Lecture 14 Data I/O and Files

1. Read/Write a data file
While a program is running, its data is in memory. When the program ends, or the computer shuts down, data in memory disappears. To store data permanently, you have to put it in a
file. Files are usually stored on a hard drive.
By reading and writing files, programs can exchange information with each other and generate printable formats like PDF.

All of this applies to files as well. To open a file, you specify its name and indicate whether you want to read or write. Opening a file creates a file object. In this example, the variable f refers to the new file object.

The open function takes two arguments. The first is the name of the file, and the second is the mode. Mode "w" means that we are opening the file for writing.

If there is no file named test.dat, it will be created (this only happens when the mode is 'w'). If there already is one, it will be replaced by the file we are writing

In order to put values into a file, the values must be in the 'string' format. 



The mode argument is "r" for reading:


Now let's write something to the data file:


Please note that if you don't do 'f.close()' in the end, the text won't be written into the text file.

Not surprisingly, the
read method reads data from the file. With no arguments, it reads the entire contents of the file:
Now, let's read it and print it in the Console:


You must do 'f.close()' every time you change the mode to either 'w' or 'r' in order to see the data in the text file.


You only read 5 characters:


The following function copies a file, reading and writing up to fifty characters at a time. The first argument is the name of the original file; the second is the name of the new file:


2. Writing variables


3. Format operator, %
In Matlab, the format operator is '\', but in Python, it is '%':

The '%d' type is formatting the data into a decimal number or an integer.
The format sequence "%f" formats the next item in the tuple as a floating-point number, and "%s" formats the next item as a string:

One more example:


The format operator enables the variables to be inserted into a string.
For more control over the format of numbers, we can specify the number of digits as part of the format sequence:

The number after the percent sign is the minimum number of spaces the number will take up. If the value provided takes fewer digits, leading spaces are added. If the number of spaces is negative, trailing spaces are added:



For example, imagine a dictionary that contains student names as keys and hourly wages as values. Here is a function that prints the contents of the dictionary as a formatted report:

By controlling the width of each value, we guarantee that the columns will line up, as long as the names contain fewer than twenty-one characters and the wages are less than one billion dollars an hour.

In order to put values into a file, you must convert the values to strings. You have already seen how to do that with str:



You can convert them into the string type as well:


Or put a list in to a file:


4. Exception

Whenever a runtime error occurs, it creates an exception. Usually, the program stops and Python prints an error message.

Sometimes we want to execute an operation that could cause an exception, but we don’t want the program to stop. We can handle the exception using the try and except statements. For example, we might prompt the user for the name of a file and then try to open it. If the file doesn’t exist, we don’t want the program to crash; we want to handle the exception:

Sometimes we want to execute an operation that could cause an exception, but we don’t want the program to stop. We can handle the exception using the try and except statements.

For example, we might prompt the user for the name of a file and then try to open it. If the file doesn’t exist, we don’t want the program to crash; we want to handle the exception:
The following function opened an existing data file 'test.dat', and read the data in it, and print it out. However, if the data file like 'testie.dat' doesn't exist, the IOError will be handled by the program and report the response you defined in the program: 'There is no file named testie.dat'.





Tasks:

1. Convert the data in the following table into a dictionary in Python. Write the data into an external/independent '.dat' file.


2. Modify the code to re-align the data in the data file in the format as follows:


3. Create a function to copy the 'test.dat' to a new data file 'test_2.dat'. (Do not mannually copy it over)

4. If we say if the monthly salary is > 5.5 k, then this employee is in the middle-class. Use the same data given in Problem 1, create a new dictionary called 'middleClass' to classify the four employees into two 'keys' - 'overMiddleClass' and 'belowMiddleClass'. Save this new dictionary into an external data file called 'test_3.dat'.