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# vDSP_vlintD

Calculates the interpolation between the neighboring elements of a double-precision vector using the specified stride.

```
extern void vDSP_vlintD(const double *__A, const double *__B, vDSP_Stride __IB, double *__C, vDSP_Stride __IC, vDSP_Length __N, vDSP_Length __M);
```

## Parameters

`__A`

The double-precision input vector.

`__B`

The vector that controls the interpolation. The integer part of each element in the control vector is the zero-based index of the first element of a pair of adjacent values in the source array. The fractional part defines the linear interpolation between the values at those indices.

`__IB`

The stride between elements in the vector `__B`.

`__C`

The double-precision output vector.

`__IC`

The stride between elements in the vector `__C`.

`__N`

The number of elements in the output vector.

`__M`

The number of elements in the input vector.

## Discussion

Performs the following operation:

```objc
for (int n = 0; n < N; ++n){
    double b = B[n*IB];
    double index = trunc(b); // The integer part of B.
    double alpha = b - index; // The fractional part of B.
 
    double a0 = A[(int)index]; // The indexed A value.
    double a1 = A[(int)index + 1]; // The next indexed A value.
 
    C[n*IC] = a0 + (alpha * (a1 -a0)); // The interpolated value.
}
```

The function generates vector `C` by interpolating between neighboring values of vector `A` as controlled by vector `B`. The operation treats the integer portion of each element in `B` as the zero-based index of the first element of a pair of adjacent values in vector `A`.

The function uses the fractional part of the value in `B` to calculate the corresponding element of `C` from the two adjacent values.

The calculation doesn’t use the argument `M`. However, the integer parts of the values in `B` must be greater than or equal to zero and less than or equal to `M` - 2.

---

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