CODEWITHSUNDEEP
python
Vincent_Cos
Vincent_Cos

Reputation: 33

Print a large integer in Python

I'd like to print an large integer without the "e+XX" at the end in Python.

For example, when I write:

n = 100
k = 18
result = 1
i = 0

while i < k:
    result = result * (n - i) / (i + 1)
    i += 1

The result is 3.066451080298821e+19, and I would like to have 30664510802988208300.

Upvotes: 2

Views: 4941

Answers (2)

wordsforthewise
wordsforthewise

Reputation: 15777

I think the question you really want to ask is 'how can I print a number in Python without scientific notation?'

The answer is, your number right now is a float. Try print(type(result)) and you will see it says float. You could type cast it to an integer by doing int(result), and it will show close to the full number, 30664510802988208128. It will be a bit off because of the memory size storage limitations of int vs float.

The better way to do this would be like:

result = 1
i = 0

while i < 18:
    result = result * (100 - i) // (i + 1)
    i += 1

print(result)

which will keep result as an int type. It now should print 30664510802988208300

Upvotes: 2

Dietrich Epp
Dietrich Epp

Reputation: 213298

If you want an integer, you have to use integer division, // instead of /, as mentioned in @farsil's deleted answer.

result = 1
k = 18
n = 100

for i in range(k):
    result = result * (n - i) // (i + 1)

print(result)

This only gives the correct result if i + 1 is always a divisor of result * (n - i). However, this is always true, so we are fine.

You cannot use / because that will perform floating-point division, which will truncate the results to 56 bits. The correct result does not fit in 56 bits:

In [1]: int(float(30664510802988208300))
Out[1]: 30664510802988208128
#                        ^^^ oops... off by 172

Why is floor division safe?

In this case, when the division by 2 is performed, we have multiplied result by n and n-1, at least one of which is a multiple of 2. When i+1 is 3, then we have multiplied by n, n-1, and n-2, at least one of which is a multiple of 3. This pattern works for all numbers.

Upvotes: 3

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