# Common pitfalls¶

## xarray initialization¶

```xt::xarray<double> a({1, 3, 4, 2});
```

does not initialize a 4D-array, but a 1D-array containing the values `1`, `3`, `4`, and `2`. It is strictly equivalent to

```xt::xarray<double> a = {1, 3, 4, 2};
```

To initialize a 4D-array with the given shape, use the `xt::xarray::from_shape()` static method:

```auto a = xt::xarray<double>::from_shape({1, 3, 4, 2});
```

The confusion often comes from the way `xt::xtensor` can be initialized:

```xt::xtensor<double, 4> a = {1, 3, 4, 2};
```

In this case, a 4D-tensor with shape `(1, 3, 4, 2)` is initialized.

## Intermediate result¶

Consider the following function:

```template <class C>
auto func(const C& c)
{
return (1 - func_tmp(c)) / (1 + func_tmp(c));
}
```

where `func_tmp` is another unary function accepting an xtensor expression. You may be tempted to simplify it a bit:

```template <class C>
auto func(const C& c)
{
auto tmp = func_tmp(c);
return (1 - tmp) / (1 + tmp);
}
```

Unfortunately, you introduced a bug; indeed, expressions in xtensor are not evaluated immediately, they capture their arguments by reference or copy depending on their nature, for future evaluation. Since `tmp` is an lvalue, it is captured by reference in the last statement; when the function returns, `tmp` is destroyed, leading to a dangling reference in the returned expression.

Replacing `auto tmp` with `xt::xarray<double> tmp` does not change anything, `tmp` is still an lvalue and thus captured by reference.

## Random numbers not consistent¶

Using a random number function from xtensor actually returns a lazy generator. That means, accessing the same element of a random number generator does not give the same random number if called twice.

```auto gen = xt::random::rand<double>({10, 10});
auto a0 = gen(0, 0);
auto a1 = gen(0, 0);

// a0 != a1 !!!
```

You need to explicitly assign or eval a random number generator, like so:

```xt::xarray<double> xr = xt::random::rand<double>({10, 10});
auto xr2 = xt::eval(xt::random::rand<double>({10, 10}));

// now xr(0, 0) == xr(0, 0) is true.
```

## variance arguments¶

When `xt::variance()` is passed an expression and an integer parameter, this latter is not the axis along which the variance must be computed, but the degree of freedom:

```xt::xtensor<double, 2> a = {{1., 2., 3.}, {4., 5., 6.}};
std::cout << xt::variance(a, 1) << std::endl;
// Outputs 3.5
```

If you want to specify an axis, you need to pass an initializer list:

```xt::xtensor<double, 2> a = {{1., 2., 3.}, {4., 5., 6.}};
std::cout << xt::variance(a, {1}) << std::endl;
.. Outputs { 0.666667, 0.666667 }
```

## fixed_shape on Windows¶

Builder functions such as `xt::empty()` or `xt::ones()` accept an initializer list as argument. If the elements of this list do not have the same type, a curious compilation error may occur on Windows:

```size_t N = 10ull;
xt::xarray<int> ages = xt::empty<int>({N, 4ul});

// error: cannot convert argument 1 from 'initializer list'
// to 'const xt::fixed_shape<> &'
```

To avoid this compiler bug (for which we don’t have a workaround), ensure all the elements in the initializer list have the same type.

## Alignment of fixed-size members¶

Note

If you are using `C++ >= 17` you should not have to worry about this.

When building with xsimd (see External dependencies), if you define a structure having members of fixed-size xtensor types, you must ensure that the buffers properly aligned. For this you can use the macro `XTENSOR_FIXED_ALIGN` available in `xtensor/xtensor_config.hpp`. Consider the following example:

```template <typename T>
class alignas(XTENSOR_FIXED_ALIGN) Foo
{
public:

using allocator_type = std::conditional_t<XTENSOR_FIXED_ALIGN != 0,
xt_simd::aligned_allocator<T, XTENSOR_FIXED_ALIGN>,
std::allocator<T>>;

Foo(T fac) : m_fac(fac)
{
m_bar.fill(fac);
}

auto get() const
{
return m_bar;
}

private:

xt::xtensor_fixed<T, xt::xshape<10, 10>> m_bar;
T m_fac;
};
```

Whereby it is important to store the fixed-sized xtensor type (in this case `xt::xtensor_fixed<T, xt::xshape<10, 10>>`) as first member.