I'm getting mad with list indexes, and can't explain what I'm doing wrong.
I have this piece of code in which I want to create a list of lists, each one containing values of the same circuit parameter (voltage, current etc..) that I'm reading from a csv file that looks like this:
Sample, V1, I1, V2, I2
0, 3, 0.01, 3, 0.02
1, 3, 0.01, 3, 0.03
And so on. What I want is to create a list that for example contains V1 and I1 (but I want to chose interactively) in the form [[V1], [I1]], so:
[[3,3], [0.01, 0.01]]
The code that I'm using is this:
plot_data = [[]]*len(positions)
for row in reader:
for place in range(len(positions)):
value = float(row[positions[place]])
plot_data[place].append(value)
plot_data is the list that contains all the values, while positions is a list with the indexes of the columns that I want to copy from the .csv file. The problem is that if I try the commands in the shell, seems to work, but if I run the script instead of appending each value to the proper sub-list, it appends all values to all lists, so I obtain 2 (or more) identical lists.
Python lists are mutable objects and here:
plot_data = [[]] * len(positions)
you are repeating the same list len(positions) times.
>>> plot_data = [[]] * 3
>>> plot_data
[[], [], []]
>>> plot_data[0].append(1)
>>> plot_data
[[1], [1], [1]]
>>>
Each list in your list is a reference to the same object. You modify one, you see the modification in all of them.
If you want different lists, you can do this way:
plot_data = [[] for _ in positions]
for example:
>>> pd = [[] for _ in range(3)]
>>> pd
[[], [], []]
>>> pd[0].append(1)
>>> pd
[[1], [], []]
import csv
cols = [' V1', ' I1'] # define your columns here, check the spaces!
data = [[] for col in cols] # this creates a list of **different** lists, not a list of pointers to the same list like you did in [[]]*len(positions)
with open('data.csv', 'r') as f:
for rec in csv.DictReader(f):
for l, col in zip(data, cols):
l.append(float(rec[col]))
print data
# [[3.0, 3.0], [0.01, 0.01]]
Source: Stackoverflow.com