I wrote a python program that has as input a matrix, in which, each element appears in each row and column once. Elements are only positive integers.
e.g.
0,2,3,1
3,1,0,2
1,3,2,0
2,0,1,3
Then i find all possible traversals. They are defined as such:
choose an element from the first column
move on to the next column and
choose the element that is not in the same line from previous elements in traversal and the element has not the same value with previous elements in traversal.
e.g.
0,*,*,*
*,*,*,2
*,3,*,*
*,*,1,*
I have constructed the code that finds the traversals for matrices 4x4, but i have trouble generalizing it for NxN matrices. My code follows below. Not looking for a solution, any tip would be helpful.
import sys # Import to input arguments from cmd.
import pprint # Import for a cool print of the graph
import itertools # Import to find all crossings' combinations
# Input of arguments
input_filename = sys.argv[1]
# Create an empty graph
g = {}
# Initialize variable for the list count
i = 0
# Opens the file to make the transfer into a matrix
with open(input_filename) as graph_input:
for line in graph_input:
# Split line into four elements.
g[i] = [int(x) for x in line.split(',')]
i += 1
# Initialize variable
var = 0
# Lists for the crossings, plus rows and cols of to use for archiving purposes
f = {}
r = {}
c = {}
l = 0
# Finds the all the crossings
if len(g) == 4:
for x in range (len(g)):
for y in range (len(g)):
# When we are in the first column
if y == 0:
# Creates the maximum number of lists that don't include the first line
max_num = len(g) -1
for z in range (max_num):
f[l] = [g[x][y]]
r[l] = [x]
c[l] = [y]
l += 1
# When on other columns
if y != 0:
for z in range(len(g)):
# Initializes a crossing archive
used = [-1]
for item in f:
# Checks if the element should go in that crossing
if f[item][0] == g[x][0]:
if g[z][y] not in f[item] and z not in r[item] and y not in c[item] and g[z][y] not in used:
# Appends the element and the archive
f[item].append(g[z][y])
used.append(g[z][y])
r[item].append(z)
c[item].append(y)
# Removes unused lists
for x in range (len(f)):
if len(f[x]) != len(g):
f.pop(x)
#Transfers the value from a dictionary to a list
f_final = f.values()
# Finds all the combinations from the existing crossings
list_comb = list(itertools.combinations(f_final, i))
# Initialize variables
x = 0
w = len(list_comb)
v = len(list_comb[0][0])
# Excludes from the combinations all invalid crossings
while x < w:
# Initialize y
y = 1
while y < v:
# Initialize z
z = 0
while z < v:
# Check if the crossings have the same element in the same position
if list_comb[x][y][z] == list_comb[x][y-1][z]:
# Removes the combination from the pool
list_comb.pop(x)
# Adjust loop variables
x -= 1
w -= 1
y = v
z = v
z += 1
y += 1
x += 1
# Inputs the first valid solution as the one to create the orthogonal latin square
final_list = list_comb[0]
# Initializes the orthogonal latin square matrix
orthogonal = [[v for x in range(v)] for y in range(v)]
# Parses through the latin square and the chosen solution
# and creates the orthogonal latin square
for x in range (v):
for y in range (v):
for z in range (v):
if final_list[x][y] == g[z][y]:
orthogonal[z][y] = int(final_list[x][0])
break
# Initializes the orthogonal latin square matrix
gr_la = [[v for x in range(v)] for y in range(v)]
# Creates the greek-latin square
for x in range (v):
for y in range (v):
coords = tuple([g[x][y],orthogonal[x][y]])
gr_la[x][y] = coords
pprint.pprint(gr_la)
Valid traversals for the 4x4 matrix above are:
[[0123],[0312],[3210],[3021],[1203],[1032],[2130],[2301]]
Related
I would like to solve the above formulation in Scipy and solve it using milp(). For a given graph (V, E), f_ij and x_ij are the decision variables. f_ij is the flow from i to j (it can be continuous). x_ij is the number of vehicles from i to j. p is the price. X is the available number vehicles in a region. c is the capacity.
