Vector-transposing function
$begingroup$
I profiled a library I'm writing that uses vector transposes and found that I am spending a good bit of time doing the following transpose. I am using a std::vector of std::vector<double>s to represent the column vectors.
What are some ways to optimize this function?
std::vector<double> transpose_vector(const std::vector<std::vector<double>> &column_vec) {
// take a column vector:
// |x1|
// |x2|
// |x3|
// and return a row vector |x1, x2, x3|
std::vector<double> row_vector;
for (auto c : column_vec) {
for (auto r : c) {
row_vector.push_back(r);
}
}
return row_vector;
}
c++ performance c++11 vectors
edited 2 hours ago
200_success
130k16153419
asked 3 hours ago
L LL L
405
$endgroup$
add a comment |
$begingroup$
I profiled a library I'm writing that uses vector transposes and found that I am spending a good bit of time doing the following transpose. I am using a std::vector of std::vector<double>s to represent the column vectors.
What are some ways to optimize this function?
std::vector<double> transpose_vector(const std::vector<std::vector<double>> &column_vec) {
// take a column vector:
// |x1|
// |x2|
// |x3|
// and return a row vector |x1, x2, x3|
std::vector<double> row_vector;
for (auto c : column_vec) {
for (auto r : c) {
row_vector.push_back(r);
}
}
return row_vector;
}
c++ performance c++11 vectors
edited 2 hours ago
200_success
130k16153419
asked 3 hours ago
L LL L
405
$endgroup$
add a comment |
$begingroup$
I profiled a library I'm writing that uses vector transposes and found that I am spending a good bit of time doing the following transpose. I am using a std::vector of std::vector<double>s to represent the column vectors.
What are some ways to optimize this function?
std::vector<double> transpose_vector(const std::vector<std::vector<double>> &column_vec) {
// take a column vector:
// |x1|
// |x2|
// |x3|
// and return a row vector |x1, x2, x3|
std::vector<double> row_vector;
for (auto c : column_vec) {
for (auto r : c) {
row_vector.push_back(r);
}
}
return row_vector;
}
c++ performance c++11 vectors
edited 2 hours ago
200_success
130k16153419
asked 3 hours ago
L LL L
405
$endgroup$
I profiled a library I'm writing that uses vector transposes and found that I am spending a good bit of time doing the following transpose. I am using a std::vector of std::vector<double>s to represent the column vectors.
What are some ways to optimize this function?
std::vector<double> transpose_vector(const std::vector<std::vector<double>> &column_vec) {
// take a column vector:
// |x1|
// |x2|
// |x3|
// and return a row vector |x1, x2, x3|
std::vector<double> row_vector;
for (auto c : column_vec) {
for (auto r : c) {
row_vector.push_back(r);
}
}
return row_vector;
}
c++ performance c++11 vectors
c++ performance c++11 vectors
edited 2 hours ago
200_success
130k16153419
asked 3 hours ago
L LL L
405
edited 2 hours ago
200_success
130k16153419
asked 3 hours ago
L LL L
405
edited 2 hours ago
200_success
130k16153419
edited 2 hours ago
200_success
130k16153419
edited 2 hours ago
200_success
130k16153419
130k16153419
asked 3 hours ago
L LL L
405
asked 3 hours ago
L LL L
405
asked 3 hours ago
L LL L
405
405
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
There's not really much here. The only thing I can think of is it may prove faster to pre-allocate the destination vector using reserve. push_back has the potential to cause several re-allocations per call to transpose, which will be slow. Try:
std::vector<double> transpose_vector(const std::vector<std::vector<double>> &column_vec) {
std::vector<double> row_vector;
row_vector.reserve(total_entries(column_vec)); // Pre-allocate the space we need
for (auto c : column_vec) {
for (auto r : c) {
row_vector.push_back(r);
}
}
return row_vector;
}
Where total_entries is a function that finds how many cells there are in the 2D vector. If each row is the same length, you could use math to figure this out. If it's ragged though, you may need to iterate column_vector summing the row lengths.
answered 3 hours ago
CarcigenicateCarcigenicate
3,63211631
$endgroup$
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
There's not really much here. The only thing I can think of is it may prove faster to pre-allocate the destination vector using reserve. push_back has the potential to cause several re-allocations per call to transpose, which will be slow. Try:
std::vector<double> transpose_vector(const std::vector<std::vector<double>> &column_vec) {
std::vector<double> row_vector;
row_vector.reserve(total_entries(column_vec)); // Pre-allocate the space we need
for (auto c : column_vec) {
for (auto r : c) {
row_vector.push_back(r);
}
}
return row_vector;
}
Where total_entries is a function that finds how many cells there are in the 2D vector. If each row is the same length, you could use math to figure this out. If it's ragged though, you may need to iterate column_vector summing the row lengths.
answered 3 hours ago
CarcigenicateCarcigenicate
3,63211631
$endgroup$
add a comment |
$begingroup$
There's not really much here. The only thing I can think of is it may prove faster to pre-allocate the destination vector using reserve. push_back has the potential to cause several re-allocations per call to transpose, which will be slow. Try:
std::vector<double> transpose_vector(const std::vector<std::vector<double>> &column_vec) {
std::vector<double> row_vector;
row_vector.reserve(total_entries(column_vec)); // Pre-allocate the space we need
for (auto c : column_vec) {
for (auto r : c) {
row_vector.push_back(r);
}
}
return row_vector;
}
Where total_entries is a function that finds how many cells there are in the 2D vector. If each row is the same length, you could use math to figure this out. If it's ragged though, you may need to iterate column_vector summing the row lengths.
answered 3 hours ago
CarcigenicateCarcigenicate
3,63211631
$endgroup$
add a comment |
$begingroup$
There's not really much here. The only thing I can think of is it may prove faster to pre-allocate the destination vector using reserve. push_back has the potential to cause several re-allocations per call to transpose, which will be slow. Try:
std::vector<double> transpose_vector(const std::vector<std::vector<double>> &column_vec) {
std::vector<double> row_vector;
row_vector.reserve(total_entries(column_vec)); // Pre-allocate the space we need
for (auto c : column_vec) {
for (auto r : c) {
row_vector.push_back(r);
}
}
return row_vector;
}
Where total_entries is a function that finds how many cells there are in the 2D vector. If each row is the same length, you could use math to figure this out. If it's ragged though, you may need to iterate column_vector summing the row lengths.
answered 3 hours ago
CarcigenicateCarcigenicate
3,63211631
$endgroup$
There's not really much here. The only thing I can think of is it may prove faster to pre-allocate the destination vector using reserve. push_back has the potential to cause several re-allocations per call to transpose, which will be slow. Try:
std::vector<double> transpose_vector(const std::vector<std::vector<double>> &column_vec) {
std::vector<double> row_vector;
row_vector.reserve(total_entries(column_vec)); // Pre-allocate the space we need
for (auto c : column_vec) {
for (auto r : c) {
row_vector.push_back(r);
}
}
return row_vector;
}
Where total_entries is a function that finds how many cells there are in the 2D vector. If each row is the same length, you could use math to figure this out. If it's ragged though, you may need to iterate column_vector summing the row lengths.
answered 3 hours ago
CarcigenicateCarcigenicate
3,63211631
answered 3 hours ago
CarcigenicateCarcigenicate
3,63211631
answered 3 hours ago
CarcigenicateCarcigenicate
3,63211631
answered 3 hours ago
CarcigenicateCarcigenicate
3,63211631
3,63211631
add a comment |
add a comment |
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