Comments on: Russian Peasant Multiplication

By: Bob Phillips

Bob Phillips — Fri, 14 Aug 2009 09:25:00 +0000

Spreadsheet formula.

=A1*B1

If you have a modern spreadsheet, absolutetly no need to conform to the old ways.

By: Lizz

Lizz — Thu, 13 Aug 2009 16:13:00 +0000

Ah, that makes sense, thank you ) I hadn’t realised the original numbers were included.

By: Dick Kusleika

Dick Kusleika — Thu, 13 Aug 2009 14:57:00 +0000

Yeah, I was thinking that the first number would only be odd once. But that’s clearly not true. I have no excuse as I was sober when I wrote this post.

By: Bob Phillips

Bob Phillips — Thu, 13 Aug 2009 14:29:00 +0000

Lizz,

You have missed the other odd element in the list.

It decomposes to

1923
946
492
2184
1368

so you add all of the odds, 368+46+23. You seem to have missed the starting element which is also odd.

By: Lizz

Lizz — Thu, 13 Aug 2009 13:40:00 +0000

I've tried doing this in a plain spreadsheet (and I'm not that great at maths so I may have missed something), but it seems to only work if the number being divided is even. It gets the wrong answer if it's odd.

So 19 * 23 comes out using this method:

19X23=437

94646
492
2184
1368368
414

Have I misunderstood this completely?

By: Tushar Mehta

Tushar Mehta — Thu, 13 Aug 2009 05:54:00 +0000

The best I can do...

Array formula in Excel:
=SUM(IF(MOD(INT(G9/2^(-1+ROW(INDIRECT("1:"&CEILING(LOG(G9)/LOG(2),1))))),2)=1,G10*INT(2^(-1+ROW(INDIRECT("1:"&CEILING(LOG(G9)/LOG(2),1)))))))
where G9 and G10 contain the numbers to be multiplied.

and in VBA

Option Explicit

Function RussianPeasant(ByVal Int1 As Long, ByVal Int2 As Long) As Long
If Int1 = 0 Or Int2 = 0 Then Exit Function
Dim SignMult As Integer: SignMult = Sgn(Int1) * Sgn(Int2)
Int1 = Abs(Int1): Int2 = Abs(Int2)
Do
RussianPeasant = RussianPeasant + IIf(Int1 Mod 2 = 1, Int2, 0)
Int2 = Int2 * 2: Int1 = Int1 2
Loop Until Int1 = 0
RussianPeasant = RussianPeasant * SignMult
End Function

By: Doug Jenkins

Doug Jenkins — Thu, 13 Aug 2009 02:02:00 +0000

Dick - your code doesn't always give the right answer. The line:
lMod = lMult
should be
lMod = lMult + lMod

Here's my effort:

Function RPMult(Num1 As Long, Num2 As Double) As Double
Do While Num1 >= 1
If Num1 Mod 2 = 1 Then RPMult = RPMult + Num2
Num1 = (Num1 2)
Num2 = Num2 * 2
Loop
End Function

I made Num2 a double so it didn't overflow so quickly.
I'll be intrerested to see the worksheet approach (I had a think about it, but my brain crashed).

By: mark nold

mark nold — Thu, 13 Aug 2009 01:55:00 +0000

Something looks wrong with your function… have you tested RussianPeasant(99, 100) ?

By: Dick Kusleika

Dick Kusleika — Wed, 12 Aug 2009 22:30:00 +0000

This guy did it in one line. I'll post a spreadsheet formula solution tomorrow.