pow work differently, explain in detail?

Question

int a=pow(100,2);//line 1
int b=ceil(pow(100,2));//line 2
cout<<pow(100,2);//line 3

line 1 gives a=9999 on printing the value of a

line 2 gives b=10000 on printing value of b

line 3 prints 10000

I understood that pow give value 9999.9999 so ceil func. in line 2 gives it the upper value.

But why didn't cout print 9999.9999

Can anyone explain why pow behave like this return decimal value pow is just a power function why doesn't it simply give 100*100 as answer?

it doesn't print 9999.9999 because it is an integer not a float.... — Dadep, Apr 11 '17 at 19:51
What is bad in this question many people doesn't know that pow return a decimal value — Shaurya Uppal, Apr 11 '17 at 19:52
Everyone with an IDE or who had read the [docs](http://en.cppreference.com/w/cpp/numeric/math/pow) would know it returns a *floating point* value. — jaggedSpire, Apr 11 '17 at 19:54
@shauryauppal I'm pretty sure your compiler gave you a warning such as VS C4244 'initializing' : conversion from 'double' to 'int', possible loss of data... — AlexG, Apr 11 '17 at 19:55
What is `ceil(100, 2)` supposed to do? `ceil()` only takes 1 argument. Did you mean `ceil(pow(100, 2))`? — Barmar, Apr 11 '17 at 19:57
@Dadep: no. Line 1 proves that the result of pow is a floating-point value, which is truncated to 9999. — , Apr 11 '17 at 20:05
[Another possible duplicate](http://stackoverflow.com/questions/25678481/why-does-pown-2-return-24-when-n-5-with-my-compiler-and-os) — PaulMcKenzie, Apr 11 '17 at 20:05
The OP made the classical mistake of combining two questions in one. One is a duplicate, while the other is not. — , Apr 12 '17 at 06:59

score 3 · Accepted Answer · 2017-04-11T20:07:07.727

3

In the cout statement, pow(100, 2) is indeed a double-precision value and slightly below 10000 (by a well-known effect of the pow function), but the default accuracy setting of cout causes rounding and output as an integer.

edited Apr 11 '17 at 20:07

answered Apr 11 '17 at 20:00

score 1 · Answer 2 · edited May 23 '17 at 12:02

1

The reason is due to pow taking two double values as arguments (and returning a double) and is typically implemented such that pow(x, y) is exp(y log x). This "goes off" for even seemingly trivial input values. See Is floating point math broken?

Note that std::pow has overloads for integral types which can be helpful when working in integer arithmetic.

edited May 23 '17 at 12:02

Community

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answered Apr 11 '17 at 19:52

Bathsheba

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This doesn't explain why the output isn't 9999.9999 – Apr 11 '17 at 20:06
Coupled with the linked question I believe it does. – Bathsheba Apr 11 '17 at 20:52
No, it does not. – Apr 12 '17 at 06:37
Well do please feel free to downvote in that case: it's what the mechanism is for. – Bathsheba Apr 12 '17 at 06:57
I don't think it would help you understandwhy I am saying that. – Apr 12 '17 at 07:01

pow work differently, explain in detail?

2 Answers2