renaming the 0 column in a dataframe

[Led_Zeppelin]

In the following screenshot, I am trying to rename the first column (0th column) in the df2 dataframe to IndexData. The python code is shown to do this. I am assuming when the csv file is read it automatically sets index to be column 0. So, I have a defined index at column 0, by just reading the csv file.

However, it i not doing this as anyone can see from the df2.head() readout.

I am including a screenshot that shows my dilemma. The code as shown:

df2.rename({'Unamed:0':'IndexData'}, inplace=True)

does not through any errors. But it does not rename the 0 column in dataframe df2.

So, what am I doing wrong?

Any help appreciated. Thaks in advance.

Screenshot attached.

Respectfuly,

LZ

[deanhystad]

What you are calling "the first column (0th column)" is the index. The first column is "sensor_00".

You can give the index (actually the row axis) a name, but you aren't going to like how that is printed.

import pandas as pd

df = pd.DataFrame({"A": [1, 2, 3], "B": [1, 2, 3]}).rename_axis("Index")
print(df)

Output:
       A  B
Index
0      1  1
1      2  2
2      3  3

If you want an index-like thing that looks like a column, make a column with index-like values. When you print out the dataframe, hide the index.

import pandas as pd

df = pd.DataFrame({"Index": [1, 2, 3], "A": [1, 2, 3], "B": [1, 2, 3]})
print(df.to_string(index=None))

Output:
 Index  A  B
     1  1  1
     2  2  2
     3  3  3

Better yet, stop letting it bother you that your index isn't a column.

[rob101]

<removed>

[Led_Zeppelin]

I am not following you. I believe that when a csv file is read in Python that the first column by default is made the index. That is column 0. Thus, explicitly calling a column 0 an index is not needed.

Is this correct. Is column 0 automatically an Index column when the csv file is read???

Now if Col 0 in the newly created DataFrame is the index column it still has no name. So, the second task I want to do is give it a name.

I beleive that by doing that I can then plot data in the Dataframe with the index column being used. But first it must have a name.

I plan to make a lot of plots of sensor 0 to 51 in all. The index is needed to keep track of each sensor's output.

Actually, I only have about 17 sensors. I ran an importance study and only 17 sensors seemed made the cut.

Common sense prevailed.



Each index is a one-minute step in the plot that to see how that sensor's values change over time. I plan to make 17 plots, one for each sensor used.

Calling it an index is unnecessary. It will function as an index only when plotting.

Call it anything. It must have name. The name 0 Column will not do it.

i could plot each of the 17 sensors against time. But why? Each sensors reading is one minute apart, and this make things much easier. Why mess with time (hours, minutes, seconds) when the data is taken every minute (for each sensor) and it is indexed.

Any help appreciated.

Respectfully,

LZ

[deanhystad]

What you are calling "column0" is not a column, it is row headings/names, just line the first "row" is column headings/names. They do not have names. They do not need names. They are attributes of their axis.

If your concern is a label when plotting, provide a name for the X axis when you plot.

import pandas as pd
import matplotlib.pyplot as plt

df = pd.DataFrame({"A": [1, 3, 2], "B": [3, 2, 3]}).rename_axis("Minutes")
df.plot(y="A", xlabel="Minutes", ylabel="Some Unit")
plt.show()

Because no x data was provided, this will plot using 0, 1, 2... for x data. I also explicitly set x axis label to "Minutes".

[deanhystad]

I think I might understand now. I think your screenshot does not really apply to your question. In your screenshot you show a dataframe where the first column (column 0) is named sensor_00. I thought you wanted to rename the leftmost column of row index names. This is not a column, so it cannot be named. You can rename the axis, but the numbers [0, 1, 2, 3, ...] are row index names, not data.

Later you mention reading a csv file. I looked back at your original post and added this information to better understand how can you rename "Unamed:0". Now your question makes more sense. You wrote a dataframe to a CSV file and read it back. This added a new, unnamed column to the dataframe.

DataFrame.to_csv() has an "index" to write the column index or not. From the docs:

indexbool, default True
Write row names (index).

I always set this to False when writing a CSV. This creates a CSV file without the row index labels. This is better for opening in Excel, and I can easily reload the dataframe from the CSV file.

df.to_csv(filename, index=None)
df = pd.read_csv(filename)

pandas.read_csv() has a similar argument, index col. From the docs:

index_colint, str, sequence of int / str, or False, optional, default None
Column(s) to use as the row labels of the DataFrame, either given as string name or column index. If a sequence of int / str is given, a MultiIndex is used.

Note: index_col=False can be used to force pandas to not use the first column as the index, e.g. when you have a malformed file with delimiters at the end of each line.

If I include row index labels in the CSV, I can use this option to tell pandas that the first column in the CSV file are the row index values, not the first column of the dataframe.

df.to_csv(filename)  # Will write row index values to file
df = pd.read_csv(filename, index_col=0)  # Tell pandas the first "column" is the row index values

You want to avoid doing this:

df.to_csv(filename)  # Writes row index values to CSV file
df = pd.read_csv(filename)  # Treat first column in CSV file as data, not an index

Now the dataframe has an extra, unnamed column at the front.

You can give the column a name.

import pandas as pd
import matplotlib.pyplot as plt

df = pd.read_csv("data.txt").rename(columns={"Unnamed: 0": "Time"})
print(df)

Output:
   Time  A  B  C
0     0  2  5  5
1     1  1  1  5
2     2  4  4  3

This gives the column a name, but you've now promoted index values to the status of data.