I have difficulty in translating the formulation to Scipy milp code. I would appreciate it if anyone could give me some pointers.
What I have done:
The code for equation (1):
f_obj = [p[i] for i in Edge]
x_obj = [0]*len(Edge)
obj = f_obj + v_obj
Integrality:
f_cont = [0 for i in Edge] # continous
x_int = [1]*len(Edge) # integer
integrality = f_cont + x_int
Equation (2):
def constraints(self):
b = []
A = []
const = [0]*len(Edge) # for f_ij
for i in v: # for x_ij
for e in Edge:
if e[0] == i:
const.append(1)
else:
const.append(0)
A.append(const)
b.append(self.accInit[i])
const = [0]*len(Edge) # for f_ij
return A, b
Equation (4):
[(0, demand[e]) for e in Edge]
I'm going to do some wild guessing, given how much you've left open to interpretation. Let's assume that
this is a maximisation problem, since the minimisation problem is trivial
Expression (1) is actually the maximisation objective function, though you failed to write it as such
p and d are floating-point vectors
X is an integer vector
c is a floating-point scalar
the graph edges, since you haven't described them at all, do not matter for problem setup
The variable names are not well-chosen and hide what they actually contain. I demonstrate potential replacements.
import numpy as np
from numpy.random._generator import Generator
from scipy.optimize import milp, Bounds, LinearConstraint
import scipy.sparse
from numpy.random import default_rng
rand: Generator = default_rng(seed=0)
N = 20
price = rand.uniform(low=0, high=10, size=N) # p
demand = rand.uniform(low=0, high=10, size=N) # d
availability = rand.integers(low=0, high=10, size=N) # X aka. accInit
capacity = rand.uniform(low=0, high=10) # c
c = np.zeros(2*N) # f and x
c[:N] = -price # (1) f maximized with coefficients of 'p'
# x not optimized
CONTINUOUS = 0
INTEGER = 1
integrality = np.empty_like(c, dtype=int)
integrality[:N] = CONTINUOUS # f
integrality[N:] = INTEGER # x
upper = np.empty_like(c)
upper[:N] = demand # (4) f
upper[N:] = availability # (2) x
eye_N = scipy.sparse.eye(N)
A = scipy.sparse.hstack((-eye_N, capacity*eye_N)) # (3) 0 <= -f + cx
result = milp(
c=c, integrality=integrality,
bounds=Bounds(lb=np.zeros_like(c), ub=upper),
constraints=LinearConstraint(lb=np.zeros(N), A=A),
)
print(result.message)
flow = result.x[:N]
vehicles = result.x[N:].astype(int)
I have a code that works perfectly well but I wish to speed up the time it takes to converge. A snippet of the code is shown below:
def myfunction(x, i):
y = x + (min(0, target[i] - data[i, :]x))*data[i]/(norm(data[i])**2))
return y
rows, columns = data.shape
start = time.time()
iterate = 0
iterate_count = []
norm_count = []
res = 5
x_not = np.ones(columns)
norm_count.append(norm(x_not))
iterate_count.append(0)
while res > 1e-8:
for row in range(rows):
y = myfunction(x_not, row)
x_not = y
iterate += 1
iterate_count.append(iterate)
norm_count.append(norm(x_not))
res = abs(norm_count[-1] - norm_count[-2])
print('Converge at {} iterations'.format(iterate))
print('Duration: {:.4f} seconds'.format(time.time() - start))
I am relatively new in Python. I will appreciate any hint/assistance.
Ax=b is the problem we wish to solve. Here, 'A' is the 'data' and 'b' is the 'target'
Ugh! After spending a while on this I don't think it can be done the way you've set up your problem. In each iteration over the row, you modify x_not and then pass the updated result to get the solution for the next row. This kind of setup can't be vectorized easily. You can learn the thought process of vectorization from the failed attempt, so I'm including it in the answer. I'm also including a different iterative method to solve linear systems of equations. I've included a vectorized version -- where the solution is updated using matrix multiplication and vector addition, and a loopy version -- where the solution is updated using a for loop to demonstrate what you can expect to gain.
