#### Overview of this book

Python for Finance
Credits
Acknowledgments
www.PacktPub.com
Preface
Free Chapter
Introduction and Installation of Python
13 Lines of Python to Price a Call Option
Introduction to Modules
Statistical Analysis of Time Series
Index

## 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...```