I am new to scipy minimize. I want to minimize a function. There are 2 vectors in play :
x : 4 element vector of spending
y : 4 element vector of cost per customer
each element in y is defined something like 50 from 0 to 100000, and 0.0005 * X from 100000 to infinity
The objective function is to minimize the spend :
def objective(x):
x1=x[0]
x2=x[1]
x3=x[2]
x4=x[3]
return x1+x2+x3+x4
As the constraint I have the number of users I have to sign up for like this :
def constraint1(x,y):
x[0]/y[0]+x[1]/y[1]+x[2]/y[2]+x[3]/y[3]>5035
bounds and definition like this
b=(0,1000000)
bnds=(b,b,b,b)
con1={'type': 'ineq', 'fun': constraint1}
x0=[20000,20000,20000,20000]
sol= minimize(objective,x0, method= "SLSQP",bounds=bnds,constraints=con1)
I simply do not know to define the Y vector properly. Any feedback or help wound be very much appreciated .
Related
I have a dataset which looks roughly as follows (and is sinusoidal in nature):
TW-240-run1.txt
Point Number Temperature
0 51.504781
1 51.487722
2 51.487722
3 51.828893
4 51.828893
5 51.436547
6 51.368312
7 51.726542
8 51.368312
9 51.317137
10 51.317137
11 51.283020
12 51.590073
.
.
.
9599 51.675366
I am tasked with finding the fundamental/first fourier coefficients, a_n and b_n for this dataset, by means of a numerical integration technique. In this case, I am simply using numpy.trapz from numpy, which aims to implement the trapezium rule. The fourier coefficients, a_n and b_n can be calculated with the following formulae:
where tau (𝛕) is the time period of the sine function. For my case, 𝛕 = 240 seconds (referring to the point number 240 on the data sheet), and thus the bounds of integration are 0 to 240. T(t) from the above formulae is the data set and n = 1.
My current code for trying to calculate the fourier coefficients is as follows:
# Packages
import numpy as np
import matplotlib.pyplot as plt
import scipy as sp
#input data from datasheet, the loadtxt below takes in the data from t = 0s to t = 240s
x1, y1 = np.loadtxt(r'C:\Users\Sidharth\Documents\y2python\y2python\thermal_4min_a.txt', unpack=True, skiprows=3)
tau_4min = 240.0
def cosine(period, t, n):
return np.cos((2*np.pi*n*t)/(period)) #defines the cos term for the a_n formula
def sine(period, t, n): #defines the sin term for the a_n formula
return np.sin((2*np.pi*n*t)/(period))
a_1_4min = (2/tau_4min)*np.trapz((y1*cos_term_4min), x1) #implement a_n formula (trapezium rule for T(t)*cos)
print('a_1 is', a_1_4min)
b_1_4min = (2/tau_4min)*np.trapz((y1*sin_term_4min), x1) #implement b_n formula (trapezium rule for T(t)*cos)
print('b_1 is', b_1_4min)
Essentially what this is doing is, it takes in the data, but only up to the row index 241 (point number 240), and then multiplies it by the sine/cosine term from each of the above formulae. However, I realise this isn't calculating the fourier coefficients properly.
My question(s) are as follows:
Will my code work if I can find a way to set limits of integration for np.trapz and then importing the entire data set, instead of only importing the data points from 0 to 240 and multiplying it by the cos or sine term, then using np trapz on that product, as I am currently doing (0 and 240 are supposed to be my limits of integration)
Suppose there are three variables that take on discrete integer values, say w1 = {1,2,3,4,5,6,7,8,9,10,11,12}, w2 = {1,2,3,4,5,6,7,8,9,10,11,12}, and w3 = {1,2,3,4,5,6,7,8,9,10,11,12}. The task is to pick one value from each set such that the resulting triplet minimizes some (black box, computationally expensive) cost function.
I've tried the surrogate optimization in Matlab but I'm not sure it is appropriate. I've also heard about simulated annealing but found no implementation applied to this instance.
Which algorithm, apart from exhaustive search, can solve this combinatorial optimization problem?
Any help would be much appreciated.
The requirement/benefit of Simulated Annealing (SA), is that the objective surface is somewhat smooth, that is, we can be close to a solution.
