Scala 2.13

Question

这应该很简单。如何将函数应用于Scala中的元组？即：

scala> def f (i : Int, j : Int) = i + j
f: (Int,Int)Int

scala> val p = (3,4)
p: (Int, Int) = (3,4)

scala> f p
:6: error: missing arguments for method f in object $iw;
follow this method with `_' if you want to treat it as a partially applied function
       f p
       ^

scala> f _ p
:6: error: value p is not a member of (Int, Int) => Int
       f _ p
           ^

scala> (f _) p
:6: error: value p is not a member of (Int, Int) => Int
       (f _) p
             ^

scala> f(p)
:7: error: wrong number of arguments for method f: (Int,Int)Int
       f(p)
       ^

scala> grr!

非常感谢提前。

Answer 1

在Scala 2.7中：

scala> def f (i : Int, j : Int) = i + j
f: (Int,Int)Int

scala> val ff = f _
ff: (Int, Int) => Int = <function>

scala> val fft = Function.tupled(ff)
fft: ((Int, Int)) => Int = <function>

在Scala 2.8中：

scala> def f (i : Int, j : Int) = i + j
f: (i: Int,j: Int)Int

scala> val ff = f _
ff: (Int, Int) => Int = <function2>

scala> val fft = ff.tupled
fft: ((Int, Int)) => Int = <function1>

Answer 2

跟进另一个答案，可以写（用2.11.4测试）：

scala> def f (i: Int, j: Int) = i + j
f: (i: Int, j: Int)Int

scala> val ff = f _
ff: (Int, Int) => Int = <function2>

scala> val p = (3,4)
p: (Int, Int) = (3,4)

scala> ff.tupled(p)
res0: Int = 7

请参阅def tupled: ((T1, T2)) ⇒ R：

创建此函数的tupled版本：而不是2个参数接受一个scala.Tuple2参数。

Answer 3

Scala 2.13

import cv2
import numpy as np 
import matplotlib.pyplot as plt

def showing_features(img1, key_points):
   plt.imshow(cv2.drawKeypoints(img1, key_points, None))
   plt.show() 

img1 = cv2.imread('img1.jpg')
img2 = cv2.imread('img2.jpg')

img1_gray = cv2.cvtColor(img1, cv2.COLOR_BGR2GRAY)
img2_gray = cv2.cvtColor(img2, cv2.COLOR_BGR2GRAY)

sift= cv2.xfeatures2d.SIFT_create(nfeatures=0,
                            nOctaveLayers=3,
                            contrastThreshold=0.05,
                            edgeThreshold=10.0,
                            sigma=1.6)

"*-------------- Create Masks --------------*"
mask1 = np.ones(img1_gray.shape)
#ignore the left half of the first image
mask1[:,:int(mask1.shape[1]/2)] = 0
#ignore the right half of the second image
mask2 = np.ones(img2_gray.shape)
mask2[:,int(mask2.shape[1]/2):] = 0

"*-------------- Change Masks to Uint8 --------------*"
mask1 = mask1.astype(np.uint8)
mask2 = mask2.astype(np.uint8)

"*-------------- Extract SIFT Features --------------*"
kp1m, des1m =sift.detectAndCompute(img1_gray,mask1)
kp2m, des2m =sift.detectAndCompute(img2_gray,mask2)

"*-------------- Display SIFT features after using MASK --------------*"
showing_features(img1, kp1m)
showing_features(img2, kp2m)

Answer 4

scala> def f (i: Int, j: Int) = i + j
f: (i: Int, j: Int)Int
scala> val p = (3,4)
p: (Int, Int) = (3,4)
scala> val ft = (f _).tupled
ft: ((Int, Int)) => Int = <function1>
scala> ft apply(p)
res0: Int = 7

Answer 5

在Scala 3中，您可以使用TupledFunction：

您可以单次使用

summon[TupledFunction[(Int, Int) => Int, ((Int, Int)) => Int]].tupled(f)((2, 3))

要使其易于使用，可以使用扩展名（从Dotty自己的文档中复制）

extension [F, T <: Tuple, R](f: F)(using tf: TupledFunction[F, T => R])
  def tupled(t: T): R = tf.tupled(f)(t)

然后您可以执行f.tupled((2, 3))来获得5。

如何将函数应用于元组？

5 个答案:

Scala 2.13