\[ [Index](index.md) | [Exercise 1.3](ex1_3.md) | [Exercise 1.5](ex1_5.md) \]

# 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.

\[ [Solution](soln1_4.md) | [Index](index.md) | [Exercise 1.3](ex1_3.md) | [Exercise 1.5](ex1_5.md) \]

----
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