is there pointers in python

abd_bela · 04-18-2013, 12:10 PM

Hi everybody,

I am new to python and I am discovering it.
I know C well,
and want to know if python knows how to manage Pointers
like pointer to function here is a C example how to write it in python
Intergration with trapeze method

When we write Trapeze ( at the compilation level) we don't know which functions
Fonc to handle. Here for example we use sin and a user defined F1
The program is attached too

#include <stdio.h>
#include <math.h>

double F1 (double x){
return x*x;
}
double Trapeze(double Fonc(double ),
double left, double right, double step){
double X1, X0, Y0, Y1, Z = 0;
for(X0=left; X0 < right ; X0 = X0 + step) {
X1 = X0 + step;
Y1 = Fonc(X1); Y0 = Fonc(X0);
Z += (Y1 + Y0) * step * 0.5;
}
return Z;
}
int main(){
double y;
y=Trapeze(sin, -2.5, 3.2, 0.1);
printf("\n\tValue for sin is : \t %8.3lf ", y);
y=Trapeze(F1, 0, 3, 0.1);
printf("\n\tValue for F1 is : \t %8.3lf ", y);
return 0;
}
/**
Value for sin is : 0.197
Value for F1 is : 9.005
*/
thanks a lot

evo2 · 04-18-2013, 12:19 PM

Hi,

basically everything in python is a reference (except for simple types like ints etc), so, yes you can do something like "pointer to a function". Eg try the following:

Code:

def x(f, a):
  f(a)

def y(s):
  print s

x( y, "Hello" )

Evo2.

PS. Please use [code] tags... it makes reading code, so much less painful.

abd_bela · 04-18-2013, 12:54 PM

thanks

but when we use the x fonction y is define too, we know it
this is not my case when I wrote trapeze I didn't care about the argument function!!!

My C program may be split in 2 parts: the library with trapeze and the test program to check it
with main , F1 and sin

Try to write same thing in python
cheers

evo2 · 04-18-2013, 01:06 PM

Hi,

sorry I don't see what the problem is. If you really can't work it out from the example I gave, please repost your c code in the simplest form, properly indeted in [code] tags, and I'll write the equivalent python. When I say simplest, I mean it just has to demonstrate the functionality you are looking for, not actually do anything useful.

Evo2.

evo2 · 04-18-2013, 01:41 PM

Ok, I'm on my lunch break so I decided to do it...

Code:

import math

def F1(x):
    return x*x

def Trapeze( f, left, right, step ):
    x1=x0=y0=y1=z=0.0
    for i in range( int((right-left)/step) ):
        x0=left+i*step
        x1=x0+step
        y1=f(x1)
        y0=f(x0)
        z += (y1+y0)*step*0.5
    return z

if __name__ == '__main__':
    print 'Value for sin is %f' % Trapeze( math.sin, -2.5, 3.2, 0.1 )
    print 'Value for F1  is %f' % Trapeze( F1, 0, 3, 0.1)

It runs and outputs the following:

Code:

Value for sin is 0.196987
Value for F1  is 9.005000

Note that I would not normally write code like this: I wrote it this way to make it as close as possible to the sample c code.

Evo2.

sundialsvcs · 04-20-2013, 08:14 PM

In high-level languages like Python, "a reference" is a very similar thing to "a pointer" ... except that it is smart.

The language system knows what sort of thing any reference is referring to.
The thing being referred to has a "reference count," which keeps it from being destroyed when something refers to it.
There is a "garbage collector" that will dispose of objects that are no longer being referenced.

This is, quite frankly, one of the most-important reasons why these language systems are routinely used. C/C++ is used to implement them, but with only a few lines of high-level language code you can (very efficiently) tap into "all sorts of language goodness" (tens of thousands of lines' worth of scrupulously optimized, magical voodoo C/C++ code) that makes your life considerably easier. The overhead of the system is predictable, and relatively inconsequential.