1. The failed attempt
Let's take a look at what you're doing here.
def myfunction(x, i):
y = x + (min(0, target[i] - data[i, :] # x)) * (data[i] / (norm(data[i])**2))
return y
You subtract
the dot product of (the ith row of data and x_not)
from the ith row of target,
limited at zero.
You multiply this result with the ith row of data divided my the norm of that row squared. Let's call this part2
Then you add this to the ith element of x_not
Now let's look at the shapes of the matrices.
data is (M, N).
target is (M, ).
x_not is (N, )
Instead of doing these operations rowwise, you can operate on the entire matrix!
1.1. Simplifying the dot product.
Instead of doing data[i, :] # x, you can do data # x_not and this gives an array with the ith element giving the dot product of the ith row with x_not. So now we have data # x_not with shape (M, )
Then, you can subtract this from the entire target array, so target - (data # x_not) has shape (M, ).
So far, we have
part1 = target - (data # x_not)
Next, if anything is greater than zero, set it to zero.
part1[part1 > 0] = 0
1.2. Finding rowwise norms.
Finally, you want to multiply this by the row of data, and divide by the square of the L2-norm of that row. To get the norm of each row of a matrix, you do
rownorms = np.linalg.norm(data, axis=1)
This is a (M, ) array, so we need to convert it to a (M, 1) array so we can divide each row. rownorms[:, None] does this. Then divide data by this.
part2 = data / (rownorms[:, None]**2)
1.3. Add to x_not
Finally, we're adding each row of part1 * part2 to the original x_not and returning the result
result = x_not + (part1 * part2).sum(axis=0)
Here's where we get stuck. In your approach, each call to myfunction() gives a value of part1 that depends on target[i], which was changed in the last call to myfunction().
2. Why vectorize?
Using numpy's inbuilt methods instead of looping allows it to offload the calculation to its C backend, so it runs faster. If your numpy is linked to a BLAS backend, you can extract even more speed by using your processor's SIMD registers
The conjugate gradient method is a simple iterative method to solve certain systems of equations. There are other more complex algorithms that can solve general systems well, but this should do for the purposes of our demo. Again, the purpose is not to have an iterative algorithm that will perfectly solve any linear system of equations, but to show what kind of speedup you can expect if you vectorize your code.
Given your system
data # x_not = target
Let's define some variables:
A = data.T # data
b = data.T # target
And we'll solve the system A # x = b
x = np.zeros((columns,)) # Initial guess. Can be anything
resid = b - A # x
p = resid
while (np.abs(resid) > tolerance).any():
Ap = A # p
alpha = (resid.T # resid) / (p.T # Ap)
x = x + alpha * p
resid_new = resid - alpha * Ap
beta = (resid_new.T # resid_new) / (resid.T # resid)
p = resid_new + beta * p
resid = resid_new + 0
To contrast the fully vectorized approach with one that uses iterations to update the rows of x and resid_new, let's define another implementation of the CG solver that does this.
