The if() function

The present value of a growing perpetuity has the following formula:

Here, C is the first cash flow occurring at the end of the first period, R is the effective periodic rate, and g is the constant growth rate. The second and the third future cash flows will be and , respectively. A necessary condition for the correctness of equation (3) is that the discount rate should be greater than the growth rate, that is, R should be greater than g. What is the present value if C is $10, R is 10 percent, and g is 12 percent? The wrong answer is -500. For these similar cases, we could use the if() function to print an error message instead of offering the wrong answer, as shown in the following code:

def pv_growing_perpetuity(c,r,g):
    if(r<g):
        print("r<g !!!!")
    else:
        return(c/(r-g))    

We could try different sets of input values, as shown in the following code:

>>>pv_growing_perpetuity(10,0.1,0.08)
499.9999999999999
>>>pv_growing_perpetuity...