Why I cannot use a decltype in range-for with multi-dimension arrays? - c++11

I have a problem here. I am trying to use decltype in range-for loop for using multi-dimension array:
int a[][4]{
{0, 1, 2, 3 },
{4, 5, 6, 7 },
{8, 9, 10, 11}
};
for (auto& row : a) { // reference is needed here to prevent array decay to pointer
cout << "{";
for (auto col : row)
cout << col << ", ";
cout << "}" << endl;
}
decltype (*a) row{ *a};
cout << sizeof(row) << endl;
cout << typeid(row).name() << endl;
// for (decltype(*a) row : *a) {
// for (int col : row)
// cout << col << ", ";
// cout << endl;
// }
With auto I can easily iterate over the the array But with decltype it doesn't work for me.
What I get above if I uncomment the code is: cannot convert from int to int(&)[4].


That is because the line for(decltype(*a) row : *a) is incorrect. Try to read it correctly: for each array of 4 int from a, not from *a.
The code may look like:
for (decltype(*a) row : a) {
for (int col : row)
cout << col << ", ";
cout << endl;
}
Dereferencing a (*a) with decltype will yield an array of 4 integers. So the type is int[4]. Unlike using keyword auto where it yields int*.

Related

How to read chunk of the data from a hdf5 file in c++?

I want to read a chunk of data which is just one frame of many frames stored in one dataset. The shape of the whole dataset is (10, 11214,3), 10 frames each frame has 11214 rows and 4 columns. Here is the file. The chunk I want to read would have the shape (11214,3). I can print the predefined array using, but I'm not sure how can I read data from a hdf5 file. Here is my code,
#include <h5xx/h5xx.hpp>
#include <boost/multi_array.hpp>
#include <iostream>
#include <vector>
#include <cstdio>
typedef boost::multi_array<int, 2> array_2d_t;
const int NI=10;
const int NJ=NI;
void print_array(array_2d_t const& array)
{
for (unsigned int j = 0; j < array.shape()[1]; j++)
{
for (unsigned int i = 0; i < array.shape()[0]; i++)
{
printf("%2d ", array[j][i]);
}
printf("\n");
}
}
void write_int_data(std::string const& filename, array_2d_t const& array)
{
h5xx::file file(filename, h5xx::file::trunc);
std::string name;
{
// --- create dataset and fill it with the default array data (positive values)
name = "integer array";
h5xx::create_dataset(file, name, array);
h5xx::write_dataset(file, name, array);
// --- create a slice object (aka hyperslab) to specify the location in the dataset to be overwritten
std::vector<int> offset; int offset_raw[2] = {4,4}; offset.assign(offset_raw, offset_raw + 2);
std::vector<int> count; int count_raw[2] = {2,2}; count.assign(count_raw, count_raw + 2);
h5xx::slice slice(offset, count);
}
}
void read_int_data(std::string const& filename)
{
h5xx::file file(filename, h5xx::file::in);
std::string name = "integer array";
// read and print the full dataset
{
array_2d_t array;
// --- read the complete dataset into array, the array is resized and overwritten internally
h5xx::read_dataset(file, name, array);
printf("original integer array read from file, negative number patch was written using a slice\n");
print_array(array);
printf("\n");
}
}
int main(int argc, char** argv)
{
std::string filename = argv[0];
filename.append(".h5");
// --- do a few demos/tests using integers
{
array_2d_t array(boost::extents[NJ][NI]);
{
const int nelem = NI*NJ;
int data[nelem];
for (int i = 0; i < nelem; i++)
data[i] = i;
array.assign(data, data + nelem);
}
write_int_data(filename, array);
read_int_data(filename);
}
return 0;
}
I'm using the h5xx — a template-based C++ wrapper for the HDF5 library link and boost library.
The datasets are stored in particles/lipids/box/positions path. The dataset name value holds the frames.

