[Tutor] Traversing lists or getting the element you want.

[Kipton Moravec]

I am new to Python, and I do not know how to traverse lists like I
traverse arrays in C. This is my first program other than "Hello World".
I have a Raspberry Pi and they say Python is the language of choice for
that little machine. So I am going to try to learn it.


I have data in the form of x, y pairs where y = f(x) and is non linear.
It comes from a .csv file.

In this case x is an integer from 165 to 660 so I have 495 data sets. 

I need to find the optimal locations of three values of x to piecewise
linear estimate the function. 


So I need to find i, j, k so 165 < i < j < k < 660 and the 4 line
segments [(165, f(165)), (i, f(i))], [(i, f(i)), (j, f(j))], [(j, f(j),
(k, f(k))], [(k, f(k)), (660, f(660))].


The value I need to minimize is the square of the difference between the
line estimate and the real value at each of the 495 points. 


I can do this in C. To keep it simple to understand I will assume the
arrays x[] and y[] and minsum, mini, minj, and mink are global.


I have not tested this but this is what I came up with in C. Assuming
x[] and y[] are already filled with the right values.


int x[495];
double y[495];
max_elements = 495;
double minsum;
int mini, minj, mink


void minimize(int max_elements)

{
    minsum = 9999999999999.0; // big big number

    for (i=2; i<max_elements-6;i++)
        for (j=i+2; j< max_elements -4; j++)
             for (k=j+2; k< max_elements-2;k++)
             {
                sum = error(0,i);
                sum += error(i,j);
                sum += error(j,k);
                sum += error(k,495)

                if (sum < minsum) 
                {
                    minsum = sum;
                    mini = i;
                    minj = j;
                    mink = k;
                } 
             }
}


double error(int istart, int iend)
{

// linear interpolation I can optimize but left 
// it not optimized for clarity of the function

    int m;

    double lin_estimate;

    double errorsq;

    errorsq = 0;

    for (m=istart; m<iend; m++)
    {
        lin_estimate = y[istart] + ((y[iend] – y[istart]) * 
                       ((x[m] – x[istart]) / (x[iend] – x[istart])));

        errorsq += (lin_estimate – y[m]) * (lin_estimate – y[m]);
    } 

    return (errorsq);

}



At the end the three values I want are mini, minj, mink;
or x[mini], x[minj], x[mink]


So how do I do this (or approach this) in Python?


Kip