我正在寻找在scipy/numpy中可以解决非线性最小二乘问题的优化例程(例如,将参数函数拟合到大型数据集中),但包括界限和约束(例如要优化的参数的最小值和最大值)。目前,我正在使用mpfit的Python版本(从idl翻译而来...):尽管它表现良好,但显然不是最优选择。
在python/scipy等中实现高效的例程将会非常棒!
非常感谢!
我正在寻找在scipy/numpy中可以解决非线性最小二乘问题的优化例程(例如,将参数函数拟合到大型数据集中),但包括界限和约束(例如要优化的参数的最小值和最大值)。目前,我正在使用mpfit的Python版本(从idl翻译而来...):尽管它表现良好,但显然不是最优选择。
在python/scipy等中实现高效的例程将会非常棒!
非常感谢!
scipy.optimize.least_squares在scipy 0.17(2016年1月)中,处理边界;使用它,而不是这个hack。
边界限制可以轻松地变为二次型,并与其余部分一起通过leastsq进行最小化。
假设你想最小化10个平方和Σf_i(p)^2的总和,因此您的func(p)是一个10向量[f0(p) ... f9(p)],
同时还想要3个参数满足0 <= p_i <= 1。
考虑“管函数”max(-p, 0, p-1),它在0..1内为0,在外部为正,就像\_____/管子一样。
如果我们给leastsq
提供一个13维向量
[ f0(p), f1(p), ... f9(p), w*tub(p0), w*tub(p1), w*tub(p2) ]
如果w = 100,它将最小化一系列数字的平方和,这些数字受到以下约束:0≤p≤1。类似地,当lo≤p≤hi时也是如此。
下面的代码只是一个包装器,它运行 leastsq
来最小化一个长度为13的向量。
# leastsq_bounds.py
# see also test_leastsq_bounds.py on gist.github.com/denis-bz
from __future__ import division
import numpy as np
from scipy.optimize import leastsq
__version__ = "2015-01-10 jan denis" # orig 2012
#...............................................................................
def leastsq_bounds( func, x0, bounds, boundsweight=10, **kwargs ):
""" leastsq with bound conatraints lo <= p <= hi
run leastsq with additional constraints to minimize the sum of squares of
[func(p) ...]
+ boundsweight * [max( lo_i - p_i, 0, p_i - hi_i ) ...]
Parameters
----------
func() : a list of function of parameters `p`, [err0 err1 ...]
bounds : an n x 2 list or array `[[lo_0,hi_0], [lo_1, hi_1] ...]`.
Use e.g. [0, inf]; do not use NaNs.
A bound e.g. [2,2] pins that x_j == 2.
boundsweight : weights the bounds constraints
kwargs : keyword args passed on to leastsq
Returns
-------
exactly as for leastsq,
http://docs.scipy.org/doc/scipy/reference/generated/scipy.optimize.leastsq.html
Notes
-----
The bounds may not be met if boundsweight is too small;
check that with e.g. check_bounds( p, bounds ) below.
To access `x` in `func(p)`, `def func( p, x=xouter )`
or make it global, or `self.x` in a class.
There are quite a few methods for box constraints;
you'll maybe sing a longer song ...
Comments are welcome, test cases most welcome.
"""
# Example: test_leastsq_bounds.py
if bounds is not None and boundsweight > 0:
check_bounds( x0, bounds )
if "args" in kwargs: # 8jan 2015
args = kwargs["args"]
del kwargs["args"]
else:
args = ()
#...............................................................................
funcbox = lambda p: \
np.hstack(( func( p, *args ),
_inbox( p, bounds, boundsweight )))
else:
funcbox = func
return leastsq( funcbox, x0, **kwargs )
def _inbox( X, box, weight=1 ):
""" -> [tub( Xj, loj, hij ) ... ]
all 0 <=> X in box, lo <= X <= hi
"""
assert len(X) == len(box), \
"len X %d != len box %d" % (len(X), len(box))
return weight * np.array([
np.fmax( lo - x, 0 ) + np.fmax( 0, x - hi )
for x, (lo,hi) in zip( X, box )])
# def tub( x, lo, hi ):
# """ \___/ down to lo, 0 lo .. hi, up from hi """
# return np.fmax( lo - x, 0 ) + np.fmax( 0, x - hi )
#...............................................................................
def check_bounds( X, box ):
""" print Xj not in box, loj <= Xj <= hij
return nr not in
"""
nX, nbox = len(X), len(box)
assert nX == nbox, \
"len X %d != len box %d" % (nX, nbox)
nnotin = 0
for j, x, (lo,hi) in zip( range(nX), X, box ):
if not (lo <= x <= hi):
print "check_bounds: x[%d] %g is not in box %g .. %g" % (j, x, lo, hi)
nnotin += 1
return nnotin
scipy在scipy.optimize中拥有多个约束优化算法。其中,约束最小二乘变体是scipy.optimize.fmin_slsqp。
这个新函数可以使用适当的信任区域算法处理边界约束,并充分利用非线性函数的平方和特性进行优化。
注意:
@denis提出的解决方案存在一个主要问题,即引入了不连续的“管函数”。当越过边界时,这使得为平滑函数设计的scipy.optimize.leastsq
优化非常低效,可能不稳定。
scipy.optimize.minimize
和method='SLSQP'
(如@f_ficarola建议的)或scipy.optimize.fmin_slsqp
(如@matt建议的),主要问题在于它们没有利用要最小化的函数的平方和性质。这些函数都是设计用来最小化标量函数的(即使对于fmin_slsqp也是如此,尽管名称有误导性)。这些方法比一个合适的方法更低效、不准确。
File "[...]/leastsq_bounds.py", line 49, in leastsq_bounds return leastsq( funcbox, x0, **kwargs ) File "[...]/minpack.py", line 369, in leastsq shape, dtype = _check_func('leastsq', 'func', func, x0, args, n) File "[...]/minpack.py", line 20, in _check_func res = atleast_1d(thefunc(*((x0[:numinputs],) + args))) TypeError: <lambda>() takes exactly 1 argument (5 given)
。 - redcrowFile "leastsq_bounds.py", line 58, in leastsq_bounds return leastsq( funcbox, x0, **kwargs ) File "minpack.py", line 369, in leastsq shape, dtype = _check_func('leastsq', 'func', func, x0, args, n) File "minpack.py", line 20, in _check_func res = atleast_1d(thefunc(*((x0[:numinputs],) + args))) File "leastsq_bounds.py", line 55, in <lambda> _inbox( p, bounds, boundsweight ))) File "leastsq_bounds.py", line 66, in _inbox "len X %d != len box %d" % (len(X), len(box)) AssertionError: len X 1 != len box 2
- redcrownp.shape(bounds)
的输出结果应该是 nparam x 2,请先运行leastsq_bounds.check_bounds(x0, bounds)
。 - denis