On Fri, 2015-09-25 at 06:02 +0000, Nicholas Wilson via Digitalmars-d
-learn wrote:
> On Friday, 25 September 2015 at 05:50:58 UTC, Charanjit Singh
> wrote:
> > import std.stdio;
> > import std.math;
> > void main()
> >
> > {
> > float sum,pi,t;
> > int n=1;
> > sum=0;
> > while (n<100 )
> > {
> > t=1/( n*n);
> > n=n+1;
> > sum=sum+t;
> >
> >
> > }
> > writeln("value of PI= " , (sum*6)^^.5);
> > that is pi program as
> > (pi^2)/6= 1/1+ 1/4 + 1/9 + 1/16 + 1/25
> > but output of my program is 2.44
This looks a bit like K&R C transliterated to D. Much better to write
more idiomatic D…
> t=1/( n*n); //<----
> Is doing integer division so is zero for n > 1 hence sqrt(1*6) =
> 2.449...
>
> change that to a
> t=1.0/( n*n);
That certainly solves the immediate problem.
Taking the original code and making it more D-like, you get something
like:
double while_loop() {
auto n = 1;
auto sum = 0.0;
while (n < 100) {
sum += 1.0 / (n * n);
n++;
}
return sum;
}
but this is bounded iteration not unbounded iteration, so let us use a
far more idiomatic form:
double foreach_loop() {
auto sum = 0.0;
foreach (n; 1..100) {
sum += 1.0 / (n * n);
}
return sum;
}
but this is still very explicit iteration and we have moved on to the
era of implicit iteration and declarative rather than imperative
expression:
double reduce_loop() {
return reduce!"1.0 / (a * a)"(iota(1, 100));
}
Unfortunately I have clearly done something silly here as:
double take_root(double x) { return (x * 6)^^0.5; }
void main() {
writeln("while = " , take_root(while_loop()));
writeln("foreach = " , take_root(foreach_loop()));
writeln("reduce = " , take_root(reduce_loop()));
}
results in:
while = 3.13198
foreach = 3.13198
reduce = 2.44949
If we worry about the string form and instead try:
double reduce_string_loop() {
return reduce!"1.0 / (a * a)"(iota(1, 100));
}
double reduce_function_loop() {
return reduce!((n) => 1.0 / (n * n))(iota(1, 100));
}
then we end up with:
/usr/include/dmd/phobos/std/algorithm/iteration.d(2565): Error: template
pi_sumInverseSquares.reduce_function_loop.__lambda1 cannot deduce function from
argument types !()(int, int), candidates are:
pi_sumInverseSquares.d(28):
pi_sumInverseSquares.reduce_function_loop.__lambda1
/usr/include/dmd/phobos/std/meta.d(546): Error: template instance
pi_sumInverseSquares.reduce_function_loop.F!(__lambda1) error instantiating
/usr/include/dmd/phobos/std/algorithm/iteration.d(2473): instantiated
from here: staticMap!(ReduceSeedType, __lambda1)
pi_sumInverseSquares.d(28): instantiated from here: reduce!(Result)
pi_sumInverseSquares.d(28): Error: template std.algorithm.iteration.reduce
cannot deduce function from argument types !((n) => 1.00000 / (n * n))(Result),
candidates are:
/usr/include/dmd/phobos/std/algorithm/iteration.d(2447):
std.algorithm.iteration.reduce(fun...) if (fun.length >= 1)
Failed: ["dmd", "-v", "-o-", "pi_sumInverseSquares.d", "-I."]
which is sort of annoying.
--
Russel.
=============================================================================
Dr Russel Winder t: +44 20 7585 2200 voip: sip:russel.win...@ekiga.net
41 Buckmaster Road m: +44 7770 465 077 xmpp: rus...@winder.org.uk
London SW11 1EN, UK w: www.russel.org.uk skype: russel_winder
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