argv[0] is not what you want (arguments start at 1, 0 is the program name). Consider bounds checking as well:
std::vector<std::string> const args(argv, argv + argc);
std::string const filename = args.at(1) + ".h5";
the initialization can be done directly, without a temporary array (what is multi_array for, otherwise?)
for (size_t i = 0; i < array.num_elements(); i++)
array.data()[i] = i;
Or indeed, make it an algorithm:
std::iota(array.data(), array.data() + array.num_elements(), 0);
same with vectors:
std::vector<int> offset; int offset_raw[2] = {4,4}; offset.assign(offset_raw, offset_raw + 2);
std::vector<int> count; int count_raw[2] = {2,2}; count.assign(count_raw, count_raw + 2);
besides being a formatting mess can be simply
std::vector offset{4,4}, count{2,2};
h5xx::slice slice(offset, count);
On To The Real Question
The code has no relevance to the file. At all. I created some debug/tracing code to dump the file contents:
void dump(h5xx::group const& g, std::string indent = "") {
auto dd = g.datasets();
auto gg = g.groups();
for (auto it = dd.begin(); it != dd.end(); ++it) {
std::cout << indent << " ds:" << it.get_name() << "\n";
}
for (auto it = gg.begin(); it != gg.end(); ++it) {
dump(*it, indent + "/" + it.get_name());
}
}
int main()
{
h5xx::file xaa("xaa.h5", h5xx::file::mode::in);
dump(xaa);
}
Prints
/particles/lipids/box/edges ds:box_size
/particles/lipids/box/edges ds:step
/particles/lipids/box/edges ds:time
/particles/lipids/box/edges ds:value
/particles/lipids/box/positions ds:step
/particles/lipids/box/positions ds:time
/particles/lipids/box/positions ds:value
Now we can drill down to the dataset. Let's see whether we can figure out the correct type. It certainly is NOT array_2d_t:
h5xx::dataset ds(xaa, "particles/lipids/box/positions/value");
array_2d_t a;
h5xx::datatype detect(a);
std::cout << "type: " << std::hex << ds.get_type() << std::dec << "\n";
std::cout << "detect: " << std::hex << detect.get_type_id() << std::dec << "\n";
Prints
type: 30000000000013b
detect: 30000000000000c
That's a type mismatch. I guess I'll have to learn to read that gibberish as well...
Let's add some diagnostics:
void diag_type(hid_t type)
{
std::cout << " Class " << ::H5Tget_class(type) << std::endl;
std::cout << " Size " << ::H5Tget_size(type) << std::endl;
std::cout << " Sign " << ::H5Tget_sign(type) << std::endl;
std::cout << " Order " << ::H5Tget_order(type) << std::endl;
std::cout << " Precision " << ::H5Tget_precision(type) << std::endl;
std::cout << " NDims " << ::H5Tget_array_ndims(type) << std::endl;
std::cout << " NMembers " << ::H5Tget_nmembers(type) << std::endl;
}
int main()
{
h5xx::file xaa("xaa.h5", h5xx::file::mode::in);
// dump(xaa);
{
h5xx::group g(xaa, "particles/lipids/box/positions");
h5xx::dataset ds(g, "value");
std::cout << "dataset: " << std::hex << ds.get_type() << std::dec << std::endl;
diag_type(ds.get_type());
}
{
array_2d_t a(boost::extents[NJ][NI]);
h5xx::datatype detect(a);
std::cout << "detect: " << std::hex << detect.get_type_id() << std::dec << std::endl;
diag_type(detect.get_type_id());
}
}
Prints
dataset: 30000000000013b
Class 1
Size 4
Sign -1
Order 0
Precision 32
NDims -1
NMembers -1
detect: 30000000000000c
Class 0
Size 4
Sign 1
Order 0
Precision 32
NDims -1
NMembers -1
At least we know that HST_FLOAT (class 1) is required. Let's modify array_2d_t:
using array_2d_t = boost::multi_array<float, 2>;
array_2d_t a(boost::extents[11214][3]);
This at least makes the data appear similarly. Let's ... naively try to read:
h5xx::read_dataset(ds, a);
Oops, that predictably throws
terminate called after throwing an instance of 'h5xx::error'
what(): /home/sehe/Projects/stackoverflow/deps/h5xx/h5xx/dataset/boost_multi_array.hpp:176:read_dataset(): dataset "/particles/lipi
ds/box/positions/value" and target array have mismatching dimensions
No worries, we can guess:
using array_3d_t = boost::multi_array<float, 3>;
array_3d_t a(boost::extents[10][11214][3]);
h5xx::read_dataset(ds, a);
