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digitalmars.D.learn - bigint and pow

reply Fausto <fausap outlook.it> writes:
Hello,

I am trying to use pow with an integer argument, but I cannot 
have a bigint result, for example, ```pow(10,72)```.
Do I have to write my pow function or is there a native solution?

thanks,
Fausto
Oct 01 2022
parent reply rassoc <rassoc posteo.de> writes:
On 10/2/22 00:04, Fausto via Digitalmars-d-learn wrote:

 Hello,
 
 I am trying to use pow with an integer argument, but I cannot have a bigint
result, for example, ```pow(10,72)```.
 Do I have to write my pow function or is there a native solution?
 
 thanks,
 Fausto
 


In contrast to certain scripting languages, there's no implicit promotion, you
have to opt in for BigInt [1] usage in D:

```d
import std;
void main()
{
     // all print the same
     writeln(BigInt(10) ^^ 72);
     writeln(10.BigInt ^^ 72);
     writeln("10".BigInt ^^ 72);
}
```

[1] https://dlang.org/phobos/std_bigint.html#.BigInt
Oct 01 2022
parent reply Fausto <fausap outlook.it> writes:
On Sunday, 2 October 2022 at 02:02:37 UTC, rassoc wrote:

 On 10/2/22 00:04, Fausto via Digitalmars-d-learn wrote:

 Hello,
 
 I am trying to use pow with an integer argument, but I cannot 
 have a bigint result, for example, ```pow(10,72)```.
 Do I have to write my pow function or is there a native 
 solution?
 
 thanks,
 Fausto
 


 In contrast to certain scripting languages, there's no implicit 
 promotion, you have to opt in for BigInt [1] usage in D:

 ```d
 import std;
 void main()
 {
     // all print the same
     writeln(BigInt(10) ^^ 72);
     writeln(10.BigInt ^^ 72);
     writeln("10".BigInt ^^ 72);
 }
 ```

 [1] https://dlang.org/phobos/std_bigint.html#.BigInt


Thanks a lot. I am to used to C and, more important, I didn't 
think to look for also another operator for the power function :)
Oct 02 2022
next sibling parent rassoc <rassoc posteo.de> writes:
On 10/2/22 09:24, Fausto via Digitalmars-d-learn wrote:

 Thanks a lot. I am to used to C and, more important, I didn't think to look
for also another operator for the power function :)
 


D does have pow and many other useful math functions [1], it's just not defined
for BitInts. Oh, and speaking of C, you also have access to all the usual C
math [1] functions with just an import:

```d
import std.stdio : writeln;
void main()
{
     import std.math : pow;
     writeln(pow(10, 3)); // pow from D
     
     import core.stdc.math : pow;
     writeln(pow(10, 3)); // pow from C
     
     // can also make it more explicit to show where it is coming from:
     import cmath = core.stdc.math;
     writeln(cmath.pow(10, 3));
}
```

Have fun with D!

[1] https://dlang.org/library/std/math.html
[2] https://dlang.org/library/core/stdc/math.html
Oct 02 2022
prev sibling parent rassoc <rassoc posteo.de> writes:
On 10/2/22 09:24, Fausto via Digitalmars-d-learn wrote:

 Thanks a lot. I am to used to C and, more important, I didn't think to look
for also another operator for the power function :)
 


Oh, and I forgot to mention that this is doing what you probably asked for
originally:

```d
import std;
import cmath = core.stdc.math;
void main()
{
     // both print 1e+72
     writeln(pow(10.0, 72));
     writeln(cmath.pow(10, 72));
}
```

But it's just floating-point scientific notation, not true BigInts.

Another math difference from C is that D has well-defined wrapping math for
signed ints.
Oct 02 2022