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digitalmars.D.learn - Seed Value for reduce function

reply Vino <vino.bheeman hotmail.com> writes:
Hi All,

   Request you help on how to add the seed value for the reduce 
function, below is the scnerio

Program 1 : Works

import std.algorithm;
void main () {
int[] ara = [1,2 ,3];
auto sum1 = ara.reduce!((a,b) => a + b);
writeln(sum1);
}

Program 2: Works
void main () {
int[] arrb = [];
auto sum1 = reduce!((a,b) => a + b)(0 , arb);
writeln(sum1);
}

So how to add seed value for the below code as same as program 1 
without calling the seed value and array at the end rather than 
calling it as arc.reduce!((a,b) => a + b);

void main () {
int[] arc = [];
auto sum1 = arc.reduce!((a,b) => a + b);
writeln(sum1);
}

From,
Vino.B
Dec 09 2017
next sibling parent Vino <vino.bheeman hotmail.com> writes:
On Sunday, 10 December 2017 at 00:39:07 UTC, Vino wrote:

 Hi All,

   Request you help on how to add the seed value for the reduce 
 function, below is the scnerio

 Program 1 : Works

 import std.algorithm;
 void main () {
 int[] ara = [1,2 ,3];
 auto sum1 = ara.reduce!((a,b) => a + b);
 writeln(sum1);
 }

 Program 2: Works
 void main () {
 int[] arrb = [];
 auto sum1 = reduce!((a,b) => a + b)(0 , arb);
 writeln(sum1);
 }

 So how to add seed value for the below code as same as program 
 1 without calling the seed value and array at the end rather 
 than calling it as arc.reduce!((a,b) => a + b);

 void main () {
 int[] arc = [];
 auto sum1 = arc.reduce!((a,b) => a + b);
 writeln(sum1);
 }

 From,
 Vino.B


Another example: The below code does errors out with the below 
error in there are any empty folders, else it works fine.

import std.stdio;
import std.file;
import std.container;
import std.algorithm;
ulong Size = 10;

auto SdFiles = Array!ulong(dirEntries("C:\\Temp\\BACKUP", 
SpanMode.depth).map!(a => a.size).reduce!((a,b) => a + 
b))[].filter!(a => a  > Size);

Error:
object.Exception C:\D\dmd2\windows\bin\..\..\src\phobos\std\algorith
\iteration.d(2794): Cannot reduce an empty input range w/o an explicit seed
value.

From,
Vino.B
Dec 09 2017
prev sibling parent reply Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:
On Sunday, December 10, 2017 00:39:07 Vino via Digitalmars-d-learn wrote:

 Hi All,

    Request you help on how to add the seed value for the reduce
 function, below is the scnerio

 Program 1 : Works

 import std.algorithm;
 void main () {
 int[] ara = [1,2 ,3];
 auto sum1 = ara.reduce!((a,b) => a + b);
 writeln(sum1);
 }

 Program 2: Works
 void main () {
 int[] arrb = [];
 auto sum1 = reduce!((a,b) => a + b)(0 , arb);
 writeln(sum1);
 }

 So how to add seed value for the below code as same as program 1
 without calling the seed value and array at the end rather than
 calling it as arc.reduce!((a,b) => a + b);

 void main () {
 int[] arc = [];
 auto sum1 = arc.reduce!((a,b) => a + b);
 writeln(sum1);
 }


So, basically, you're trying to use reduce with UFCS, and that doesn't work
in the case where you want to provide an explicit seed value? In that's the
case, then use fold:

https://dlang.org/phobos/std_algorithm_iteration.html#fold

reduce predates UFCS in the language, so there wasn't really a reason to
prefer having the range as the first argument at the time. fold has since
been added to fix that problem. They're identical except for the order of
arguments (in fact, fold just calls reduce internally).

- Jonathan M Davis
Dec 09 2017
parent reply Vino <vino.bheeman hotmail.com> writes:
On Sunday, 10 December 2017 at 01:42:46 UTC, Jonathan M Davis 
wrote:

 On Sunday, December 10, 2017 00:39:07 Vino via 
 Digitalmars-d-learn wrote:

 [...]


 So, basically, you're trying to use reduce with UFCS, and that 
 doesn't work in the case where you want to provide an explicit 
 seed value? In that's the case, then use fold:

 https://dlang.org/phobos/std_algorithm_iteration.html#fold

 reduce predates UFCS in the language, so there wasn't really a 
 reason to prefer having the range as the first argument at the 
 time. fold has since been added to fix that problem. They're 
 identical except for the order of arguments (in fact, fold just 
 calls reduce internally).

 - Jonathan M Davis


Hi Jonathan,

  Tried with fold even then it is failing same as reduce with the 
same error when there is empty folder

auto SdFiles = Array!ulong(dirEntries(d, SpanMode.depth).map!(a 
=> a.size).fold!((a,b) => a + b))[].filter!(a => a  > Size);

From,
Vino.B
Dec 09 2017
parent reply =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 12/09/2017 06:19 PM, Vino wrote:


   Tried with fold even then it is failing same as reduce with the same 
 error when there is empty folder
 
 auto SdFiles = Array!ulong(dirEntries(d, SpanMode.depth).map!(a => 
 a.size).fold!((a,b) => a + b))[].filter!(a => a  > Size);


When no seed value is specified, fold (and reduce) take the first 
element as the seed. When the range is empty, then there is a run-time 
failure:

object.Exception /usr/include/dmd/phobos/std/algorithm/iteration.d(2794): 
Cannot reduce an empty input range w/o an explicit seed value.

The solution is to provide an explicit seed value:

import std.container;
import std.file;
import std.algorithm;

void main() {
     const d = "/tmp/empty_folder";
     const Size = 42;
     auto SdFiles = Array!ulong(dirEntries(d, SpanMode.depth).map!(a => 
a.size).fold!((a,b) => a + b)(size_t(0)))[].filter!(a => a  > Size);
}

I chose size_t(0) but you can use a variable, size_t.init, etc.

Ali
Dec 09 2017
parent Vino <vino.bheeman hotmail.com> writes:
On Sunday, 10 December 2017 at 06:01:49 UTC, Ali Çehreli wrote:

 On 12/09/2017 06:19 PM, Vino wrote:


 [...]


 When no seed value is specified, fold (and reduce) take the 
 first element as the seed. When the range is empty, then there 
 is a run-time failure:

 object.Exception /usr/include/dmd/phobos/std/algorithm/iteration.d(2794):
Cannot reduce an empty input range w/o an explicit seed value.

 The solution is to provide an explicit seed value:

 import std.container;
 import std.file;
 import std.algorithm;

 void main() {
     const d = "/tmp/empty_folder";
     const Size = 42;
     auto SdFiles = Array!ulong(dirEntries(d, 
 SpanMode.depth).map!(a => a.size).fold!((a,b) => a + 
 b)(size_t(0)))[].filter!(a => a  > Size);
 }

 I chose size_t(0) but you can use a variable, size_t.init, etc.

 Ali


Hi Ali,
  Thank you very much, was able to apply your logic which resolved 
the issue.

From,
Vino.B
Dec 11 2017