At least this does work. Adapting the print function:
template <typename T> void print_array(T const& array) {
for (auto const& row : array) {
for (auto v : row) printf("%5f ", v);
printf("\n");
}
}
Now we can print the first frame:
h5xx::read_dataset(ds, a);
print_array(*a.begin()); // print the first frame
This prints:
80.480003 35.360001 4.250000
37.450001 3.920000 3.960000
18.530001 -9.690000 4.680000
55.389999 74.339996 4.600000
22.110001 68.709999 3.850000
-4.130000 24.040001 3.730000
40.160000 6.390000 4.730000
-5.400000 35.730000 4.850000
36.669998 22.450001 4.080000
-3.680000 -10.660000 4.180000
(...)
That checks out with h5ls -r -d xaa.h5/particles/lipids/box/positions/value:
particles/lipids/box/positions/value Dataset {75/Inf, 11214, 3}
Data:
(0,0,0) 80.48, 35.36, 4.25, 37.45, 3.92, 3.96, 18.53, -9.69, 4.68,
(0,3,0) 55.39, 74.34, 4.6, 22.11, 68.71, 3.85, -4.13, 24.04, 3.73,
(0,6,0) 40.16, 6.39, 4.73, -5.4, 35.73, 4.85, 36.67, 22.45, 4.08, -3.68,
(0,9,1) -10.66, 4.18, 35.95, 36.43, 5.15, 57.17, 3.88, 5.08, -23.64,
(0,12,1) 50.44, 4.32, 6.78, 8.24, 4.36, 21.34, 50.63, 5.21, 16.29,
(0,15,1) -1.34, 5.28, 22.26, 71.25, 5.4, 19.76, 10.38, 5.34, 78.62,
(0,18,1) 11.13, 5.69, 22.14, 59.7, 4.92, 15.65, 47.28, 5.22, 82.41,
(0,21,1) 2.09, 5.24, 16.87, -11.68, 5.35, 15.54, -0.63, 5.2, 81.25,
(...)
The Home Stretch: Adding The Slice
array_2d_t read_frame(int frame_no) {
h5xx::file xaa("xaa.h5", h5xx::file::mode::in);
h5xx::group g(xaa, "particles/lipids/box/positions");
h5xx::dataset ds(g, "value");
array_2d_t a(boost::extents[11214][3]);
std::vector offsets{frame_no, 0, 0}, counts{1, 11214, 3};
h5xx::slice slice(offsets, counts);
h5xx::read_dataset(ds, a, slice);
return a;
}
There you have it. Now we can print any frame:
print_array(read_frame(0));
Printing the same as before. Let's try the last frame:
print_array(read_frame(9));
Prints
79.040001 36.349998 3.990000
37.250000 3.470000 4.140000
18.600000 -9.270000 4.900000
55.669998 75.070000 5.370000
21.920000 67.709999 3.790000
-4.670000 24.770000 3.690000
40.000000 6.060000 5.240000
-5.340000 36.320000 5.410000
36.369999 22.490000 4.130000
-3.520000 -10.430000 4.280000
(...)
Checking again with h5ls -r -d xaa.h5/particles/lipids/box/positions/value |& grep '(9' | head confirms:
(9,0,0) 79.04, 36.35, 3.99, 37.25, 3.47, 4.14, 18.6, -9.27, 4.9, 55.67,
(9,3,1) 75.07, 5.37, 21.92, 67.71, 3.79, -4.67, 24.77, 3.69, 40, 6.06,
(9,6,2) 5.24, -5.34, 36.32, 5.41, 36.37, 22.49, 4.13, -3.52, -10.43,
(9,9,2) 4.28, 35.8, 36.43, 4.99, 56.6, 4.09, 5.04, -23.37, 49.42, 3.81,
(9,13,0) 6.31, 8.83, 4.56, 22.01, 50.38, 5.43, 16.3, -2.92, 5.4, 22.02,
(9,16,1) 70.09, 5.36, 20.23, 11.12, 5.66, 78.48, 11.34, 6.09, 20.26,
(9,19,1) 61.45, 5.35, 14.25, 48.32, 5.35, 79.95, 1.71, 5.38, 17.56,
(9,22,1) -11.61, 5.39, 15.64, -0.19, 5.06, 80.43, 71.77, 5.29, 75.54,
(9,25,1) 35.14, 5.26, 22.45, 56.86, 5.56, 16.47, 52.97, 6.16, 20.62,
(9,28,1) 65.12, 5.26, 19.68, 71.2, 5.52, 23.39, 49.84, 5.28, 22.7,
Full Listing
#include <boost/multi_array.hpp>
#include <h5xx/h5xx.hpp>
#include <iostream>
using array_2d_t = boost::multi_array<float, 2>;
template <typename T> void print_array(T const& array)
{
for (auto const& row : array) { for (auto v : row)
printf("%5f ", v);
printf("\n");
}
}
void dump(h5xx::group const& g, std::string indent = "") {
auto dd = g.datasets();
auto gg = g.groups();
for (auto it = dd.begin(); it != dd.end(); ++it) {
std::cout << indent << " ds:" << it.get_name() << std::endl;
}
for (auto it = gg.begin(); it != gg.end(); ++it) {
dump(*it, indent + "/" + it.get_name());
}
}
array_2d_t read_frame(int frame_no) {
h5xx::file xaa("xaa.h5", h5xx::file::mode::in);
h5xx::group g(xaa, "particles/lipids/box/positions");
h5xx::dataset ds(g, "value");
array_2d_t arr(boost::extents[11214][3]);
std::vector offsets{frame_no, 0, 0}, counts{1, 11214, 3};
h5xx::slice slice(offsets, counts);
h5xx::read_dataset(ds, arr, slice);
return arr;
}
int main()
{
print_array(read_frame(9));
}

