sicp每日一题[2.50]
1.sicp每日一题[1.42]2.sicp每日一题[1.43]3.sicp每日一题[1.44]4.sicp每日一题[1.45]5.sicp每日一题[1.46]6.sicp每日一题[2.1]7.sicp每日一题[2.2]8.sicp每日一题[2.3]9.sicp每日一题[2.4]10.sicp每日一题[2.5]11.sicp每日一题[2.6]12.sicp每日一题[2.7]13.sicp每日一题[2.8]14.sicp每日一题[2.9]15.sicp每日一题[2.11]16.sicp每日一题[2.10]17.sicp每日一题[2.12]18.sicp每日一题[2.13-2.16]19.sicp每日一题[2.17]20.sicp每日一题[2.18]21.sicp每日一题[2.19]22.sicp每日一题[2.20]23.sicp每日一题[2.21]24.sicp每日一题[2.22-2.23]25.sicp每日一题[2.24-2.27]26.sicp每日一题[2.28]27.sicp每日一题[2.29]28.sicp每日一题[2.30]29.sicp每日一题[2.31]30.sicp每日一题[2.32]31.sicp每日一题[2.33]32.sicp每日一题[2.34]33.sicp每日一题[2.35]34.sicp每日一题[2.36-2.37]35.sicp每日一题[2.38-2.39]36.sicp每日一题[2.40]37.sicp每日一题[2.41]38.sicp每日一题[2.42]39.sicp每日一题[2.43]40.sicp每日一题[2.44]41.sicp每日一题[2.45]42.sicp每日一题[2.46]43.sicp每日一题[2.47]44.sicp每日一题[2.48]45.sicp每日一题[2.49]
46.sicp每日一题[2.50]
47.sicp每日一题[2.51]48.sicp每日一题[2.52]49.sicp每日一题[2.53-2.55]50.sicp每日一题[2.56]51.sicp每日一题[2.57]52.sicp每日一题[2.58]53.sicp每日一题[2.59]54.sicp每日一题[2.60]55.sicp每日一题[2.61]56.sicp每日一题[2.62]57.sicp每日一题[2.63-2.64]58.sicp每日一题[2.65]59.sicp每日一题[2.66]60.sicp每日一题[2.67-2.68]61.sicp每日一题[2.69]62.sicp每日一题[2.70]63.sicp每日一题[2.71]64.sicp每日一题[2.72]65.sicp每日一题[2.73]66.sicp每日一题[2.74]67.sicp每日一题[2.75]68.sicp每日一题[2.76]69.sicp每日一题[2.77]70.sicp每日一题[2.78]71.sicp每日一题[2.79]72.sicp每日一题[2.80]73.sicp每日一题[2.81]74.sicp每日一题[2.82]75.sicp每日一题[2.83]76.sicp每日一题[2.84]77.sicp每日一题[2.85-2.86]Exercise 2.50
Define the transformation flip-horiz, which flips painters horizontally, and transformations that rotate painters counterclockwise by 180 degrees and 270 degrees.
这道题挺有意思的，搞明白这道题就明白了 frame 的3个点的位置。如上图所示，为了更好区分，特意用了长方形而不是正方形，第一幅图是原图，O 表示 origin, A 表示 edge1，B 表示 edge2。无论进行何种变换，左下坐标都是 (0, 0)，左上都是(0, 1)， 右下都是(1, 0)， 右上都是(1, 1)，我们只要把变换后的图形中 O, A, B 的新位置作为参数传进去就行。
#lang racket
(require (planet "sicp.ss" ("soegaard" "sicp.plt" 2 1)))
(define sub-vect vector-sub)
; Transforming and combining painters
(define (transform-painter painter origin corner1 corner2)
(lambda (frame)
(let ((m (frame-coord-map frame)))
(let ((new-origin (m origin)))
(painter (make-frame
new-origin
(sub-vect (m corner1) new-origin)
(sub-vect (m corner2) new-origin)))))))
(define (flip-horiz painter)
(transform-painter painter
(make-vect 1.0 0.0) ; new origin
(make-vect 0.0 0.0) ; new end of edge1
(make-vect 1.0 1.0))) ; new end of edge2
(define (rotate90 painter)
(transform-painter painter
(make-vect 1.0 0.0) ; new origin
(make-vect 1.0 1.0) ; new end of edge1
(make-vect 0.0 0.0))) ; new end of edge2
(define (rotate180 painter)
(transform-painter painter
(make-vect 0.0 1.0) ; new origin
(make-vect 1.0 1.0) ; new end of edge1
(make-vect 0.0 0.0))) ; new end of edge2
(define (rotate270 painter)
(transform-painter painter
(make-vect 0.0 1.0) ; new origin
(make-vect 0.0 0.0) ; new end of edge1
(make-vect 1.0 1.0))) ; new end of edge2
(paint einstein)
(paint (flip-horiz einstein))
(paint (rotate90 einstein))
(paint (rotate180 einstein))
(paint (rotate270 einstein))
效果如下图所示，为了跟水平翻转对比，特意把原图加上了：
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