For a completely random spiky surface- you might as well do a random search
If it is anything smooth, or even sometimes, it makes sense to try SA.
The idea is that (sometimes) changing only 1 of the 3 values, we have little effect on out blackbox function.
Here is a basic example to do this with Simulated Annealing, using frigidum in Python
import numpy as np
w1 = np.array( [1,2,3,4,5,6,7,8,9,10,11,12] )
w2 = np.array( [1,2,3,4,5,6,7,8,9,10,11,12] )
w3 = np.array( [1,2,3,4,5,6,7,8,9,10,11,12] )
W = np.array([w1,w2,w3])
LENGTH = 12
I define a black-box using the Rastrigin function.
def rastrigin_function_n( x ):
"""
N-dimensional Rastrigin
https://en.wikipedia.org/wiki/Rastrigin_function
x_i is in [-5.12, 5.12]
"""
A = 10
n = x.shape[0]
return A*n + np.sum( x**2- A*np.cos(2*np.pi * x) )
def black_box( x ):
"""
Transform from domain [1,12] to [-5,5]
to be able to push to rastrigin
"""
x = (x - 6.5) * (5/5.5)
return rastrigin_function_n(x)
Simulated Annealing needs to modify state X. Instead of taking/modifying values directly, we keep track of indices. This simplifies creating new proposals as an index is always an integer we can simply add/subtract 1 modulo LENGTH.
def random_start():
"""
returns 3 random indices
"""
return np.random.randint(0, LENGTH, size=3)
def random_small_step(x):
"""
change only 1 index
"""
d = np.array( [1,0,0] )
if np.random.random() < .5:
d = np.array( [-1,0,0] )
np.random.shuffle(d)
return (x+d) % LENGTH
def random_big_step(x):
"""
change 2 indici
"""
d = np.array( [1,-1,0] )
np.random.shuffle(d)
return (x+d) % LENGTH
def obj(x):
"""
We have a triplet of indici,
1. Calculate corresponding values in W = [w1,w2,w3]
2. Push the values in out black-box function
"""
indices = x
values = W[np.array([0,1,2]), indices]
return black_box(values)
And throw a SA Scheme at it
import frigidum
local_opt = frigidum.sa(random_start=random_start,
neighbours=[random_small_step, random_big_step],
objective_function=obj,
T_start=10**4,
T_stop=0.000001,
repeats=10**3,
copy_state=frigidum.annealing.naked)
I am not sure what the minimum for this function should be, but it found a objective with 47.9095 with indicis np.array([9, 2, 2])
Edit:
For frigidum to change the cooling schedule, use alpha=.9. My experience is that all the work of experiment which cooling scheme works best doesn't out-weight simply let it run a little longer. The multiplication you proposed, (sometimes called geometric) is the standard one, also implemented in frigidum. So to implement Tn+1 = 0.9*Tn you need a alpha=.9. Be aware this cooling step is done after N repeats, so if repeats=100, it will first do 100 proposals before lowering the temperature with factor alpha
Simple variations on current state often works best. Since its best practice to set the initial temperature high enough to make most proposals (>90%) accepted, it doesn't matter the steps are small. But if you fear its soo small, try 2 or 3 variations. Frigidum accepts a list of proposal functions, and combinations can enforce each other.
I have no experience with MINLP. But even if, so many times experiments can surprise us. So if time/cost is small to bring another competitor to the table, yes!
Try every possible combination of the three values and see which has the lowest cost.
I have curve that initially Y increases linearly with X, then reach a plateau at point C.
In other words, the curve can be defined as:
if X < C:
Y = k * X + b
else:
Y = k * C + b
The training data is a list of X ~ Y values. I need to determine k, b and C through a machine learning approach (or similar), since the data is noisy and refection point C changes over time. I want something more robust than get C through observing the current sample data.
How can I do it using sklearn or maybe scipy?
WLOG you can say the second equation is
Y = C
looks like you have a linear regression to fit the line and then a detection point to find the constant.
You know that in the high values of X, as in X > C you are already at the constant. So just check how far back down the values of X you get the same constant.