How to append more items to an existing vector contained in the value field of a std::map?

I have a std::vector<std::string>>. Following is my full program:
#include <iostream>
#include <vector>
#include <string>
#include <map>
int main() {
std::cout << " -- Beginining of program -- " << std::endl;
std::map<std::string, std::vector<std::string>> my_map_2;
std::vector<std::string> s = {"a", "b", "c"};
my_map_2.insert(std::make_pair("key1", s));
std::vector<std::string> s2 = {"d", "e", "f"};
my_map_2.insert(std::make_pair("key1", s2));
for(auto const &map_item: my_map_2) {
std::cout << map_item.first << " " << map_item.second[0] << std::endl;
std::cout << map_item.first << " " << map_item.second[1] << std::endl;
std::cout << map_item.first << " " << map_item.second[2] << std::endl;
std::cout << map_item.first << " " << map_item.second[3] << std::endl;
std::cout << map_item.first << " " << map_item.second[4] << std::endl;
std::cout << map_item.first << " " << map_item.second[5] << std::endl;
}
std::cout << " -- End of program -- " << std::endl;
return 0;
}
Problem:
I don't see the items of s2 when I print values of my_map_2. I see them only if I add s2 with a new key! If I do my_map_2.insert(std::make_pair("key2", s2)) instead of my_map_2.insert(std::make_pair("key1", s2)), I do see the items.
Question:
So, my question is, how to I append more items to the vector pointed to by key1 of my_map_2?

The below fails because the key is already taken:
std::vector<std::string> s2 = {"d", "e", "f"};
my_map_2.insert(std::make_pair("key1", s2)); // fails
To append to the mapped vector, you could do like this:
auto& vec = my_map_2["key1"]; // get reference to the existing vector
vec.insert(vec.end(), s2.begin(), s2.end()); // append to it
To view the keys and all the values in the vector you could change your loop to this:
for(auto const&[key, value]: my_map_2) {
for(const std::string& str : value) {
std::cout << key << ' ' << str << '\n';
}
}

my_map_2["key1"] is always a valid vector. You can insert into it directly
#include <iostream>
#include <vector>
#include <string>
#include <map>
int main() {
std::cout << " -- Beginining of program -- " << std::endl;
std::map<std::string, std::vector<std::string>> my_map_2;
std::vector<std::string> s = {"a", "b", "c"};
my_map_2["key1"].insert(my_map_2["key1"].end(), s.begin(), s.end());
std::vector<std::string> s2 = {"d", "e", "f"};
my_map_2["key1"].insert(my_map_2["key1"].end(), s2.begin(), s2.end());
for(auto const &map_item: my_map_2) {
for(auto const &value: map_item.second) {
std::cout << map_item.first << " " << value << std::endl;
}
}
std::cout << " -- End of program -- " << std::endl;
return 0;
}

Get iterator to key1, and just pushs back new items to existing vector:
std::vector<std::string> s2 = {"d", "e", "f"};
auto it = my_map_2.find("key1");
if (it != my_map_2.end())
std::move(s2.begin(), s2.end(), std::back_inserter(it->second));
else
my_map_2.insert(std::make_pair("key1",std::move(s2)));
To see: d,e,f you have to access 3,4 and 5 indices of vector. (You want to append new items, or just override existed items for given key?)

