Allen Wang - 6 months ago 24

Scala Question

This is probably a basic question, but I'm trying to create a simple function to find the two closest pairs in a list. Since this operation involves comparing TWO elements in the list, I am not sure using a for loop works if it really is just sugar for map and flatmap. So far, I have the structure of the expression as this:

`val points: List[(Double, Double)] = List((2.0,3.1),(3.5,2.3),(1.2,0.2),(6.4,2.4))`

var minDistance = Double.PositiveInfinity

var closestPoints = ((Double.NaN, Double.NaN), (Double.NaN, Double.NaN))

for {

point1 <- points

point2 <- points if (point1 != point2)

if (distance(point1, point2) < minDistance): {

minDistance = distance(point1, point2)

closestPoints = (point1, point2)

}

} yield (I guess I don't want a yield here?)

Note that the

`if (point1 != point2)`

`for {`

index1 <- 0 until points.length

index2 <- 0 until index1

...

This still seems unsatisfactory because of the yield? I guess there is some foldleft implementation that works, but I also don't know how to iterate on smaller and smaller subsets in the inner loop. I find foldLeft to be confusing to reason about compared to a simple double loop.

Answer

You can achieve this using `combinations`

and `foldLeft`

:

```
points.combinations(2).foldLeft((Double.PositiveInfinity,
((Double.NaN, Double.NaN), (Double.NaN, Double.NaN))))
((acc, points) => {
val List(firstPoint, secondPoint) = points
val dist = distance(firstPoint, secondPoint)
if (dist < acc._1) (dist, (firstPoint, secondPoint)) else acc
})
```