def solve_loopy(data, target, itermax = 100, tolerance = 1e-8):
A = data.T # data
b = data.T # target
rows, columns = data.shape
x = np.zeros((columns,)) # Initial guess. Can be anything
resid = b - A # x
resid_new = b - A # x
p = resid
niter = 0
while (np.abs(resid) > tolerance).any() and niter < itermax:
Ap = A # p
alpha = (resid.T # resid) / (p.T # Ap)
for i in range(len(x)):
x[i] = x[i] + alpha * p[i]
resid_new[i] = resid[i] - alpha * Ap[i]
# resid_new = resid - alpha * A # p
beta = (resid_new.T # resid_new) / (resid.T # resid)
p = resid_new + beta * p
resid = resid_new + 0
niter += 1
return x
And our original vector method:
def solve_vect(data, target, itermax = 100, tolerance = 1e-8):
A = data.T # data
b = data.T # target
rows, columns = data.shape
x = np.zeros((columns,)) # Initial guess. Can be anything
resid = b - A # x
resid_new = b - A # x
p = resid
niter = 0
while (np.abs(resid) > tolerance).any() and niter < itermax:
Ap = A # p
alpha = (resid.T # resid) / (p.T # Ap)
x = x + alpha * p
resid_new = resid - alpha * Ap
beta = (resid_new.T # resid_new) / (resid.T # resid)
p = resid_new + beta * p
resid = resid_new + 0
niter += 1
return x
Let's solve a simple system to see if this works first:
2x1 + x2 = -5
−x1 + x2 = -2
should give a solution of [-1, -3]
data = np.array([[ 2, 1],
[-1, 1]])
target = np.array([-5, -2])
print(solve_loopy(data, target))
print(solve_vect(data, target))
Both give the correct solution [-1, -3], yay! Now on to bigger things:
data = np.random.random((100, 100))
target = np.random.random((100, ))
Let's ensure the solution is still correct:
sol1 = solve_loopy(data, target)
np.allclose(data # sol1, target)
# Output: False
sol2 = solve_vect(data, target)
np.allclose(data # sol2, target)
# Output: False
Hmm, looks like the CG method doesn't work for badly conditioned random matrices we created. Well, at least both give the same result.
np.allclose(sol1, sol2)
# Output: True
But let's not get discouraged! We don't really care if it works perfectly, the point of this is to demonstrate how amazing vectorization is. So let's time this:
import timeit
timeit.timeit('solve_loopy(data, target)', number=10, setup='from __main__ import solve_loopy, data, target')
# Output: 0.25586539999994784
timeit.timeit('solve_vect(data, target)', number=10, setup='from __main__ import solve_vect, data, target')
# Output: 0.12008900000000722
Nice! A ~2x speedup simply by avoiding a loop while updating our solution!
For larger systems, this will be even better.
for N in [10, 50, 100, 500, 1000]:
data = np.random.random((N, N))
target = np.random.random((N, ))
t_loopy = timeit.timeit('solve_loopy(data, target)', number=10, setup='from __main__ import solve_loopy, data, target')
t_vect = timeit.timeit('solve_vect(data, target)', number=10, setup='from __main__ import solve_vect, data, target')
print(N, t_loopy, t_vect, t_loopy/t_vect)
This gives us:
N t_loopy t_vect speedup
00010 0.002823 0.002099 1.345390
00050 0.051209 0.014486 3.535048
00100 0.260348 0.114601 2.271773
00500 0.980453 0.240151 4.082644
01000 1.769959 0.508197 3.482822
I built a grid that generates random obstacles for pathfinding algorithm, but with fixed starting and ending points as shown in my snippet below:
import random
import numpy as np
#grid format
# 0 = navigable space
# 1 = occupied space
x = [[random.uniform(0,1) for i in range(50)]for j in range(50)]
grid = np.array([[0 for i in range(len(x[0]))]for j in range(len(x))])
for i in range(len(x)):
for j in range(len(x[0])):
if x[i][j] <= 0.7:
grid[i][j] = 0
else:
grid[i][j] = 1
init = [5,5] #Start location
goal = [45,45] #Our goal
# clear starting and end point of potential obstacles
def clear_grid(grid, x, y):
if x != 0 and y != 0:
grid[x-1:x+2,y-1:y+2]=0
elif x == 0 and y != 0:
grid[x:x+2,y-1:y+2]=0
elif x != 0 and y == 0:
grid[x-1:x+2,y:y+2]=0
elif x ==0 and y == 0:
grid[x:x+2,y:y+2]=0
clear_grid(grid, init[0], init[1])
clear_grid(grid, goal[0], goal[1])
I need to generate also the starting and ending points randomly every time I run the code instead of making them fixed. How could I make it? Any assistance, please?.