How to mix auto with brace-initialization to return a std::pair properly?

I have this example which is a function that returns a std::pair which holds a string value and its size.
std::pair<std::string, int> getLastPair(const vector<string>& vec) {
return{ vec.back(), vec.back().size() };
}
int main(){
vector<string> names{ "An", "Apple", "A", "Day", "Keeps", "The",
"Doctor", "Away"};
auto p{getLastPair(names)}; // ok p is a pair<string, int>
std::cout << p.first << " " << p.second << std::endl;
std::cout << typeid(p).name() << std::endl;
auto p2 = { getLastPair(names) }; // why it is not a pair but an initializer list?
std::cout << typeid(p2).name() << std::endl; // initializer list
std::pair<std::string, int> p3 = { getLastPair(names) }; // ok a pair
std::cout << p3.first << " : " << p3.second << std::endl;
decltype (getLastPair(names)) p4 = { getLastPair(names) }; // ok
std::cout << p4.first << " : " << p4.second << std::endl; // ok
}
Why the first initialization returns a pair as expected but the second one p2 is not a pair but an initializer_list instead?
As you can see p3 works fine (returns a pair) aslong as I've provided the type explicitly!?
Is the problem in Type Specifier auto?
Also p4 works fine with decltype.

Eigen: Concatenating matrices and vectors

Using the Eigen C++ library, I have a Matrix3f A, a Vector4f b, and a Vector4f c. I want to create a Matrix4f M out of these. I want the top 3-by-3 corner of M to be A, I want to final column of M to be b, and I want the bottom row of M to be c.
I know how to do this by simply creating a Matrix4f and assigning each element individually. But is there a more elegant solution that Eigen supports?

Does this count as elegant enough?
#include <Eigen/Sparse>
#include <iostream>
using namespace Eigen;
using std::cout;
using std::endl;
int main(int argc, char *argv[])
{
Matrix4f m = Matrix4f::Random();
Matrix3f A = Matrix3f::Constant(0.1);
Vector4f b = Vector4f::Constant(0.2), c = Vector4f::Constant(0.3);
cout << m << endl << endl;
cout << A << endl << endl;
cout << b << endl << endl;
cout << c << endl << endl;
m.block(0, 0, 3, 3) = A;
m.col(3) = b;
m.row(3) = c;
cout << m << endl << endl;
return 0;
}
Note that your question is kinda ambiguous, as the (3,3) position will be determined by the order of assignment between b and c.

find chars/string in string from vector c++

I have a vector of strings, and I want to count all 'Ace' in the vector. Right now I can only find one...
int main()
{
std::vector<string> vec;
vec.push_back("Ace of Spades");
vec.push_back("Ace");
string value = "Ace";
int cnt = 0;
auto iter = find_if(begin(vec), end(vec), [&](const string &str)
{
return str.find(value) != str.npos;
});
if(iter == end(vec))
cout << "no found" << endl;
else
{
cout << *iter << endl;
cnt++;
cout << cnt++ << endl;
}
}

You could use std::count_if:
auto cnt = count_if(begin(vec),
end(vec),
[&](const string& str) {
return str.find(value) != std::string::npos;
});
Note that this only counts the number of strings containing "Ace", not the total number of occurrences of "Ace" in the vector's elements.

If you just want to count the number of matching elements, you could use std::count_if.
If you also need to do something with them, it would probably be best to forget about the standard library algorithms and use a ranged for like so:
int count = 0;
for (const auto& element : vec) {
if (element.find(value) != std::string::npos) {
std::cout << element << std::endl;
++count;
}
}
std::cout << count << std::endl;

Resources