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Replying to Topic: simple C code
Created On Thu Feb 03, 05 04:48 PM by SPAAGG

SPAAGG
Senior Member

Posts: 779
Joined: Mar 2003

Thu Feb 03, 05 04:48 PM

Hi !

I am very weak in C, and need to compute a function which does the following (i write it in R):
v is a vector (unknown size)
a is a scalar

theFunction = function(v, a)
{
n = length(v)

y[1] = 10
for (i in 2:n)
{
y = v + a * v[i-1]
}

return(y)
}

that is, I enter a vector of unknown size, floating type, and a scalar, floating type, and want that the C function returns me a new vector. I think that in C, a function cannot return a vector. We need to use a procedure, that is void.

Could you help me

Tx in advance

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Cuchulainn
Senior Member

Posts: 16364
Joined: Jul 2004

Thu Feb 03, 05 05:09 PM

removed, just confused the issue

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Edited: Sat Feb 05, 05 at 08:14 PM by Cuchulainn

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daveangel
Senior Member

Posts: 4978
Joined: Oct 2003

Thu Feb 03, 05 07:59 PM

SPAAG

this might do what you want ...

double�*�myFunc(const�double�*v,int�n,double�a,double�*y)
{
��y[0]�=�10;
��for�(int�i�=�1;i�<�n;�i++)
��{
��y[i]�=�v[i]�+�a*v[i-1];
��}

��return�y;
}

int�main()
{
��double�v[]�=�{�1,�2,�3,�4,�5,�6,�7,�8,�9,�10}�;�//�create�v
��double�y[sizeof(v)/sizeof(double)];�//�allocate�y

��myFunc(v,10,5,y);�//�a�=�5

��return�1;
}

-------------------------
Knowledge comes, but wisdom lingers.

Edited: Thu Feb 03, 05 at 08:00 PM by daveangel

Posts: 94
Joined: Apr 2003

Thu Feb 03, 05 08:06 PM

Quote
Originally posted by: daveangel
SPAAG
double * myFunc(const double *v,int n,double a,double *y)
{
y[0] = 10;
for (int i = 1;i < n; i++)
{
y = v + a*v[i-1];
}

return y;
}

int main()
{
double v[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10} ; // create v
double y[sizeof(v)/sizeof(double)]; // allocate y

myFunc(v,10,5,y); // a = 5

return 1;
}

If SPAAG wants to do dynamic memory stuff then that's another story of course. If he is new to C, those pointers are going to be great fun, not.
I suggested vector because all that memory stuff is hidden.
this might do what you want ...

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SPAAGG
Senior Member

Posts: 779
Joined: Mar 2003

Thu Feb 03, 05 09:56 PM

thank you so much !

David

Posts: 178
Joined: Jan 2005

Thu Feb 03, 05 10:12 PM

��Objective�Caml�version�3.08.2

#�let�testfun�v�a�=
��let�n�=�Array.length�v�in
��let�y�=�Array.create�n�0.�in
��y.(0)�<-�10.;
��for�i�=�1�to�n-1�do
��y.(i)�<-�v.(i)�+.�(a�*.�v.(i-1))
��done;
��y;;
��val�testfun�:�float�array�->�float�->�float�array�=�<fun>
#�let�a�=�[|1.;2.;3.;4.;5.;6.;7.;8.;9.;10.|];;
val�a�:�float�array�=�[|1.;�2.;�3.;�4.;�5.;�6.;�7.;�8.;�9.;�10.|]
#�testfun�a�1.;;
-�:�float�array�=�[|10.;�3.;�5.;�7.;�9.;�11.;�13.;�15.;�17.;�19.|]
#�

Edited: Wed Mar 30, 05 at 01:51 PM by henny

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Cuchulainn
Senior Member

Posts: 16364
Joined: Jul 2004

Fri Feb 04, 05 11:00 AM

removed, confused the issue, excuses

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Edited: Sat Feb 05, 05 at 08:13 PM by Cuchulainn

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Cuchulainn
Senior Member

Posts: 16364
Joined: Jul 2004

Fri Feb 04, 05 11:26 AM

Correct (now pasted from VS, not Word)
--

// testvalarray.cpp
//
// Show some of the functionality of the STL
// 'valarray'. This is a dynamic maths vector class.
//

#include <valarray> // Reusable vector class
#include <iostream> // I/O stuff
using namespace std; // STL namespace, STANDARD C++

template <class T> valarray<T> multiply(const valarray<T>& vec1, const T& factor)
{ // Multiply a vector by a scalar factor

valarray<T> result (vec1.size()); // Same size

for (int j = 0; j < result.size(); j++)
{
result[j] = vec1[j] * factor;
}

return result;
}

// Usually work with doubles, define handy alias
typedef valarray<double> DoubleArray;

int main()
{
int N;
cout << "Give size: ";
cin >> N;

double factor = 0.5;

DoubleArray inputArray(N);

// Initialisation of input array
for (int j = 0; j < inputArray.size(); j++)
{
inputArray[j] = double (j+1); // Simple values
}
DoubleArray outputArray = multiply(inputArray, factor);

for (int i = 0; i < outputArray.size(); i++)
{
cout << outputArray << ", ";
}

return 0;
}

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Edited: Fri Feb 04, 05 at 11:57 AM by Cuchulainn

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Cuchulainn
Senior Member

Posts: 16364
Joined: Jul 2004

Sat Feb 05, 05 04:36 PM

Quote
Originally posted by: daveangel
SPAAG
double * myFunc(const double *v,int n,double a,double *y)
{
y[0] = 10;
for (int i = 1;i < n; i++)
{
y = v + a*v[i-1];
}

return y;
}

int main()
{
double v[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10} ; // create v
double y[sizeof(v)/sizeof(double)]; // allocate y

myFunc(v,10,5,y); // a = 5

return 1;
}

this might do what you want ...

What happens if the CLIENT, i.e. main program inadvertently does ...

double a = 5.0;
myFunc(v,20,a,y); // overwrites someone else's memory

or even

double a = 5.0;
myFunc(v,6,a,y); // not all elements iterated over

So this code works but very brittle, unfortunately. In the first place I get a run-time error and 2nd the wrong answer.

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Edited: Sat Feb 05, 05 at 04:56 PM by Cuchulainn

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daveangel
Senior Member

Posts: 4978
Joined: Oct 2003

Sat Feb 05, 05 05:17 PM

Look Cuchulain,

You have made three attempts at posting a solution to the SPAAGs problem .. remember the title of the thread "simple C Code".

I have no doubt that the design isnt foolproof but that was not what SPAAG wanted.

Cheers

-------------------------
Knowledge comes, but wisdom lingers.

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Cuchulainn
Senior Member

Posts: 16364
Joined: Jul 2004

Sat Feb 05, 05 07:46 PM

Quote
Originally posted by: daveangel
Look Cuchulain,

You have made three attempts at posting a solution to the SPAAGs problem .. remember the title of the thread "simple C Code".

I have no doubt that the design isnt foolproof but that was not what SPAAG wanted.

Cheers

Daveangel
Ok, point taken. My real reason was to warn about the difficult problem of memory management in C, not a slur on your code, certainly not.

I come across people who write C/C++ programs who are actually unaware of the C nasties.

Cheers also

-------------------------
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Cuchulainn
Senior Member

Posts: 16364
Joined: Jul 2004

Sat Feb 05, 05 08:17 PM

Quote
Originally posted by: daveangel
Look Cuchulain,

You have made three attempts at posting a solution to the SPAAGs problem ..Cheers

first 2 versions have been removed. Solution in C++.

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