Recently I decided to learn C and have been diligently working my way through tutorials and websites, and since I’m more familiar with high level languages (C# for example) getting used to the more low level C is proving to be a challenge.
After getting to grips with the dreaded pointers I immediately ran into the problem of how to return an array from a function – the familiar methods used by anyone familiar with C# or VB.Net don’t work, and attempts at implementing return unusual results. Basically, populating an array in a function and passing a reference to it is doomed to fail it’s a regular local variable, with automatic duration, which means that it springs into existence when the function is called and disappears when the function returns. Passing a reference out of a function to a variable that has expired points to an junk.
After some googling, I found a helpful document that describes how to deal with the problem on http://c-faq.com, which describes in the three methods of returning arrays from functions which, briefly, are:
- The use of a static array within the function,
- Passing an array into the function and operating on it, and
- Creating an array in the function usign malloc.
All three methods have their own positives and negatives, but for the purposes on what I’m doing I’m going for the second one.
'
' Process a column of values, colouring alternate ranges of the same value.
'
' Work down a column of cells, stopping when a blank is hit.
'
' If a cell is a different value to the one preceding it, invert the
' value of the toggle variable.
'
' If the toggle variable is true colour the cell.
'
Public Sub main()
Dim w As Worksheet
Set w = Worksheets("Sheet1")
Dim rowCtr As Integer
rowCtr = 2
Dim toggle As Boolean
toggle = False
While w.Cells(rowCtr, 1).Value <> ""
If rowCtr > 2 Then
If w.Cells(rowCtr, 2).Value <> w.Cells(rowCtr - 1, 2).Value Then
toggle = Not toggle
End If
End If
If toggle Then
With w.Cells(rowCtr, 2).Interior
.ColorIndex = 6
.Pattern = xlSolid
End With
End If
rowCtr = rowCtr + 1
Wend
End Sub
I worked out an implementation of the Sieve of Atkin for use in solving Project Euler problems. It’s probably not as efficient as it could be, but it does the trick for me.
set nocount on
drop table #atkin
create table #atkin
(
num float
, isPrime bit
)
create clustered index primenums
on #atkin(num)
declare @limit as bigint
declare @sqrt as float
declare @x as int
declare @y as int
declare @n as int
set @limit = 1000000
set @sqrt = sqrt(@limit)
set @x = 1
print 'begin outer loop'
/*
put in candidate primes:
integers which have an odd number of representations by certain quadratic forms
*/
while (@x <= @sqrt)
begin
set @y = 1
while (@y <=@sqrt)
begin
set @n = (4 * (@x * @x)) + (@y * @y)
if ((select num from #atkin where num =@n) is null)
insert into #atkin (num, isPrime) values (@n,0)
if (@n <= @limit AND (@n % 12 = 1 or @n % 12 =5))
update #atkin set isPrime = isPrime ^ 1 where num = @n
set @n = (3 * (@x * @x)) + (@y * @y)
if ((select num from #atkin where num =@n) is null)
insert into #atkin (num, isPrime) values (@n,0)
if (@n <= @limit AND @n % 12 = 7)
update #atkin set isPrime = isPrime ^ 1 where num = @n
set @n = (3 * (@x * @x)) - (@y * @y)
if ((select num from #atkin where num =@n) is null)
insert into #atkin (num, isPrime) values (@n,0)
if (@x >= @y AND @n <= @limit AND @n % 12 = 11)
update #atkin set isPrime = isPrime ^ 1 where num = @n
set @y = @y + 1
end
set @x = @x + 1
end
declare @nsqr as bigint
declare @k as bigint
/*
eliminate composites by sieving
*/
set @n = 5
while ( @n <= @sqrt)
begin
print @n
if ((select isprime from #atkin where num = @n)=1)
begin
set @nsqr = @n * @n
set @k = @nsqr
while (@k <= @limit)
begin
update #atkin set isPrime = 0 where num = @k
set @k = @k + @nsqr
print '@k increment ' + str(@k)
end
end
set @n = @n + 1
print '@n increment ' + str(@n)
end
insert into #atkin (num, isPrime) values (2,1)
insert into #atkin (num, isPrime) values (3,1)
delete from #atkin where isprime <> 1
This is a solution for Project Euler Problem 37. It makes uses of a temporary table called #atkin which is populated with prime numbers using the TSQL routine that uses the Sieve of Atkin, which is also located on this site.
