Changing output statement in python

Question

I have created the following code. Everything is correct besides my output statement. On the end of the x: array there is a 79 which stands for the number of iterations. I am trying to make a statement which uses

print("The number of iterations is", )

I tried to put iter_ct in there but it gave me an error. Looking for some help with making this adjustment, thanks! import numpy as np from pprint import pprint from numpy import array, zeros, diag, diagflat, dot

def jacobi(A,b,N=100,x=None):
    """Solves the equation Ax=b via the Jacobi iterative method."""
    # Create an initial guess if needed                                                                       
    if x is None:
        x = zeros(len(A[0]))

    # Create a vector of the diagonal elements of A                                                                                                                                                
    # and subtract them from A                                                                                                                                                                     
    D = diag(A)
    R = A - diagflat(D)

    x_old = x
    error = 1.0     # Dummy value
    iter_ct = 0
    while error > 10 ** (-15):
        x = (b - dot(R, x_old)) / D
        error = np.linalg.norm(x - x_old)
        iter_ct += 1
        x_old = x
    return x, iter_ct

A = np.array([[3.0, 1.0, 0., 0., 0., 0., 0., 0., 0., 0.],[1.0, 3.0, 1.0, 0., 0., 0., 0., 0., 0., 0.], [0., 1.0, 3.0, 1.0, 0., 0., 0., 0., 0., 0.], [0., 0, 1.0, 3.0, 1.0, 0., 0., 0., 0., 0.], [0., 0., 0., 1.0, 3.0, 1.0, 0., 0., 0., 0.], [0., 0., 0., 0., 1.0, 3.0, 1.0, 0., 0., 0.], [0., 0., 0., 0., 0., 1.0, 3.0, 1.0, 0., 0.], [0., 0., 0., 0., 0., 0., 1.0, 3.0, 1.0, 0.], [0., 0., 0., 0., 0., 0., 0., 1.0, 3.0, 1.0], [0., 0., 0., 0., 0., 0., 0., 0., 1.0, 3.0]])
b = np.array([1.0,1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0])
guess = np.array([0., 0., 0., 0., 0., 0., 0., 0., 0., 0.])

sol, iter = jacobi(A,b,N=100,x=guess)

print ("A:")
pprint(A)

print ("b:")
pprint(b)

print ("x:")
pprint(sol)

print("It took",sol[1], "iterations.")

This is the current output I am getting now. Notice the 79 on the end of the x: array

A:
array([[3., 1., 0., 0., 0., 0., 0., 0., 0., 0.],
   [1., 3., 1., 0., 0., 0., 0., 0., 0., 0.],
   [0., 1., 3., 1., 0., 0., 0., 0., 0., 0.],
   [0., 0., 1., 3., 1., 0., 0., 0., 0., 0.],
   [0., 0., 0., 1., 3., 1., 0., 0., 0., 0.],
   [0., 0., 0., 0., 1., 3., 1., 0., 0., 0.],
   [0., 0., 0., 0., 0., 1., 3., 1., 0., 0.],
   [0., 0., 0., 0., 0., 0., 1., 3., 1., 0.],
   [0., 0., 0., 0., 0., 0., 0., 1., 3., 1.],
   [0., 0., 0., 0., 0., 0., 0., 0., 1., 3.]])
b:
array([1., 1., 1., 1., 1., 1., 1., 1., 1., 1.])
x:
(array([0.27638191, 0.17085427, 0.21105528, 0.1959799 , 0.20100503,
   0.20100503, 0.1959799 , 0.21105528, 0.17085427, 0.27638191]),
 79)

Loocid · Accepted Answer · 2018-10-23 00:53:27Z

1

Your jacobi function is returning a tuple (x, iter_ct) so if you want to just print iter_ct you could do it like so: pprint(sol[1])

Alternatively you could unpack the tuple when you return it:

sol, iter = jacobi(A,b,N=100,x=guess)
pprint(iter)

answered Oct 23, 2018 at 0:53

Loocid

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1 Comment

fr14 Over a year ago

thank you that works perfectly! one more question, how could I take that 79 out of the output statement for the x: array?

troymyname00 · Accepted Answer · 2018-10-23 00:55:45Z

1

print("The number of iterations is {}".format(x[-1]))

Use the statement above after you've executed the function.

answered Oct 23, 2018 at 0:55

troymyname00

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2 Comments

fr14 Over a year ago

would you know how to take the 79 out of the output from the x array?

troymyname00 Over a year ago

I am assuming this is how it should be: x[-1]

Kingsley · Accepted Answer · 2018-10-23 02:46:19Z

1

You are returning two values from your jacobi function - return x, iter_ct. This is assigned into sol.

perhaps you could try:

sol, iter = jacobi(A,b,N=100,x=guess)
pprint(sol)
#pprint(iter)  - don't print the '79'

edited Oct 23, 2018 at 2:46

answered Oct 23, 2018 at 0:53

Kingsley

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9 Comments

fr14 Over a year ago

would you know how to take the 79 out of the output from the x array?

Kingsley Over a year ago

With the return like that (sol, iter = ...) the 97 is no longer in the array sol. You could always do a del(sol[1]).

fr14 Over a year ago

where would I put the del(x[1])?

Kingsley Over a year ago

Sorry I got confused by x. Add del(sol[1]) before you pprint(sol) - which you call x.

fr14 Over a year ago

'tuple' object doesn't support item deletion

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