Help with vectors

The only sense that it's "easier" with an array is that you can't physically do it in the first place; an array has a fixed size.

Using erase like that is error-prone because let's say you're currently at i = 5, and you erase element Skaiciai[5]... so what's now at Skaiciai[5] after the erase? Well, Skaiciai[6] (or perhaps nothing, if you were already at the end of the array).

So in a sense, your current index becomes invalidated when you erase something.

To avoid this, the best thing would to avoid writing your own loop in the first place. Use the standard library to help you.
See: https://en.wikipedia.org/wiki/Erase%E2%80%93remove_idiom

// Example program
#include <iostream>
#include <vector>
#include <algorithm>

using std::vector;

bool isLessThan15(int n) { return n < 15; }

void removeLessThan15(vector<int>& vec)
{
    // Removes all numbers less than 15.
    vec.erase(std::remove_if(vec.begin(), vec.end(), isLessThan15), vec.end());
}

void print(const vector<int>& vec)
{
    for (const int i : vec)
    {
        std::cout << i << ' ';   
    }
    std::cout << '\n';
}

int main()
{
    vector<int> vec = { 16, 3, 14, 15, 17 };
    removeLessThan15(vec);
    print(vec);
}

Last edited on Oct 12, 2019 at 5:50pm

Oct 12, 2019 at 6:03pm

Okay, but let's say that we don't know what to remove yet. Let's say we need to find first the sum of the elements and then it's average. And then remove every element that is lower than average. Since I'm new I don't know how would I change this

bool isLessThan15(int n) { return n < 15; }

Would I just pass the function that calculated the average to this boolean ?

Oct 12, 2019 at 6:23pm

JLBorges (13770)

#include <iostream>
#include <vector>
#include <algorithm>
#include <numeric>

int main()
{
    std::vector<int> seq { 12, 24, 58, 67, 92, 45, 77, 22, 56, 89, 21, 80 } ;
    for( int v : seq ) std::cout << v << ' ' ;
    std::cout << '\n' ;

    // find the sum of all the elements
    // https://en.cppreference.com/w/cpp/algorithm/accumulate
    const double sum = std::accumulate( seq.begin(), seq.end(), 0.0 ) ;

    const double average = sum / seq.size() ; // find the average
    std::cout << "average: " << average << '\n' ;

    // remove all elements less than average
    // https://en.wikipedia.org/wiki/Erase%E2%80%93remove_idiom
    const auto less_than_avg = [average] ( int v ) { return v < average ; } ;
    seq.erase( std::remove_if( seq.begin(), seq.end(), less_than_avg ), seq.end() ) ;

    for( int v : seq ) std::cout << v << ' ' ;
    std::cout << '\n' ;
}

http://coliru.stacked-crooked.com/a/339d6bfc976a77c6

Oct 12, 2019 at 6:28pm

we need to find first the sum of the elements and then it's average. And then remove every element that is lower than average.

Well, there's many ways to skin this cat. I would still prefer to use remove_if. If I choose to go with the same pattern as before, I could do something like the following. Note that you wouldn't re-calculate the average each time, just once (unless you want it to be a rolling-average or something like that).

// Example program
#include <iostream>
#include <vector>
#include <algorithm>

using std::vector;

struct LessThanAverage {
    LessThanAverage(double average)
    {
        this->average = average;   
    }
    bool operator()(double n)
    {
        return n < average;   
    }
  private:
    double average;
};

double average(const vector<int>& vec)
{
    double sum = 0.0;
    for (int i : vec)
        sum += i;
    return (vec.size() == 0) ? 0.0 : sum / vec.size();
}

void removeLessThanAverage(vector<int>& vec)
{
    double avg = average(vec);
    vec.erase(std::remove_if(vec.begin(), vec.end(), LessThanAverage(avg)), vec.end());
}

void print(const vector<int>& vec)
{
    for (const int i : vec)
    {
        std::cout << i << ' ';   
    }
    std::cout << '\n';
}

int main()
{
    vector<int> vec = { 16, 3, 14, 15, 4 };
    removeLessThanAverage(vec);
    print(vec);
}

remove_if's UnaryPredicate can use a classes's operator() returning a bool as the predicate.
https://en.cppreference.com/w/cpp/algorithm/remove_if

Edit: Of course, JLBorges' example, which uses a lambda to capture the average, is much more concise than mine. (I need to get more comfortable with using those...!)

Last edited on Oct 12, 2019 at 6:35pm

Oct 12, 2019 at 7:00pm

Every time I'm trying to do the line

const auto lessThen = [average] (int v) {return v < average;};

It just gives me an error

'lessThen' was not declared in this scope"
'lessThen' does not name a type"

Oct 12, 2019 at 7:03pm

Does this compile for you?

#include <iostream>

int main()
{
    int arr[3] = {3, 2, 1};
    for (auto i : arr) std::cout << i;
}

What is your compiler version?

Last edited on Oct 12, 2019 at 7:04pm

Oct 12, 2019 at 7:10pm

No. this not compile for me. I'm using Code::Blocks 17.12

Oct 12, 2019 at 7:21pm

Go to Settings --> Compiler --> Compiler Flags tab, and check "Have g++ follow the C++14 ISO C++ language standard [-std=c++14]". (If -std=c++17 is available, you can also try that. If nothing else, -std=c++11 should be available)

My previous post will compile with -std=c++11 or higher.

Last edited on Oct 12, 2019 at 7:30pm

Oct 12, 2019 at 7:26pm

In the long term, I suggest updating your compiler.

CodeBlocks download page wrote:
The codeblocks-17.12mingw-setup.exe file includes additionally the GCC/G++ compiler and GDB debugger from TDM-GCC (version 5.1.0, 32 bit, SJLJ).

Plugging an updated compiler to CodeBlocks or another IDE is a good skill to learn.

You can download an even newer MinGW compiler from sites like here: https://nuwen.net/mingw.html
And I think these directions tell you how to point CodeBlocks to it: http://wiki.codeblocks.org/index.php/MinGW_installation

Last edited on Oct 12, 2019 at 7:29pm

Oct 12, 2019 at 7:34pm