DataManipulation.txt

In this lesson, we'll write a script to import data from a text file, process the data, and plot it.
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From the Notebook:
	Data...
	Upload or create file...
	Select data.txt

The contents of data.txt will appear in the Notebook.
Click on the 'Worksheet' button to return to the Worksheet.

To access the contents of data.txt, type DATA+'data.txt'.
If you evaluate this expression, it will list the location of this file.
You can use this to open the file as you normally would in Python.
To open data.txt in read-only mode and assign it to variable 'dataFile':
	dataFile = open(DATA+'data.txt', 'r')
To read the first line of data.txt and assign its contents to variable 'line':
	line = dataFile.readline()
You can verify that this worked by showing 'line:
	show(line)
	>>> 0;0.1
This data is semicolon-separated; we can make use of this fact to move the data from data.txt into an array for further manipulations and graphing:
	while(line != '')
		xy = line.split(',')
		xList.append(xyList[0])
		yList.append(xyList[1])
		line=dataFile.readline()

Take a look at xArray to verify that your data is in there:
	show(xList)

If you attempt to sum xArray, you'll run into a problem:
	sum(xList)
	>>> TypeError: unsupported operand type(s) for +: 'int' and 'str'

A TypeError? Let's check the type of the elements of xArray:
	typeList = []
	for item in xList:
		typeList.append(type(item))
	show(typeList)

Turns out, all of your elements are of type 'str' (string), not numbers!
(You could have also figured this out by identifying the line in which the problem was occurring by clicking to the left of the error traceback; 'sum()' was the source of the error!)

We can fix this by adding one thing to the loop that reads in the data:
	DATA+'data.txt'
	dataFile = open(DATA+'data.txt', 'r')
	line = dataFile.readline()
	#show(line)

	xList = []
	yList = []
	while(line != ''):
		xy = line.split(',')
		xList.append(float(xy[0]))
		yList.append(float(xy[1]))
		line=dataFile.readline()
	sum(xArray)
	>>> 55.0
'float()' recasts its argument to a floating-point number.  No problems!

Now that we have our x and y datapoints in lists, how do we combine them to plot the data?
With a powerful built-in function called 'zip()'!
'zip()' takes lists as arguments and returns a single list that combines the elements in each list's i'th position into a tuple.
A tuple is a datatype composed of a series of values separated by commas.
See it in action:
	zippedList = zip(xList,yList)
	show(zippedList)
	>>> [(0.0,0.0),(1.0,1.0),...,(10.0,5.0)]

Ready for plotting... with 'list_plot()'!
	p = list_plot(zippedList)
	show(p)
That plot's a little large, so let's shrink it down a bit:
	p = list_plot(zippedList,figsize=(4,3))
	show(p)

To learn more about plotting options, see: http://learningvirus.com/sage/plotting.html