如何将 3 个元素的向量推回 C++ 中的向量？

Question

I have a long vector of values, a specified user input of row/column size. I need to assign a set of 3 numbers into a vector, from the long list of vectors. The vector with 3 number set will be pushed back into another vector, with user input row/column size. 1 column = the 3 number vector set and so on, until every column is filled out. I have trouble making this code (it needs to be in a loop). Any help please?

The picture is an example of a 4x4 vector, with each column a vector of 3 numbers

Answer 1

It sounds as if you want nested vectors, where each smaller vector inside your "long vector" represents a column of 3 values. If so you could do so like:

std::vector<int> columnVec = { 1, 2, 3 };
std::vector<std::vector<int>> longVector;
longVector.push_back(columnVec);

In the first line we declare a vector representing our column and place three integers inside it. On line two we declare another vector, but this time containing vectors which themselves contain ints, ie a vector full of column vectors. We then used push_back() to push the column vector into our vector of vectors. If you needed to print the values you could do so like:

for(auto& vec : longVector) { //Walk through our vector of vectors.
  for(int value : vec) {      //Walk through our column vectors of values.
    std::cout << value;       //Print out each value of the column.
  }
  std::cout << std::endl;     //Add a newline.
}

Note that if you print them, the columns will appear as rows in the console. If you care about the formatting in the console it will take a bit more effort and might be worth asking as a separate question.

Answer 2

One possible approach might look something like this:

#include <iostream>
#include <cstdlib>
#include <vector>

using namespace std;

int main()
{
    typedef std::vector<int> VecInt;
    typedef VecInt::iterator VecIntIter;

    typedef std::vector<VecInt> VecVecInt;
    typedef VecVecInt::iterator VecVecIntIter;

    VecVecInt rows;
    const int maxRows = 10, maxCols = 10;

    cout << "Values during creation" << endl;
    cout << "----------------------" << endl;
    for (int rowNum=0; rowNum<maxRows; rowNum++)
    {
        VecInt curRow;
        for (int colNum=0; colNum<maxCols; colNum++)
        {
            if (colNum != 0)
                cout << " ";
            int cellValue = rand() % 32;
            cout << cellValue;
            curRow.push_back( cellValue );
        }
        cout << endl;
        rows.push_back(curRow);
    }

    cout << endl;
    cout << "Values during retrieval" << endl;
    cout << "----------------------" << endl;
    for (VecVecIntIter rowIter=rows.begin(); rowIter!=rows.end(); rowIter++)
    {
        VecInt curRow = (*rowIter);
        for (VecIntIter colIter=curRow.begin(); colIter!=curRow.end(); colIter++)
        {
            if (colIter != curRow.begin())
                cout << " ";
            cout << (*colIter);
        }
        cout << endl;
    }
}

Though, this will store a collection of rows, rather than a collection of columns. Easy enough to change the for loops.