CODEWITHSUNDEEP
c++vector
WewillSee
WewillSee

Reputation: 299

How do you get the max value of a given column of a 2D std::vector?

I know how to get the max value of the row of a 2D std::vector as follows:

std::vector<std::vector<int>> a = {{ 1, 45, 54}, {71, 76, 12 }, {74451, 756, 125 }};}

std::cout << " The max value of row 0 is: " << *max_element(begin(a[0]), end(a[0]));

How do you do call max_element to get the max value of a 2D vector column instead? (without copying the column into a 1D vector)

Thanks!

Upvotes: 1

Views: 1397

Answers (3)

anastaciu
anastaciu

Reputation: 23792

As an alternative to std::max_element you can use a vector of vectors iterator to iterate over any column and find the max value, something like:

int maxValue(const std::vector<std::vector<int>> &a, size_t col)
{
    int max_value = 0; //assuming non negative values in the vector

    try
    {
        for (auto it = a.begin(); it != a.end(); it++)
            max_value = it->at(col) > max_value ? it->at(col) : max_value;
    }
    catch (std::out_of_range &e)
    {
        return -1; // or whatever you'd like
    }

    return max_value;
}

int main()
{
    std::vector<std::vector<int>> a = {{1, 45, 54}, {71, 76, 12}, {74451, 756, 125}};

    std::cout << maxValue(a, 0) << " "; // first column
    std::cout << maxValue(a, 1) << " "; // second column
    std::cout << maxValue(a, 2) << " "; // third column
    std::cout << maxValue(a, 3);        // out of range
}

Output:

74451 756 125 -1

Upvotes: 1

Galik
Galik

Reputation: 48605

What you can do is provide std::max_element with a user-defined comparator that compares each std::vector<int> in a by the values in the desired column to search.

Like this:

std::vector<std::vector<int>> a = {{ 1, 45, 54}, {71, 76, 12 }, {74451, 756, 125 }};

std::cout << " The max value of row 0 is: " << *max_element(begin(a[0]), end(a[0])) << '\n';

auto max_col_0 = (*std::max_element(begin(a), end(a), [](auto& a, auto& b){ return a[0] < b[0]; }))[0];
auto max_col_1 = (*std::max_element(begin(a), end(a), [](auto& a, auto& b){ return a[1] < b[1]; }))[1];

std::cout << " The max value of col 0 is: " << max_col_0 << '\n';
std::cout << " The max value of col 1 is: " << max_col_1 << '\n';

Here the llambda function is called with two std::vector<int>objects to compare and you need to compare the values in the given column you are searching. Resolving the auto types will give this for the comparator:

[](std::vector<int>& a, std::vector<int>& b){ return a[1] < b[1]; }

The std::max_element function calls the comparator with each of vector a's stored vectors so that the relevant column in each vector can be compared for which is the lesser.

The returned iterator is to the winning std::vector<int> from which you need to extract the winning column after dereferencing the iterator.

Upvotes: 2

risingStark
risingStark

Reputation: 1155

Let's say the column for which you need to find the max value is c. You can then just iterate through that column.

int m = a[0][c]; // initialize the maximum to be value of column c in first row.
for(int i = 1; i < a.size(); i++){ // loop through all rows.
    m = max(m, a[i][c]);
}
cout<<"Maximum value in column "<<c<<" is "<<m<<endl;

Upvotes: 2

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