Easy! There are, however, a couple of deliberate errors, that can be easily located with some patience. I don’t want to make things to easy for you little scamps, do I?
/*
process the prime number list
*/
/*
the number we are examining can't be single digits or contain zeros
*/
drop table #candidates
select num
into #candidates
from #atkin
where charindex('0', cast(num as varchar)) = 0
declare @ctr as integer
declare @del as integer
set @ctr = 1
set @del = 0
/*
delete from the candidate list any number that doesn't have a
shorter counterpart in the list of prime numbers, increasing
the string size by 1 with every loop, and bailing out of when
we stop deleting things
*/
while (@del > 0)
begin
delete from #candidates
from #candidates c1
left outer join #atkin a1
on left(c1.num,@ctr) = a1.num
left outer join #atkin a2
on right(c1.num,@ctr) = a2.num
where a1.num is null
or a2.num is not null
set @del = @@rowCount
set @ctr = @ctr + 1
end
/*
output the answer
*/
select sum(num)
from #candidates
/*
generate the number range
*/
create table #polygonalnumbers
(
ctr int
, val bigint
, poltype varchar(3)
)
declare @ctr as integer
set @ctr = 1
while (@ctr < 1000)
begin
insert into #polygonalnumbers
select @ctr,(@ctr*(@ctr+1))/2,'tri'
insert into #polygonalnumbers
select @ctr,@ctr * @ctr *@ctr,'sqr'
insert into #polygonalnumbers
select @ctr,(@ctr*((3*@ctr)-1))/2 ,'pnt'
insert into #polygonalnumbers
select @ctr,(@ctr*((2*@ctr)-1)) ,'hex'
insert into #polygonalnumbers
select @ctr,(@ctr*((5*@ctr)-3))/2 ,'hep'
insert into #polygonalnumbers
select @ctr,(@ctr*((3*@ctr)-2)) ,'oct'
set @ctr = @ctr + 1
end
/*
disregard
*/
delete from #polygonalnumbers
where LEN(val) <> 4
/*
output the answer
*/
select distinct num1.val + num2.val + num3.val + num4.val + num5.val + num6.val
from #polygonalnumbers num1
inner join #polygonalnumbers num2
on RIGHT (num1.val,2) = LEFT(num2.val,1)
and num2.poltype <> num1.poltype
inner join #polygonalnumbers num3
on RIGHT (num2.val,2) = LEFT(num3.val,2)
and num3.poltype <> num2.poltype
and num3.poltype <> num1.poltype
inner join #polygonalnumbers num4
on RIGHT (num3.val,2) = LEFT(num4.val,2)
and num4.poltype <> num3.poltype
and num4.poltype <> num2.poltype
and num4.poltype <> num1.poltype
inner join #polygonalnumbers num5
on RIGHT (num4.val,2) = LEFT(num5.val,2)
and num5.poltype <> num4.poltype
and num5.poltype <> num3.poltype
and num5.poltype <> num2.poltype
and num5.poltype <> num1.poltype
inner join #polygonalnumbers num6
on RIGHT (num5.val,2) = LEFT(num6.val,2)
and num6.poltype <> num5.poltype
and num6.poltype <> num4.poltype
and num6.poltype <> num3.poltype
and num6.poltype <> num2.poltype
and num6.poltype <> num1.poltype
and RIGHT (num6.val,2) = LEFT(num1.val,2)
Easy! There are, however, a couple of deliberate errors, that can be easily located with some patience. I don’t want to make things to easy for you little scamps, do I?
With WinClone which you can find here.
A relationship counsellor’s bid to challenge his sacking for refusing to give sex therapy to gay couples has been turned down by the High Court.
Gary McFarlane, 48, from Bristol, was sacked by Relate Avon in 2008. He claimed the service had refused to accommodate his Christian beliefs.
Lord Justice Laws said legislation for the protection of views held purely on religious grounds cannot be justified.
He said it was irrational and “also divisive, capricious and arbitrary”.
Continued in this BBC article.
