9

How do you sort, operate on, and then unsort the result?

Assume I have a float array p1 = 0.15,0.3, 0.25, 0.12, .... It is sorted to: p2 = sort(p1). A function (operation with p2 as input) results in p3: p3 = f(p2, x, y, ...) for some function f.

How can I unsort p3 in the smartest way? (reverse of how p1 was sorted)

i.e: p4 = unsort(p3) <- p4 unsorted to same order as p1, for comparison (x-plot) with p1?

Praveen
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Trond.H.H
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3 Answers3

9

You need a double argsort here to keep the order:

In [6]: a
Out[6]: array([5, 4, 8, 3, 6, 1, 2, 4, 9, 6])

In [7]: b=sort(a)

In [8]: b
Out[8]: array([1, 2, 3, 4, 4, 5, 6, 6, 8, 9])

In [9]: ii=a.argsort().argsort()

In [10]: c=b*b

In [11]: c
Out[11]: array([ 1,  4,  9, 16, 16, 25, 36, 36, 64, 81])

In [12]: c[ii]
Out[12]: array([25, 16, 64,  9, 36,  1,  4, 16, 81, 36])
B. M.
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1

One way is to use numpy.argsort to find the indices that will sort your initial array. The same indices can be used to un-sort your array into its result, as follows:

a = np.array([5, 2, 4, 3, 1])
i = np.argsort(a)    # i = array([4, 1, 3, 2, 0])

# b will be the sorted version of a    
b = a[i]             # b = array([1, 2, 3, 4, 5])
# c is the function on b
c = b**2             # c = array([ 1,  4,  9, 16, 25])

# d will hold the un-sorted result
d = np.empty(a.shape)
d[i] = c             # d = array([ 25.,   4.,  16.,   9.,   1.])

But this will require that you pre-declare d before indexing.

Praveen
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1

To unsort a list in python using built in functions:

Program:

a=[589,273,981,642,702,883,319,128]
print("a",a)
b=[(p[1],p[0]) for p in enumerate(a)]
print("b",b)
c=sorted(b)
print("c",c)
d=[p[1] for p in c]
z=[p[0] for p in c]
print("d",d)
print("z",z)
y=zip(d,z)
print("y",y)
x=list(y)
print("x",x)
w=sorted(x)
print("w",w)
v=[p[1] for p in w]
print("v",v)
# unsort of z in one statement:
u=[r[1] for r in
    sorted(list(zip([q[1] for q in
        sorted([(p[1],p[0]) for p in
            enumerate(a)])],z)))]

Output:

a [589, 273, 981, 642, 702, 883, 319, 128]
b [(589, 0), (273, 1), (981, 2), (642, 3), (702, 4), (883, 5), (319, 6), (128, 7)]
c [(128, 7), (273, 1), (319, 6), (589, 0), (642, 3), (702, 4), (883, 5), (981, 2)]
d [7, 1, 6, 0, 3, 4, 5, 2]
z [128, 273, 319, 589, 642, 702, 883, 981]
y <zip object at 0x035E38A0>
x [(7, 128), (1, 273), (6, 319), (0, 589), (3, 642), (4, 702), (5, 883), (2, 981)]
w [(0, 589), (1, 273), (2, 981), (3, 642), (4, 702), (5, 883), (6, 319), (7, 128)]
v [589, 273, 981, 642, 702, 883, 319, 128]   
u [589, 273, 981, 642, 702, 883, 319, 128]
miracle173
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