I am attempting to subtract values in a nested list (a list of historical stock price data from Yahoo finance) and I have been running into problems. I am attempting simple subtraction (i.e. high - low), but I am unable to implement this. I am probably missing something fundamental on the nature of lists, but I am stumped.
An example of the nested list I am using:
[['2012-07-31', '16.00', '16.06', '15.81', '15.84', '13753800', '15.8'],
['2012-07-30', '16.15', '16.15', '15.90', '15.98', '10187600', '15.9'],
['2012-07-27', '15.88', '16.17', '15.84', '16.11', '14220800', '16.1'],
['2012-07-26', '15.69', '15.88', '15.62', '15.80', '11033300', '15.8'],
['2012-07-25', '15.52', '15.64', '15.40', '15.50', '15092000', '15.5'],
['2012-07-24', '15.74', '15.76', '15.23', '15.43', '19733400', '15.4'],
['2012-07-23', '15.70', '15.81', '15.59', '15.76', '14825800', '15.7'],
['2012-07-20', '15.75', '15.94', '15.68', '15.92', '16919700', '15.9'],
['2012-07-19', '15.71', '15.86', '15.64', '15.73', '15985300', '15.7'],
...]
I want to subtract the 4th 'column' from the third 'column' and populate another list with the results (order IS important.) What is the best way to implement this?
In native Python, if you want to leave the nested list (call it 'table'; each list within it is 'row') intact, the concise, idiomatic way to create a list of differences is:
differences = [float(row[3]) - float(row[4]) for row in table]
so that differences[i] == table[i][3] - table[i][4].
If the numeric data in the table will be used by other code, you might want to convert the strings to floats within the table:
table = [[r[0], float(r[1]), float(r[2]),
float(r[3]), float(r[4]), r[5], float(r[6])] for r in table]
so that the differences table would just be created by
differences = [r[3] - r[4] for r in table]