Replace,
init = [5,5] #Start location
goal = [45,45] #Our goal
with,
init = np.random.randint(0, high = 49, size = 2)
goal = np.random.randint(0, high = 49, size = 2)
Assuming your grid goes from 0-49 on each axis. Personally I would add grid size variables, i_length & j_length
EDIT #1
i_length = 50
j_length = 50
x = [[random.uniform(0,1) for i in range(i_length)]for j in range(j_length)]
grid = np.array([[0 for i in range(i_length)]for j in range(j_length)])
The code is being executed but the output is not shown nor the variables are created
import numpy as np
def magicsquares():
n=input('enter the order of squares')
n=int(n)
m=np.zeros((n,n))
s=n*(n**2+1)/2 #sum of each row or diagonal
p=int(n/2)
q=(n-1)
for i in range(n**2):
m[p][q]=1 #assigning postion of 1
P=p-1
Q=q+1
if(i>=2): #assigning remaining positions
if(P==-1):
P=n-1
if(Q==n):
Q=0
there is not output showing because you are just declaring the function but not calling the function and there is no print/return inside the function. Here is a solution which you can use to see the output and work on:
import numpy as np
def magicsquares():
n = input('enter the order of squares')
n = int(n)
m = np.zeros((n, n))
s = n*(n**2+1)/2 # sum of each row or diagonal
p = int(n/2)
q = (n-1)
for i in range(n**2):
m[p][q] = 1 # assigning postion of 1
P = p-1
Q = q+1
if i >= 2: # assigning remaining positions
if P == -1:
P = n-1
if Q == n:
Q = 0
print(m)
magicsquares()
It is not the ultimate solution to find magic_square. It's just an updated version of your code so that you can see the outputs and work on.
Instructions: Compute and store R=1000 random values from 0-1 as x. moving_window_average(x, n_neighbors) is pre-loaded into memory from 3a. Compute the moving window average for x for the range of n_neighbors 1-9. Store x as well as each of these averages as consecutive lists in a list called Y.
My solution:
R = 1000
n_neighbors = 9
x = [random.uniform(0,1) for i in range(R)]
Y = [moving_window_average(x, n_neighbors) for n_neighbors in range(1,n_neighbors)]
where moving_window_average(x, n_neighbors) is a function as follows:
def moving_window_average(x, n_neighbors=1):
n = len(x)
width = n_neighbors*2 + 1
x = [x[0]]*n_neighbors + x + [x[-1]]*n_neighbors
# To complete the function,
# return a list of the mean of values from i to i+width for all values i from 0 to n-1.
mean_values=[]
for i in range(1,n+1):
mean_values.append((x[i-1] + x[i] + x[i+1])/width)
return (mean_values)
This gives me an error, Check your usage of Y again. Even though I've tested for a few values, I did not get yet why there is a problem with this exercise. Did I just misunderstand something?
The instruction tells you to compute moving averages for all neighbors ranging from 1 to 9. So the below code should work:
import random
random.seed(1)
R = 1000
x = []
for i in range(R):
num = random.uniform(0,1)
x.append(num)
Y = []
Y.append(x)
for i in range(1,10):
mov_avg = moving_window_average(x, n_neighbors=i)
Y.append(mov_avg)
Actually your moving_window_average(list, n_neighbors) function is not going to work with a n_neighbors bigger than one, I mean, the interpreter won't say a thing, but you're not delivering correctness on what you have been asked.
I suggest you to use something like:
def moving_window_average(x, n_neighbors=1):
n = len(x)
width = n_neighbors*2 + 1
x = [x[0]]*n_neighbors + x + [x[-1]]*n_neighbors
mean_values = []
for i in range(n):
temp = x[i: i+width]
sum_= 0
for elm in temp:
sum_+= elm
mean_values.append(sum_ / width)
return mean_values
My solution for +100XP
import random
random.seed(1)
R=1000
Y = list()
x = [random.uniform(0, 1) for num in range(R)]
for n_neighbors in range(10):
Y.append(moving_window_average(x, n_neighbors))