I made a simple Fibonacci sequence calculator for the first 22 terms:
i=1
n=0
while i<=20000:
i = i + n
n = i - n
print(i)
Looks like the result is correct
1
2
3
5
8
13
21
34
55
89
144
233
377
610
987
1597
2584
4181
6765
10946
17711
28657
but I can't seem to find similar code anywhere online. I think that's a big red flag. Can someone tell me what is wrong here? Is this inefficient code?
No, that code is fine. The probable reason you can't find similar code online is that it's unusual to use the subtraction operator in Fibonacci, which is a purely additive function, tn = tn-2 + tn-1
.
It works, of course, since addition/subtraction is both commutative and associative, meaning that order and grouping of terms is unimportant:
i = i + n # iNew = iOld + nOld
n = i - n # nNew = (iNew) - nOld
# = (iOld + nOld) - nOld
# = iOld + (nOld - nOld)
# = iOld + (0)
# = iOld
Use of subtraction allows you to bypass needing a third variable, which would be something like this in a lesser language than Python:
nextN = i + n
i = n
n = nextN
In Python, you don't actually need that since you can use tuple assignment such as:
(n, i) = (i, n + i)
With that, everything on the right of the =
is evaluated before any assignments to the left.
It's an unusual way to do it, but it's correct. Your lines:
i = i + n
n = i - n
are the same as doing:
new_i = i + n
n = i
i = new_i
or,
i, n = i + n, i
which would be the usual way in Python.
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