lagint_old.py

#!/usr/bin/env python
# shitty way of doing a Lagrange interpolation

def lagint(xarr,yarr):
    p = []
    k = len(xarr) # rank+1
    for i in range(k):
        Li = []
        for j in range(k):
            if (j != i):
                Lij = {
                    "xj" : str(xarr[j]), 
                    "div" : xarr[i] - xarr[j]
                }
                Li.append(Lij)
        pi = {
            'yi' : yarr[i],
            'Li' : Li
        }
        p.append(pi)
    return p
    
def pprintli(lagint):
    # pretty print of given p(x)
    # found via previous lagint func
    p = ''
    for pi in lagint:
        yid = pi['yi']
        for Lij in pi['Li']:
            yid /= Lij['div']
        if yid>0:
            p += '\n + '
        else:
            yid = abs(yid)
            p += '\n - '
        p += '%.4f'%(yid)+'*'
        p += ''.join('(x-'+Lij['xj']+')' for Lij in pi['Li'])
    print('  '+p[3:])

# xarr = [x1,x2,...,xn]
# yarr = [y1,y1,...,yn]
# pprint(lagint(xarr,yarr))