Permutations by swapping: Difference between revisions

When saved in a file called spermutations.py it is used in the Python example to the [[Matrix arithmetic#Python|Matrix arithmetic]] task.

 

 

def spermutations(n):

sign = 1

for i in range(n)]

 

yield tuple(pp[0] for pp in p), sign

while any(pp[1] for pp in p): # moving

sign *= -1

if d1 == -1:

# Swap down

p[i1], p[i2] = p[i2], p[i1]

# If this causes the chosen element to reach the First or last

# position within the permutation, or if the next element in the

# same direction is larger than the chosen element:

# The direction of the chosen element is set to zero

p[i2][1] = 0

elif d1 == 1:

# Swap up

p[i1], p[i2] = p[i2], p[i1]

# If this causes the chosen element to reach the first or Last

# position within the permutation, or if the next element in the

# same direction is larger than the chosen element:

# The direction of the chosen element is set to zero

p[i2][1] = 0

yield tuple(pp[0] for pp in p), sign

 

if n3 > n1:

pp[1] = 1 if i3 < i2 else -1

 

if __name__ == '__main__':

sp.add(i[0])

print('Perm: %r Sign: %2i' % i)

# Test

p = set(permutations(range(n)))

assert sp == p, 'Two methods of generating permutations do not agree'</lang>

<pre>Permutations and sign of 3 items

Perm: (0, 1, 2) Sign: 1