IRR/IRRv7 at master · JasonRobertC/IRR · GitHub

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# v0.0 - non-working attempt
# v0.1 - non-working attempt
# v0.2 - non-working attempt
# v0.3 - first working version (uses Numpy add-in)
# v0.3n - re-wrote to avoid using the Numpy add-in
# v0.4 - rebuild to make the long-form code into functions (upStep & downStep)
# v0.5 - replace all repetitive statements in the IRR evaluation section with a funcion
# v0.6 - addded error trapping on inputs
# v0.7 - improved the initial input section (no longer requires user to pre-specify number of cash flows)

print('\nInternal Rate of Return (IRR) Calculator v0.7')
print('By Jason R. Carroll\n')

# build the list of cash flows from user input

cf = []
i = 1
inputErrorMsg = ('\n---- Invalid entry, please try again.\n')

while True:
    try:
        c0 = float(input('Enter initial investment (time-zero).\nInput as negative value: '))
        cf.append(c0)
        if c0 < 0:
            break
        print(inputErrorMsg)
        cf = []
    except ValueError:
        print(inputErrorMsg)
        cf = []

print('\nEnter future cash flows (input blank line to finish).')

while True:
    try:
        c = float(input('Cash Flow ' + str(i) + ': '))
    except ValueError:
        print('\nOK, ' + str(i) + ' total cash flows (including initial investment).')
        break
    cf.append(c)
    i = i + 1

# define functions required for IRR evaluation

def message(result):
    print('Converging on answer... %.10f' % (result * 100) + '%.')

def upStep(increment):
    global r
    pv = []
    while True:
        r = r + increment
        for i in range(len(cf)):
            pva = cf[i] / (( 1 + r ) ** i )
            pv.append(pva)
        # print(pv)
        # print(sum(pv))
        if sum(pv) < 0:
            break
        pv = []

def downStep(increment):
    global r
    pv = []
    while True:
        r = r - increment
        for i in range(len(cf)):
            pva = cf[i] / (( 1 + r ) ** i )
            pv.append(pva)
        # print(pv)
        # print(sum(pv))
        if sum(pv) > 0:
            break
        pv = []

# evaluate the IRR

print()
r = 0
upInc = 0.1
dnInc = 0.01

while True:
    upStep(upInc) # "upward" pass
    message(r)
    downStep(dnInc) # "downward" pass
    message(r)
    upInc = upInc / 100
    dnInc = dnInc / 100
    if dnInc <= 0.0000000000001: # stop when you get to the ten-trillionths place
        break

# output the result

answer = round(r * 100, 12)
print('\nFor the cash flows: ' + str(cf) + ' ...')
print('... the IRR is ' + str(answer) + '%.')
print('\nHave a nice day.\n')
input('[Press Enter to terminate program] ... ')