如何在 GEKKO (Python) 中使用 log base 2

Question

I'm trying to solve an optimization problem with GEKKO using python, I'm trying to develop some mathematical function with log and sqrt and I figure out that I should use gekko operator instead of using numpy or math function. I wanted to know how to implement log base 2 instead of log or log10 using gekko.

gk = GEKKO()
gk.log(...) # work
gk.sqrt(...) # work
gk.log2(...) # does not work!

Error :

AttributeError: 'GEKKO' object has no attribute 'log2'

Answer 1

Instead you can change the log base using the rule:

log2(x) = gk.log(x)/gk.log(2)

You can't expect for it to have all of the log bases implemented.

Answer 2

You can create a log2 function in Gekko, especially if you need to use it multiple times throughout your model.

def glog2(x):
    return gk.log(x)/np.log(2) 

Below is a complete script that demonstrates the use of the new log2 function and shows the agreement with the Numpy log2 function.

from gekko import GEKKO
import numpy as np

# compare with numpy
xnp = np.logspace(-1,4,100)
ynp = np.log2(xnp)

gk = GEKKO(remote=False)
x = gk.Param(xnp)
y,z = gk.Array(gk.Var,2)

# define a new log2 function
def glog2(x):
    return gk.log(x)/np.log(2) 

gk.Equation(y==glog2(x))
gk.options.IMODE=2; gk.solve(disp=False)

import matplotlib.pyplot as plt
plt.semilogx(xnp,ynp,'b-',lw=4,label=r'$y=\log_2(x)$ Numpy')
plt.semilogx(x.value,y.value,'r--',lw=2,label=r'$y=\log_2(x)$ Gekko')
plt.legend(); plt.xlabel('x'); plt.ylabel('y'); plt.grid(); plt.show()