Square-root Square-root - 1 year ago 182
Java Question

Inverse of a permutation?

would someone please explain this example and its answer. I understand if an array has {3,2,6} then its inverse would be {6,2,3}. Is this what being asked because the piece of code below doesn't seem to achieve this?

Suppose that the array int pi[128] contains a permutation of the numbers 0, 1, ...., 127.

What one or two lines of Java code would you use to efficiently produce the inverse of
pi in the array ip[128]?


for (i=0; i<128; ++i)
ip[pi[i]] = i

Answer Source

Don't know where you saw that answer, because it is wrong.

To illustrate, let's use array of size 4.
With pi = {3,0,1,2}, result is ip = {1,2,3,0}.
Or pi = {0,1,2,3} becomes ip = {0,1,2,3}.
Don't know in what definition of the word "inverse", those would be the right results.

Correct answer is:

for (int i = 0; i < 128; ++i)
    ip[i] = pi[127 - i];
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