CODEWITHSUNDEEP
pythonlist
aam
aam

Reputation: 25

Trying to add previous element in list to next element, but keep getting zero

I have a formula that uses the previous element to find the current element:

p(i) = 1 / (- 2 + p(i-1))

The code I have to find this is:

p = []
po = 0
i = .01

while i < 1:
    po = (-1) / (-2 + po)
    p.append(po)
    i = i + .01

print p

What I'm getting is :

[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

My first value (if po is originally zero) should be 1/2.

Any ideas why I'm only getting zeros?

Upvotes: 0

Views: 94

Answers (2)

Ralf
Ralf

Reputation: 16515

As user @blhsing pointed out in his answer, the problem seems to be interger division: when dividing integers, the result is rounded down to yield a new integer.

By converting one of the arguments of the division into a float, the division will also return a float. It does not matter which of the 3 numbers you change into a float, changing one of them is sufficient to produce the correct result:

  • change the initialization from po = 0 to po = 0.0
  • or change the divisiion to po = (-1.0) / (-2 + po)
  • or change the divisiion to po = (-1) / (-2.0 + po)

Furthermore, I find your use of the i variable a bit strange. May I suggest a forloop using xrange()?

p = []
po = 0

# the '_' is used to say "I dont care about this value", so this
# runs the loop the specified amout of times
for _ in xrange(10):
    po = (-1.0) / (-2 + po)
    p.append(po)

print p

Upvotes: 1

blhsing
blhsing

Reputation: 107085

In Python 2, a division involving only integers would always result in an integer, rounded down. You should use a floating number for division instead:

po = -1.0 / (-2 + po)

Upvotes: 3

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