why left shift operator prints different results for hard coded values and Big-integer

Question

when using left shift and Big-integer i get different results to when i use hard coded number, why is that and how can it be overcome?

    BigInteger num = 1779033703;
    BigInteger result = num << 30;
    Console.WriteLine(result);
    Console.WriteLine(1779033703 << 30);

Result

1910222893216694272

-1073741824

Because the "hard coded value" is an `int` and they can only hold 32 bits to represent a number. `BigInteger` exists specifically to allow you to work with larger integer values. — juharr, Dec 31 '17 at 21:39
Possible duplicate of [No overflow exception for int in C#?](https://stackoverflow.com/questions/2056445/no-overflow-exception-for-int-in-c) — Progman, Dec 31 '17 at 22:02

score 2 · Answer 1 · answered Dec 31 '17 at 21:38

2

Because with 1779033703 << 30 on a regular int (a 32-bit signed integer), you are hitting the overflow, hence it starts from int.MinValue again. The BigInteger can handle much larger values, and is thus not affected by overflow.

Try to use a long (a 64-bit signed integer), and you will see you get the same result as with the BigInteger.

answered Dec 31 '17 at 21:38

Patrick Hofman

153,850
22
249
325

Thanks for the answer. how can i use << and big number(bigger than int) but get the same results as it was int but not getting the overflow exception? with JS it is possible.. but i can't with c# – harel bracha Dec 31 '17 at 21:44
1

In this case a `long` would do, as stated in my answer. – Patrick Hofman Dec 31 '17 at 21:45
let me clarify my question. if i use JS compare to BigInteger in C# i get different results. the JS give me negative numbers while C# very large ones – harel bracha Dec 31 '17 at 21:47
@Camilo Terevinto so what can i do with big numbers to get the same results as JS gives me? – harel bracha Dec 31 '17 at 21:49
2

@harelbracha You should [edit] your question and include the JS code and output – Camilo Terevinto Dec 31 '17 at 21:49
If I understand you correctly you want the wrong result in JS? The one calculated by `BigInteger` is the correct one. – Patrick Hofman Dec 31 '17 at 21:50
@ Patrick Hofman long is giving me the same results as BigIntegers – harel bracha Dec 31 '17 at 21:51
So that one is correct. The negative numbers are wrong. – Patrick Hofman Dec 31 '17 at 21:51
What JS result? – Patrick Hofman Dec 31 '17 at 21:54
2

So you have that code in C# already, right? What more do you ask? – Patrick Hofman Dec 31 '17 at 21:57
i made a mess. what i want is to be able to deal with big numbers in C# using << operator but get the same results as JS(the wrong ones) without getting overflow exception – harel bracha Dec 31 '17 at 21:59
1

@harelbracha That's not possible since the exact same is happening in JS and C#, an integer overflow. – Camilo Terevinto Dec 31 '17 at 21:59
@Patrick Hofman i don't have it yet, as if i put the hard coded value in int variable i get overflow exception(the code is much more complex and i can't write hard coded numbers) – harel bracha Dec 31 '17 at 22:00
@ Camilo Terevinto var a = 1779033703; print(a << 30) results: -1073741824 – harel bracha Dec 31 '17 at 22:13

why left shift operator prints different results for hard coded values and Big-integer

1 Answers1