Mathews24 - 8 months ago 48

Python Question

I am running the code below to build a one-dimensional array for each z and t. At the present moment, I am trying to make their sizes equivalent so that they each have a length of 501.

`import numpy as np`

#constants & parameters

omega = 1.

eps = 1.

c = 3.*(10.**8.)

hbar = 1.

eta = 0.01

nn = 10.**7.

n = eta*nn

lambdaOH = c/(1612.*10.**(6.))

gamma = 1.282*(10.**(-11.))

Tsp = 1./gamma

TR = 604800.

L = (Tsp/TR)*(np.pi)/((3.*(lambdaOH**2.))*n)

#time

Ngridt = 500.

tmax = 1.

dt = tmax/Ngridt

intervalt = tmax/dt + 1

t = np.linspace(0.01,tmax,intervalt)

#z space

Ngridz = 500.

zmax = L

dz = zmax/Ngridz

intervalz = zmax/dz + 1

z = np.linspace(0.01,zmax,intervalz)

When running the code, both intervalt and intervalz equal 501.0, but when checking the length of both z and t, len(z) = 500 while len(t) = 501. I have played around with the code above to yield len(z) = 501 by modifying certain parts. For example, if I insert the code

`zmax = int(zmax)`

then len(z) = 501. But I am wondering why the initial code, exactly as written, does not yield an array z with length 501?

(I am using Python 2.7.)

Answer

It is a problem of rounding. If you try to subtract 501 from intervalz you will find a very small negative number, -5.68e-14; linspace just takes the integer part of it, that is 500, and provides a 500-long list.

Notice two other problems with your code:

`dt`

does not provide the correct spacing because you don't remove the initial`t`

(same for`dz`

)`Ngridt`

and`Ngridz`

are conceptually integers, while you initialize them as floats. Just remove the dot at the end.

I think that your code could be simplified by writing (notice that `Ngridt`

and `Ngridz`

are initialized to 501)

```
#time
Ngridt = 501
tmax = 1.
t, dt = np.linspace(0.01,tmax,Ngridt,retstep=True)
#z space
Ngridz = 501
zmax = L
z, dz = np.linspace(0.01,zmax,Ngridz,retstep=True)
```