Calculation using power function gave wrong answers for large numbers

Question

I have two values p=19, q=14. I want to calculate p^q using the power function pow(p, q).

Here is my code:

long long p=19,q=14;
cout<<pow(p,q);

The correct answer is: 799006685782884121 but my code gives me 799006685782884096 which is incorrect.

I have also tried doing these calculations using unsigned long long instead of long long, but this didn't help.

Possible duplicate of [pow() seems to be out by one here](https://stackoverflow.com/questions/21452711/pow-seems-to-be-out-by-one-here) — phuclv, Jan 24 '18 at 06:14

score 3 · Accepted Answer · answered Jan 23 '18 at 14:21

3

The pow function is defined as:

double pow(double x, double y);

Which means that it takes floating point arguments and returns a floating point result. Due to the nature of floating point numbers, some numbers can not be exactly represented. The result you're getting is probably the closest match possible.

Note also that you're doing two (probably lossy) conversions:

converting the arguments from long long to double, and
converting the result of the function from double to long long.

answered Jan 23 '18 at 14:21

jotik

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still getting the same result could u please show me – Rakesh Sharma Jan 23 '18 at 14:30
1

@RakeshSharma if you're using MSVC then `long double` is the same as `double` which can't store all 64-bit int precisely – phuclv Jan 23 '18 at 14:35
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@RakeshSharma You were given multiple good answers. Have you read them? What is it that you still do not understand? – Ron Jan 23 '18 at 14:43
double can only store 54 bit, or 17 decimal digits with any accuracy. – Gem Taylor Jan 23 '18 at 15:25
@GemTaylor it's 53 bits and ~15-17 digits, not 54 – phuclv Jan 24 '18 at 06:12

Calculation using power function gave wrong answers for large numbers

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