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Merged
merged 3 commits into from
Aug 12, 2022
Merged

Improve STL exercise #222

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a830165
Improve STL exercise
bernhardmgruber Aug 12, 2022
a52a424
Update code/stl/randomize.cpp
bernhardmgruber Aug 12, 2022
e2a7e6c
Update code/stl/solution/randomize.sol.cpp
bernhardmgruber Aug 12, 2022
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18 changes: 8 additions & 10 deletions code/stl/randomize.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5,25 +5,23 @@
#include <random>
#include "Complex.hpp"

using namespace std;

template<typename T>
void compute(int len, T initial, T step) {
// allocate vectors
std::vector<T> v(len+1), diffs(len+1);

// fill and randomize v
generate(, , [](...) { ...; });
shuffle(..., std::default_random_engine{});
std::generate(, , [](...) { ...; });
std::shuffle(..., std::default_random_engine{});

// compute differences
adjacent_difference(...);
std::adjacent_difference(...);

// compute standard deviation of it
T sum = reduce(...);
T sumsq = reduce(..., [](...) { ...; });
T mean = sum/len;
T variance = sumsq/len - mean*mean;
// compute standard deviation of all differences
const T sum = std::reduce(...);
const T sumsq = std::reduce(..., [](...) { ...; });
const T mean = sum/len;
const T variance = sumsq/len - mean*mean;

std::cout << "Range = [" << initial << ", " << step*len << "]\n"
<< "Mean = " << mean << '\n'
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31 changes: 20 additions & 11 deletions code/stl/solution/randomize.sol.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5,8 +5,6 @@
#include <random>
#include "Complex.hpp"

using namespace std;

template<typename T>
struct Generator {
T m_value, m_step;
Expand All @@ -29,17 +27,28 @@ void compute(int len, T initial, T step) {
std::vector<T> v(len+1), diffs(len+1);

// fill and randomize v
generate(v.begin(), v.end(), Generator<T>(initial, step));
shuffle(v.begin(), v.end(), std::default_random_engine{});
std::generate(v.begin(), v.end(), Generator<T>(initial, step));
// Alternatively:
// std::generate(v.begin(), v.end(), [value = initial, step]() mutable {
// const T cur = value;
// value += step;
// return cur;
// });

// compute differences
adjacent_difference(v.begin(), v.end(), diffs.begin());
std::shuffle(v.begin(), v.end(), std::default_random_engine{});

// compute standard deviation of it
T sum = reduce(diffs.begin()+1, diffs.end(), T());
T sumsq = reduce(diffs.begin()+1, diffs.end(), T(), sumsquare<T>());
T mean = sum/len;
T variance = sumsq/len - mean*mean;
// compute differences
std::adjacent_difference(v.begin(), v.end(), diffs.begin());

// compute standard deviation of all differences.
// Note that the first element is just the original element itself, so we need to skip it.
const T sum = std::reduce(diffs.begin()+1, diffs.end(), T());
const T sumsq = std::reduce(diffs.begin()+1, diffs.end(), T(), sumsquare<T>());
Comment on lines +45 to +46
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Suggested change
const T sum = std::reduce(diffs.begin()+1, diffs.end(), T());
const T sumsq = std::reduce(diffs.begin()+1, diffs.end(), T(), sumsquare<T>());
const T sum = std::reduce(diffs.begin(), diffs.end(), T());
const T sumsq = std::reduce(diffs.begin(), diffs.end(), T(), sumsquare<T>());

Why do you skip the first element? Was this meant to be

    const T sum = std::reduce(diffs.begin(), diffs.end(), diffs[0]);

?

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I think this is correct, since std::adjacent_difference only writes differences starting at element 1. So diffs[0] is just v[0] and diffs[1] is the first difference (v[1] - v[0]).

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Sorry, I ignored where those values came from. This is correct, but not very intuitive without any explanation.

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With your suggestions (that I committed), this is resolved now I guess?

// Alternatively:
// const T sumsq = std::reduce(diffs.begin()+1, diffs.end(), T(),
// [](const T& s, const T& a) { return s + a * a; });
const T mean = sum/len;
const T variance = sumsq/len - mean*mean;

std::cout << "Range = [" << initial << ", " << step*len << "]\n"
<< "Mean = " << mean << '\n'
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