in the set, if a duplicate is found
#include <iostream>
#include <set>
#include <string>
#include <vector>
#include <atlstr.h>
#include <sstream>
using namespace std;
int _tmain(int argc, _TCHAR* argv[])
{
set<string> test;
set<string>::iterator it;
vector<int> crossproduct(9, 0);
for (int i = 0; i < 6; i++)
{
crossproduct[i] = i+1;
}
crossproduct[6] = 1;
crossproduct[7] = 2;
crossproduct[8] = 3;
for (int i = 0; i < 3; i++)
{
ostringstream cp; cp.precision(1); cp << fixed;
ostringstream cp1; cp1.precision(1); cp1 << fixed;
ostringstream cp2; cp2.precision(1); cp2 << fixed;
cp << crossproduct[i*3];
cp1 << crossproduct[i*3+1];
cp2 << crossproduct[i*3+2];
string cps(cp.str());
string cps1(cp1.str());
string cps2(cp2.str());
string cpstot = cps + " " + cps1 + " " + cps2;
cout << "cpstot: " << cpstot << endl;
it = test.find(cpstot);
if (it != test.end())
{
//Display here the line where "1 2 3" was found
cout << "i: " << i << endl;
}
test.insert(cpstot);
}
set<string>::iterator it2;
for (it2 = test.begin(); it2 != test.end(); ++it2)
{
cout << *it2 << endl;
}
cin.get();
return 0;
}
"Line number" is not very meaningful to a std::set<string>,
because as you add more strings to the set you may change the
order in which the existing strings are iterated through
(which is about as much of a "line number" as the set::set template
itself will give you).
Here's an alternative that may work better:
std::map<std::string, int> test.
The way you use this is you keep a "line counter" n somewhere.
Each time you need to put a new string cpstot in your set,
you have code like this:
std::map<std::string>::iterator it = test.find(cpstot);
if (it == test.end())
{
test[cpstot] = n;
// alternatively, test.insert(std::pair<std::string, int>(cpstot, n))
++n;
}
else
{
// this prints out the integer that was associated with cpstot in the map
std::cout << "i: " << it->second;
// Notice that we don't try to insert cpstot into the map in this case.
// It's already there, and we don't want to change its "line number",
// so there is nothing good we can accomplish by an insertion.
// It's a waste of effort to even try.
}
If you set n = 0 before you started putting any strings in test then
(and don't mess with the value of n in any other way)
then you will end up with strings at "line numbers" 0, 1, 2, etc.
in test and n will be the number of strings stored in test.
By the way, neither std::map<std::string, int>::iterator nor
std::set<std::string>::iterator is guaranteed to iterate through
the strings in the sequence in which they were first inserted.
Instead, what you'll get is the strings in whatever order the
template's comparison object puts the string values.
(I think by default you get them back in lexicographic order,
that is, "alphabetized".)
But when you store the original "line number" of each string in
std::map<std::string, int> test, when you are ready to
print out the list of strings you can copy the string-integer pairs
from test to a new object, std::map<int, std::string> output_sequence,
and now (assuming you do not override the default comparison object)
when you iterate through output_sequence you will get its
contents sorted by line number.
(You will then probably want to get the string
from the second field of the iterator.)

How to use find in linkedlist

Here is my code using STL library, where I try inserting a node at the end, in the middle and in front. For inserting in the middle, I want to provide insertion after a specific node, and not by incrementing the iterator by 2, as I might not know what to increment it by if it is a long list,
Kindly help why is find function not working:
#include <iostream>
#include <list>
#include <string>
using namespace std;
void printlist(list<int> l)
{
list<int>::iterator it = l.begin();
for (it; it != l.end(); ++it)
{
cout << "printlist function call list items: " << *it << endl;
}
}
int main()
{
list<int> l;
l.push_back(1);
l.push_back(2);
l.push_back(3);
list<int>::iterator it = l.begin();
cout << 1 << endl;
printlist(l);
l.push_front(0);
cout << 2 << endl;
printlist(l);
it = l.find(l.begin(), l.end(), 2);
l.insert(it, 25);
cout << 3 << endl;
printlist(l);
return 0;
}
Thanks...
std::list<> doesn't have a find() method. You can use the standard algorithm std::find() declared in <algorithm>:
it = std::find(l.begin(), l.end(), 2);
See the answer by #0x499602D2.
But to elaborate on an important point raised in a comment by #NeilKirk, you wrote:
void printlist(list<int> l)
{
list<int>::iterator it = l.begin();
for (it; it != l.end(); ++it)
{
cout << "printlist function call list items: " << *it << endl;
}
}
Note that you are passing the list l by value, not by reference. Passing a class by value (that has not been designed to use implicit sharing) will make a copy. Thus, l will be a copy of the parameter passed. If your list contained a million elements, then passing it by value will make a million-element-copy. You can fix that with:
void printlist(list<int> & l) { ... }
Or if you don't plan on making any changes, it's always nice to announce that with:
void printlist(list<int> const & l) { ... }
Also, C++11 has a range-based for which does the iterator begin/end stuff under the hood for you, and automatic variable typing:
void printlist(list<int> const & l)
{
for (auto i : l)
{
cout << "printlist function call list items: " << i << endl;
}
}
Lots of ways to get fancy in that spirit. But the more critical thing is not go making copies of your data structures, passing them by value when you don't need to!