Then do a linear regression to find the line with value of X, X <= C
Your model is nonlinear
I think the smartest way to solve this is to do these steps:
find the maximum value of Y which is equal to k*C+b
M=max(Y)
drop this maximum value from your dataset
df1 = df[df.Y != M]
and then you have simple dataset to fit your X to Y and you can use sklearn for that
I can't find any example anywhere on the internet .
I would like to learn using the exponential law to calculate a probability.
This my exponential lambda : 0.0035
What is the probability that my object becomes defectuous before 100 hours of work ? P(X < 100)
How could I write this with numpy or sci kit ? Thanks !
Edit : this is the math :
P(X < 100) = 1 - e ** -0.0035 * 100 = 0.3 = 30%
Edit 2 :
Hey guys, I maybe have found something there, hi hi :
http://web.stanford.edu/class/archive/cs/cs109/cs109.1192/handouts/pythonForProbability.html
Edit 3 :
This is my attempt with scipy :
from scipy import stats
B = stats.expon(0.0035) # Declare B to be a normal random variable
print(B.pdf(1)) # f(1), the probability density at 1
print(B.cdf(100)) # F(2) which is also P(B < 100)
print(B.rvs()) # Get a random sample from B
but B.cdf is wrong : it prints 1, while it should print 0.30, please help !
B.pdf prints 0.369 : What is this ?
Edit 4 : I've done it with the python math lib like this :
lambdaCalcul = - 0.0035 * 100
MyExponentialProbability = 1 - math.exp(lambdaCalcul)
print("My probability is",MyExponentialProbability * 100 , "%");
Any other solution with numpy os scipy is appreciated, thank you
The expon(..) function takes as parameters loc and scale (which correspond to the mean and the standard deviation. Since the standard deviation is the inverse of the variance, we thus can construct such distribution with:
B = stats.expon(scale=1/0.0035)
Then the cummulative distribution function says for P(X < 100):
>>> print(B.cdf(100))
0.2953119102812866
I am processing a series of points which all have the same Y value, but different X values. I go through the points by incrementing X by one. For example, I might have Y = 50 and X is the integers from -30 to 30. Part of my algorithm involves finding the distance to the origin from each point and then doing further processing.
After profiling, I've found that the sqrt call in the distance calculation is taking a significant amount of my time. Is there an iterative way to calculate the distance?
In other words:
I want to efficiently calculate: r[n] = sqrt(x[n]*x[n] + y*y)). I can save information from the previous iteration. Each iteration changes by incrementing x, so x[n] = x[n-1] + 1. I can not use sqrt or trig functions because they are too slow except at the beginning of each scanline.
I can use approximations as long as they are good enough (less than 0.l% error) and the errors introduced are smooth (I can't bin to a pre-calculated table of approximations).
Additional information:
x and y are always integers between -150 and 150
I'm going to try a couple ideas out tomorrow and mark the best answer based on which is fastest.
Results
I did some timings
Distance formula: 16 ms / iteration
Pete's interperlating solution: 8 ms / iteration
wrang-wrang pre-calculation solution: 8ms / iteration
I was hoping the test would decide between the two, because I like both answers. I'm going to go with Pete's because it uses less memory.
Just to get a feel for it, for your range y = 50, x = 0 gives r = 50 and y = 50, x = +/- 30 gives r ~= 58.3. You want an approximation good for +/- 0.1%, or +/- 0.05 absolute. That's a lot lower accuracy than most library sqrts do.
Two approximate approaches - you calculate r based on interpolating from the previous value, or use a few terms of a suitable series.
Interpolating from previous r
r = ( x2 + y2 ) 1/2
dr/dx = 1/2 . 2x . ( x2 + y2 ) -1/2 = x/r
double r = 50;
for ( int x = 0; x <= 30; ++x ) {
double r_true = Math.sqrt ( 50*50 + x*x );
System.out.printf ( "x: %d r_true: %f r_approx: %f error: %f%%\n", x, r, r_true, 100 * Math.abs ( r_true - r ) / r );
r = r + ( x + 0.5 ) / r;
}
Gives:
x: 0 r_true: 50.000000 r_approx: 50.000000 error: 0.000000%
x: 1 r_true: 50.010000 r_approx: 50.009999 error: 0.000002%
....
x: 29 r_true: 57.825065 r_approx: 57.801384 error: 0.040953%
x: 30 r_true: 58.335225 r_approx: 58.309519 error: 0.044065%
which seems to meet the requirement of 0.1% error, so I didn't bother coding the next one, as it would require quite a bit more calculation steps.
