Is there a inner function that can calculate x=f-1 (y) when I know y=f(x)

Asked Apr 17 '20 at 20:47

Active Apr 17 '20 at 21:19

Viewed 57 times

for example, I define a function like that:

def trapezoid(x, a, b, c, d):
    if x <= a:
        return 0
    if a < x <= b:
        return (x - a)/(b - a)
    if b < x <= c:
        return 1
    if c < x <= d:
        return (d - x)/(d - c)
    if d < x:
    return 0

Now I can calculate the value of y according to input x, but if I know the value of y,how can I calculate x directly? Is there some way in numpy or other library?

for example:

print(trapezoid(12, 10, 30, 50, 90))

0.1

but if I know the result= 0.1, how to calculate the input x(12)? I know there may be a large number of x can get same y, but if I want to know the field of x?

edited Apr 17 '20 at 21:19

asked Apr 17 '20 at 20:47

4daJKong

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21

Are `a`, `b`, `c`, `d` constants relative to `x` and `y`? – Green Cloak Guy Apr 17 '20 at 20:49
The problem is that your function is not bijective: y=0, for example, has no unique solution. As for the linear parts, consider this answer to a similar question: https://stackoverflow.com/a/48126944/1622937 – j-i-l Apr 17 '20 at 20:51
If you can frame the function as an equation, then you might be able to use [`pynverse`](https://pypi.org/project/pynverse/) – Green Cloak Guy Apr 17 '20 at 20:54
@Green Cloak Guy a,b,c,d just some constant like 1,2,3,4 as parameter – 4daJKong Apr 17 '20 at 21:12
What is `y`? It isn't mentioned in the code – Simon Crane Apr 17 '20 at 21:13
@jojo Actually, I want to calculate all possible value of x when I know the value of y – 4daJKong Apr 17 '20 at 21:14
@SimonCrane the return result is y – 4daJKong Apr 17 '20 at 21:14
With just an arbitrary black-box function `f`, this is impossible. The more you know about `f`, the more possible it is. – user2357112 Apr 17 '20 at 21:22
@user2357112supportsMonica thanks again, if I only want to know the f in example, is there some good method? – 4daJKong Apr 17 '20 at 21:25
From y to x would depend on a,b,c,d. You need to specify or consider a,b,c,d. – J. Linne Apr 17 '20 at 21:50

Is there a inner function that can calculate x=f-1 (y) when I know y=f(x)

0 Answers0