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Exercises/ex1_4.md

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+\[ [Index](index.md) | [Exercise 1.3](ex1_3.md) | [Exercise 1.5](ex1_5.md) \]
+
+# Exercise 1.4
+
+*Objectives:*
+
+- Review of how to define simple functions
+- Exception handling
+
+*Files Created:* None
+
+*Files Modified:* `pcost.py`
+
+## (a) Defining a function
+
+Take the program `pcost.py` that you wrote in the last exercise and
+convert it into a function `portfolio_cost(filename)` that takes a
+filename as input, reads the portfolio data in that file, and returns
+the total cost of the portfolio as a floating point number. Once you
+written the function, have your program call the function by simply
+adding this statement at the end:
+
+```python
+print(portfolio_cost('Data/portfolio.dat'))
+```
+
+Run your program and make sure it produces the same output as
+before.
+
+
+## (b) Adding Error Handling
+
+When writing programs that process data, it is common to encounter
+errors related to bad data (malformed, missing fields, etc.).  Modify
+your `pcost.py` program to read the data file `Data/portfolio3.dat`
+and run it (hint: it should crash).
+
+Modify your function slightly so that it is able to recover from lines
+with bad data.  For example, the conversion functions `int()` and
+`float()` raise a `ValueError` exception if they can't convert the
+input.  Use `try` and `except` to catch and print a warning message
+about lines that can't be parsed.  For example:
+
+```
+Couldn't parse: 'C - 53.08\n'
+Reason: invalid literal for int() with base 10: '-'
+Couldn't parse: 'DIS - 34.20\n'
+Reason: invalid literal for int() with base 10: '-'
+...
+```
+
+Try running your program on the `Data/portfolio3.dat` file
+again.   It should run successfully despite printed warning messages.
+
+## (c) Interactive Experimentation
+
+Run your `pcost.py` program and call the
+`portfolio_cost()` function directly from the interactive
+interpreter.
+
+```python
+>>> portfolio_cost('Data/portfolio.dat')
+44671.15
+>>> portfolio_cost('Data/portfolio2.dat')
+19908.75
+>>>
+```
+
+Note: To do this, you might have to run python using the `-i`
+option.  For example:
+
+```
+bash % python3 -i pcost.py
+```
+
+We are going to be writing a lot of programs where you define
+functions and experiment interactively.  Make sure you know how to do
+this.
+
+\[ [Solution](soln1_4.md) | [Index](index.md) | [Exercise 1.3](ex1_3.md) | [Exercise 1.5](ex1_5.md) \]
+
+----
+`>>>` Advanced Python Mastery  
+`...` A course by [dabeaz](https://www.dabeaz.com)  
+`...` Copyright 2007-2023  
+
+![](https://i.creativecommons.org/l/by-sa/4.0/88x31.png). This work is licensed under a [Creative Commons Attribution-ShareAlike 4.0 International License](http://creativecommons.org/licenses/by-sa/4.0/)