Truncated Series
The taylor series for sqrt ( 1 + x ) for x near zero is
sqrt ( 1 + x ) = 1 + 1/2 x - 1/8 x2 ... + ( - 1 / 2 )n+1 xn
Using r = y sqrt ( 1 + (x/y)2 ) then you're looking for a term t = ( - 1 / 2 )n+1 0.36n with magnitude less that a 0.001, log ( 0.002 ) > n log ( 0.18 ) or n > 3.6, so taking terms to x^4 should be Ok.
Y=10000
Y2=Y*Y
for x=0..Y2 do
D[x]=sqrt(Y2+x*x)
norm(x,y)=
if (y==0) x
else if (x>y) norm(y,x)
else {
s=Y/y
D[round(x*s)]/s
}
If your coordinates are smooth, then the idea can be extended with linear interpolation. For more precision, increase Y.
The idea is that s*(x,y) is on the line y=Y, which you've precomputed distances for. Get the distance, then divide it by s.
I assume you really do need the distance and not its square.
You may also be able to find a general sqrt implementation that sacrifices some accuracy for speed, but I have a hard time imagining that beating what the FPU can do.
By linear interpolation, I mean to change D[round(x)] to:
f=floor(x)
a=x-f
D[f]*(1-a)+D[f+1]*a
This doesn't really answer your question, but may help...
The first questions I would ask would be:
"do I need the sqrt at all?".
"If not, how can I reduce the number of sqrts?"
then yours: "Can I replace the remaining sqrts with a clever calculation?"
So I'd start with:
Do you need the exact radius, or would radius-squared be acceptable? There are fast approximatiosn to sqrt, but probably not accurate enough for your spec.
Can you process the image using mirrored quadrants or eighths? By processing all pixels at the same radius value in a batch, you can reduce the number of calculations by 8x.
Can you precalculate the radius values? You only need a table that is a quarter (or possibly an eighth) of the size of the image you are processing, and the table would only need to be precalculated once and then re-used for many runs of the algorithm.
So clever maths may not be the fastest solution.
Well there's always trying optimize your sqrt, the fastest one I've seen is the old carmack quake 3 sqrt:
http://betterexplained.com/articles/understanding-quakes-fast-inverse-square-root/
That said, since sqrt is non-linear, you're not going to be able to do simple linear interpolation along your line to get your result. The best idea is to use a table lookup since that will give you blazing fast access to the data. And, since you appear to be iterating by whole integers, a table lookup should be exceedingly accurate.
Well, you can mirror around x=0 to start with (you need only compute n>=0, and the dupe those results to corresponding n<0). After that, I'd take a look at using the derivative on sqrt(a^2+b^2) (or the corresponding sin) to take advantage of the constant dx.
If that's not accurate enough, may I point out that this is a pretty good job for SIMD, which will provide you with a reciprocal square root op on both SSE and VMX (and shader model 2).
This is sort of related to a HAKMEM item:
ITEM 149 (Minsky): CIRCLE ALGORITHM
Here is an elegant way to draw almost
circles on a point-plotting display:
NEW X = OLD X - epsilon * OLD Y
NEW Y = OLD Y + epsilon * NEW(!) X
This makes a very round ellipse
centered at the origin with its size
determined by the initial point.
epsilon determines the angular
velocity of the circulating point, and
slightly affects the eccentricity. If
epsilon is a power of 2, then we don't
even need multiplication, let alone
square roots, sines, and cosines! The
"circle" will be perfectly stable
because the points soon become
periodic.
The circle algorithm was invented by
mistake when I tried to save one
register in a display hack! Ben Gurley
had an amazing display hack using only
about six or seven instructions, and
it was a great wonder. But it was
basically line-oriented. It occurred
to me that it would be exciting to
have curves, and I was trying to get a
curve display hack with minimal
instructions.