Hunter Ratliff

Hunter Ratliff

HVL postdoctoral fellow

A calculator for redistributing costs to make individual expenses equal

2 minute read

Published:

Provided here is a simple calculator for redistributing funds among a group of people. The general application is for scenarios where a few people split the bill for a larger group and then everyone wants to determine how much they need to pay and to whom to make the total expense of each person equal.

Enter into the first field below the list of names of all participants, and in the second field enter the amount which has already been paid by each of the participants. Select the desired decimal precision of the results (useful if concerned about cents), and then click the "Calculate" button to see how money should be transferred among the participants.


Input people and the amounts expended by each into the fields below with entries separated by commas.

People:

Amounts:

Decimal precision of results:

^ click 'Calculate'


Python function for cost redistribution
[Download link of Python file] 
'''
Cost redistribution function
written by Hunter Ratliff
'''
def caclulate_cost_distribution(people,expenses,decimal_precision=0,sorting_on=False):
    '''
    Inputs:
       people = list of strings of names
       expenses = list of integers/floats of the amounts paid by each person (length must be same as people)
    Outputs:
       cost_str = string describing the transfers necessary to make expenditures even across all people
    '''
    p,x = people,expenses
    dpadd = 0
    if decimal_precision > 0:
        dpadd = decimal_precision + 1
    if sorting_on:
        p = [i for _,i in sorted(zip(x,p))] # reorder people based on amount
        x.sort() # reorder corresponding amount list
    T = sum(x) # total expended
    o = [(T/len(x)) - i for i in x] # amount owed by each
    d  = [] # debtors
    da = [] # amounts owed (positive)
    c  = [] # collectors
    ca = [] # amounts to be collected (negative)
    for i in range(len(x)):
        if o[i]<0:
            c.append(p[i])
            ca.append(o[i])
        else:
            d.append(p[i])
            da.append(o[i])
    if sorting_on:
        c.reverse()
        ca.reverse()
    ci, di = 0, 0
    cost_str = ''
    max_name_len = max([len(i) for i in p])
    name_format_lstr = '{:>' + str(int(max_name_len)) + '}'
    name_format_rstr = '{:' + str(int(max_name_len)) + '}'
    transactions = []
    while abs(sum(da))>=1e-3:
        if da[di] <= -1*ca[ci]:
            d2c = da[di]
        else:
            d2c = -1*ca[ci]
        ca[ci] += d2c
        da[di] += -1*d2c
        transactions.append([d[di],d2c,c[ci]])
        if ca[ci]==0: ci += 1
        if da[di]==0: di += 1
    max_transfer = max([transactions[i][1] for i in range(len(transactions))])
    number_format_str = ' --({:' + str(dpadd+len(str(int(max_transfer)))) + '.' + str(int(decimal_precision)) + 'f})--> '
    for i in range(len(transactions)):
        cost_str += name_format_lstr.format(transactions[i][0]) + number_format_str.format(transactions[i][1]) + name_format_rstr.format(transactions[i][2]) + '\n'
    return cost_str

sorting_on = False # order such that people who owe the most will pay first to who owes the most
p = ['Rick', 'Chris', 'Sohla', 'Brad']
x = [600, 300, 120, 0]
dp = 0

pstr = caclulate_cost_distribution(p,x,decimal_precision=dp,sorting_on=sorting_on)
print(pstr)

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