r中的fminsearch比matlab更糟糕
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21-12-2019 - |
题
有我的数据(x和y列是相关的): https://www.dropbox.com/s/b61a7enhoa0p57p/simple1.csv
我需要的是用折线拟合数据。MATLAB代码这是:
spline_fit.m:
function [score, params] = spline_fit (points, x, y)
min_f = min(x)-1;
max_f = max(x);
points = [min_f points max_f];
params = zeros(length(points)-1, 2);
score = 0;
for i = 1:length(points)-1
in = (x > points(i)) & (x <= points(i+1));
if sum(in) > 2
p = polyfit(x(in), y(in), 1);
pred = p(1)*x(in) + p(2);
score = score + norm(pred - y(in));
params(i, :) = p;
else
params(i, :) = nan;
end
end
test.m:
%Find the parameters
r = [100,250,400];
p = fminsearch('spline_fit', r, [], x, y)
[score, param] = spline_fit(p, x, y)
%Plot the result
y1 = zeros(size(x));
p1 = [-inf, p, inf];
for i = 1:size(param, 1)
in = (x > p1(i)) & (x <= p1(i+1));
y1(in) = x(in)*param(i,1) + param(i,2);
end
[x1, I] = sort(x);
y1 = y1(I);
plot(x,y,'x',x1,y1,'k','LineWidth', 2)
.
,这确实可以很好,产生以下优化:[102.9842,191.0006,421.9912]
我在R:
中实现了相同的想法library(pracma);
spline_fit <- function(x, xx, yy) {
min_f = min(xx)-1;
max_f = max(xx);
points = c(min_f, x, max_f)
params = array(0, c(length(points)-1, 2));
score = 0;
for( i in 1:length(points)-1)
{
inn <- (xx > points[i]) & (xx <= points[i+1]);
if (sum(inn) > 2)
{
p <- polyfit(xx[inn], yy[inn], 1);
pred <- p[1]*xx[inn] + p[2];
score <- score + norm(as.matrix(pred - yy[inn]),"F");
params[i,] <- p;
}
else
params[i,] <- NA;
}
score
}
.
但结果非常糟糕:
> fminsearch(spline_fit,c(100,250,400), xx = Simple1$x, yy = Simple1$y)
$xval
[1] 100.1667 250.0000 400.0000
$fval
[1] 4452.761
$niter
[1] 2
.
正如您所看到的,它在2次迭代后停止,不会产生好点。
我很高兴解决解决这个问题的帮助。
此外,如果有人知道如何使用任何免费库中的C#在C#中实现这一点,那将更好。我知道whereto得到polyfit,但不是fminsearch。
解决方案
这里的问题是,可能性表面表现得非常严重 - 有多个最小值和不连续的跳跃 - 这将使您使用不同的优化器几乎是任意的结果。我将承认,Matlab的优化器是非常强大的,但我会说这几乎是机会(以及您开始的地方),除非您使用某种形式的随机全局优化,否则优化器将获得全局最小值。如模拟退火。
我选择使用R的内置优化器(默认使用Nelder-Mead)而不是来自fminsearch
包的pracma
。
spline_fit <- function(x, xx = Simple1$x, yy=Simple1$y) {
min_f = min(xx)-1
max_f = max(xx)
points = c(min_f, x, max_f)
params = array(0, c(length(points)-1, 2))
score = 0
for( i in 1:(length(points)-1))
{
inn <- (xx > points[i]) & (xx <= points[i+1]);
if (sum(inn) > 2)
{
p <- polyfit(xx[inn], yy[inn], 1);
pred <- p[1]*xx[inn] + p[2];
score <- score + norm(as.matrix(pred - yy[inn]),"F");
params[i,] <- p;
}
else
params[i,] <- NA;
}
score
}
library(pracma) ## for polyfit
Simple1 <- read.csv("Simple1.csv")
opt1 <- optim(fn=spline_fit,c(100,250,400), xx = Simple1$x, yy = Simple1$y)
## [1] 102.4365 201.5835 422.2503
.
这比世代古代替代代码结果更好,但仍然不同于MATLAB结果,而且比它们更糟糕:
## Matlab results:
matlab_fit <- c(102.9842, 191.0006, 421.9912)
spline_fit(matlab_fit, xx = Simple1$x, yy = Simple1$y)
## 3724.3
opt1$val
## 3755.5 (worse)
.
fminsearch
封装提供了一种用于探索优化表面的实验/不是非常好的文档工具:
library(bbmle)
ss <- slice2D(fun=spline_fit,opt1$par,nt=51)
library(lattice)
.
在bbmle
估计参数周围的2D“切片”。圆圈显示出优化拟合(固体)和每个切片内的最小值(打开)。
png("splom1.png")
print(splom(ss))
dev.off()
.
Matlab和Optim Fits之间的“切片”表明表面非常坚固:
ss2 <- bbmle:::slicetrans(matlab_fit,opt1$par,spline_fit)
png("slice1.png")
print(plot(ss2))
dev.off()
.
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