Taskflow Algorithms » Parallel Find | Taskflow QuickStart

# Taskflow Algorithms » Parallel Find

Taskflow provides template functions for constructing tasks to perform parallel iterations over ranges of items.

## Include the Header

You need to include the header file, `taskflow/algorithm/find.hpp`, for using parallel-find algorithms.

    #include <taskflow/algorithm/find.hpp>

## What is a Find Algorithm?

A find algorithm allows you to find an element in a range `[first, last)` that satisfies a specific criteria. The algorithm returns an iterator to the first found element in the range or returns `last` if there is no such iterator. Taskflow provides the following parallel-find algorithms:

- tf::Taskflow::find_if(B first, E last, T& result, UOP predicate, P&& part)
- tf::Taskflow::find_if_not(B first, E last, T& result, UOP predicate, P&& part)
- tf::Taskflow::min_element(B first, E last, T& result, C comp, P&& part)
- tf::Taskflow::max_element(B first, E last, T& result, C comp, P&& part)

## Create a Parallel Find-If Task

tf::Taskflow::find_if performs parallel iterations to find the first element in the range `[first, last)` that makes the given predicate return `true`. It resembles a parallel implementation of the following loop:

    template<typename InputIt, typename UnaryPredicate>
    InputIt find_if(InputIt first, InputIt last, UnaryPredicate predicate) {
      for(; first != last; ++first) {
        if(predicate(*first)) {
          return first;
        }
      }
      return last;
    }

The example below creates a task to find the element that is equal to 22 from an input range of 10 elements. The result will be stored in the forth argument passed by reference:

    std::vector<int> input = {1, 9, 22, 3, -6, 13, 12, 0, 9, 11};
    std::vector<int>::iterator result;
    taskflow.find_if(
      input.begin(), input.end(), [](int i){ return i == 22; }, result
    );
    executor.run(taskflow);
    assert(*result == 22);

## Capture Iterators by Reference

You can pass iterators by reference using std::ref to marshal parameters update between dependent tasks. This is especially useful when the range iterators are not known at the time of creating a find-if task, but need initialization from another task.

    std::vector<int> input;
    std::vector<int>::iterator result, first, last;

    // task to set up the range iterators
    tf::Task init = taskflow.emplace([&](){
      input = {1, 9, 22, 3, -6, 13, 12, 0, 9, 11};
      first = input.begin(),
      last  = input.end();
    });

    // task to perform parallel find
    tf::Task task = taskflow.find_if(
      std::ref(first), std::ref(last), result, [](int i){ return i == 22; }
    );

    init.precede(task);

    executor.run(taskflow);
    assert(*result == 22);

In the above example, when `init` finishes, `input` has been initialized to 10 elements with `first` and `last` pointing to the data range of `input`. The find-if task will then work on this initialized range as a result of passing iterators by reference.

## Create a Parallel Find-If-Not Task

tf::Taskflow::find_if_not performs parallel iterations to find the first element in the range `[first, last)` that makes the given predicate return `false`. It resembles a parallel implementation of the following loop:

    template<typename InputIt, typename UnaryPredicate>
    InputIt find_if(InputIt first, InputIt last, UnaryPredicate predicate) {
      for(; first != last; ++first) {
        if(!predicate(*first)) {
          return first;
        }
      }
      return last;
    }

The example below creates a task to find the element that is NOT equal to 22 from an input range of 10 elements. The result will be stored in the forth argument passed by reference:

    std::vector<int> input = {1, 1, 22, 1, 1, 1, 1, 1, 1, 1};
    std::vector<int>::iterator result;
    taskflow.find_if_not(
      input.begin(), input.end(), result, [](int i){ return i == 1; }
    );
    executor.run(taskflow);
    assert(*result == 22);

Similar to Capture Iterators by Reference, iterators of tf::Taskflow::find_if_not are templated to allow passing iterators by reference using std::ref. This is especially useful when the range iterators are not known at the time of creating a find-if-not task, but need initialization from another task.

## Find the Smallest and the Largest Elements

tf::Taskflow::min_element finds the smallest element in a range `[first, last)` using the given comparison function object. The example below finds the smallest element, i.e., -1, from an input range of 10 elements and stores the iterator to that smallest element in `result:`

    std::vector<int> input = {1, 1, 1, 1, 1, -1, 1, 1, 1, 1};
    std::vector<int>::iterator result;
    taskflow.min_element(
      input.begin(), input.end(), std::less<int>(), result
    );
    executor.run(taskflow).wait();
    assert(*result == -1);

Similarly, tf::Taskflow::max_element finds the largest element in a range `[first, last)` using the given comparison function object. The example below finds the largest element, i.e., 2, from an input range of 10 elements and stores the iterator to that largest element in `result:`

    std::vector<int> input = {1, 1, 1, 1, 1, 2, 1, 1, 1, 1};
    std::vector<int>::iterator result;
    taskflow.max_element(
      input.begin(), input.end(), std::less<int>(), result
    );
    executor.run(taskflow).wait();
    assert(*result == 2);

#### Attention

When using tf::Taskflow::max_element to find the large element, we will still need to use std::less as our comparison function. Details can be referred to std::max_element.

## Configure a Partitioner

You can configure a partitioner for parallel-find tasks (tf::Taskflow::find_if, tf::Taskflow::find_if_not, tf::Taskflow::min_element, tf::Taskflow::max_element) to run with different scheduling methods, such as guided partitioning, dynamic partitioning, and static partitioning. The following example creates two parallel-find tasks using two different partitioners, one with the static partitioning algorithm and another one with the guided partitioning algorithm:

    std::vector<int> vec(1024, -1);
    std::vector<int>::iterator result;

    tf::ExecutionPolicy<tf::StaticPartitioner> static_partitioner;
    tf::ExecutionPolicy<tf::GuidedPartitioner> guided_partitioner;

    // create a parallel-find task with a static partitioner
    taskflow.find_if(
      vec.begin(), vec.end(), result, [&](int i) { return i == -1; }, static_partitioner
    );

    // create a parallel-find task with a guided partitioner
    taskflow.find_if(
      vec.begin(), vec.end(), result, [&](int i) { return i == -1; }, guided_partitioner
    );

#### Attention

By default, parallel-find tasks use tf::DefaultPartitioner if no partitioner is specified.

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