CODEWITHSUNDEEP
delphimath
NoPyGod
NoPyGod

Reputation: 5067

Strange Delphi integer multiplication behavior

I'm working on some ancient Delphi code and I've come across something which I don't quite understand.

[bla is set to 130245932]

outresult := ((bla * 1103516849) + 12359);

[outresult is equal to -413953101]

How does multiplying two positive numbers result in a negative number? And why is it that when I take the bla variable out of the equation and just use the integer directly (like this)

outresult := ((130245932 * 1103516849) + 12359);

I receive an error before the app even compiles

[DCC Error] Unit1.pas(60): E2099 Overflow in conversion or arithmetic operation

Some genius would be appreciated. Thanks.

Upvotes: 4

Views: 1299

Answers (3)

Argalatyr
Argalatyr

Reputation: 4659

You're seeing an integer overflow, and I thought you might be curious about the specific result you saw.

Your initial multiplication

(130245932 * 1103516849) = 143728580475708268

...results in integer overflow, and in 32-bit math the resulting "wrap-around" generates:

(143728580475708268 mod 2^32) - 2^32 = -413965460

...and then your equation adds 12359:

-413965460 + 12359 = -413953101 (the result you saw, Q.E.D.)

Hope this helps

Upvotes: 2

Mysticial
Mysticial

Reputation: 471199

Alright, I'll make this an answer.

The error message should be pretty clear. You have an integer overflow here:

130245932 * 1103516849

because 130245932 * 1103516849 = 143728580475708268 which is too large to fit into a 32-bit integer.

Upvotes: 11

Mason Wheeler
Mason Wheeler

Reputation: 84540

It's based on the way memory is represented inside your system. Basically, you've only got 32 bits per integer. For a signed integer, one bit is used for the sign; this gives you a value range from negative to positive 2^31 (approximately 2 billion). If you go outside that range, the system breaks down.

If you need large integers, try using Int64 instead of integer. If you need larger integers than that, check out the BigInteger type in DeHL.

Upvotes: 6

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