tnt - 1 year ago 67
Python Question

# Is a "for" loop necessary if elements of the a numpy vector are dependant upon the previous element?

``````T(i) = Tm(i) + (T(i-1)-Tm(i))**(-tau(i))
``````

Tm and tau are numpy vectors of the same length that have been previously calculated and the desire is to create a new vector T. The "i" is included only to indicate the element indexs for what is desired.

Is a for loop necessary for this case?

You might think this would work:

``````import numpy as np
n = len(Tm)
t = np.empty(n)

t[0] = 0  # or whatever the initial condition is
t[1:] = Tm[1:] + (t[0:n-1] - Tm[1:])**(-tau[1:])
``````

but it doesn't: you can't actually do recursion in numpy this way (since numpy calculates the whole RHS and then assigns it to the LHS).

So unless you can come up with a non-recursive version of this formula, you're stuck with an explicit loop:

``````tt = np.empty(n)
tt[0] = 0.
for i in range(1,n):
tt[i] = Tm[i] + (tt[i-1] - Tm[i])**(-tau[i])
``````
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