Sorting a Vector of Pair Vectors by Multiple Columns with C

How to Sort a Vector of Pair Vectors with C 's `std::sort` Function

Sorting a vector of pair vectors can be a common task in data manipulation, especially if you need to organize your data based on multiple columns. This article provides a detailed explanation on how to sort such a vector using the `std::sort` function in C , with custom predicates to define sorting orders for each column. This technique can be particularly useful if you are looking to mimic spreadsheet functionality, where data is often sorted based on specific columns in a predefined order.

Sorting by Multiple Columns with a Single Call to `std::sort`

In this example, we will use the `std::sort` function from the C Standard Library to sort a vector of pair vectors. The goal is to sort the first column in descending order and the second column in ascending order. This can be achieved by providing a custom predicate function to the `std::sort` function.

Example Code

Let's consider a vector of pair vectors as follows:

#include iostream#include vector#include algorithmint main() {    std::vector> v  {        {1, 5},        {3, 1},        {2, 6},        {1, 3},        {2, 2},        {1, 1}    };    // Custom predicate function    auto comp  [](const std::pair lhs, const std::pair rhs) {        if ( ! ) {            // First column sorted in descending order            return  > ;        } else {            // Second column sorted in ascending order            return  

In this example, the `comp` function serves as the custom predicate. It checks the first element of the pair vectors first. If they are not equal, it sorts them in descending order (largest to smallest). If the first elements are equal, it compares the second elements and sorts them in ascending order (smallest to largest).

Explanation of the Custom Predicate

The custom predicate function `comp` is defined as follows:

element type comp(const std::pairint, int lhs, const std::pairint, int rhs) {    if ( ! ) {        // First column sorted in descending order        return   ;    } else {        // Second column sorted in ascending order        return   ;    }}

The function takes two `std::pair` objects as arguments: `lhs` (left-hand side) and `rhs` (right-hand side). Each `pair` has two integer elements.

Step-by-Step Breakdown

Check if the first elements of the left-hand side and right-hand side pairs are not equal.If they are not equal, return `true` if the left-hand side first element is greater than the right-hand side first element. This sorts the first column in descending order.Otherwise, if the first elements are equal, compare the second elements of the pairs. Return `true` if the left-hand side second element is smaller than the right-hand side second element. This sorts the second column in ascending order.

This approach ensures that the vector is sorted based on the specified conditions in a single call to `std::sort`.

Alternative Approach: Sorting by a Single Column

If you prefer a more straightforward method, you can sort the vector by a single column at a time using `std::stable_sort`. This method can be useful for clarity or if you want to perform sorting in multiple steps.

Example Code for `std::stable_sort`

To sort the vector by the first column in descending order, you can modify the code as follows:

#include iostream#include vector#include algorithmint main() {    std::vector> v  {        {1, 5},        {3, 1},        {2, 6},        {1, 3},        {2, 2},        {1, 1}    };    // Custom predicate function for sorting the first column in descending order    auto comp  [](const std::pair lhs, const std::pair rhs) {        return  > ;    };    // Sort the vector using the custom predicate for the first column    std::stable_sort((), v.end(), comp);    // Display the sorted vector    for (const auto p : v) {        std::cout  "("    ", "    ") ";    }    return 0;}

In this example, the `comp` function is used to sort the vector by the first column. The `std::stable_sort` function is used instead of `std::sort` to maintain the relative order of equal elements.

Conclusion

Sorting a vector of pair vectors with conditions on multiple columns can be effectively achieved using the `std::sort` function with a custom predicate. The provided examples demonstrate how to implement this in C for both sorting by multiple columns and sorting by a single column.