Negative indexing/selecting working and not working

[deanhystad]

Maybe this will make the indexing clearer.

I made a small change to your code. Instead of computing a numeric type_code I left type_code as a string ("Strongly disagree"...). Now when compute_percentages() reindexes the dataframe to use the "type_code", the row indices are words, not ints. This required a change to the "percentage_end" calculation because there is no perc[-2], perc[-1], or perc[0]. These are now perc["Strongly disagree"], perc["Disagree"] and perc["Neither agree nor disagree"].

source["type_code"] = source["type"]  # Changed so type_code is words, not -2, -1, 0, 1, 2

def compute_percentages(df):
    # Set type_code as index and sort
    df = df.set_index("type")

    # Compute percentage of value with question group
    perc = (df["value"] / df["value"].sum()) * 100
    df["percentage"] = perc

    # Compute percentage end, centered on "Neither agree nor disagree" (type_code 0)
    # Notice that we have to use words to index perc, because the row indices are words, not numbers.
    df["percentage_end"] = perc.cumsum() - (
        perc["Strongly disagree"]
        + perc["Disagree"]
        + perc["Neither agree nor disagree"] / 2
    )

    # Compute percentage start by subtracting percent
    df["percentage_start"] = df["percentage_end"] - perc

    return df

When computing perc outside the dataframe, you still created a series. The row indices just happened to be numbers from 0 to 39, but they were still keys, not positions in some array or list.

One more attempt to make this clear. Here I sort the source dataframe by the "value" column.

source = source.sort_values("value")
print(source)

Output:
      question  value                   type_code  percentage  percentage_end  percentage_start
30  Question 7      0           Strongly disagree    0.000000      -10.000000        -10.000000
37  Question 8      0  Neither agree nor disagree    0.000000        0.000000          0.000000
36  Question 8      0                    Disagree    0.000000        0.000000          0.000000
35  Question 8      0           Strongly disagree    0.000000        0.000000          0.000000
34  Question 7      0              Strongly agree    0.000000       90.000000         90.000000
31  Question 7      0                    Disagree    0.000000      -10.000000        -10.000000
38  Question 8      0                       Agree    0.000000        0.000000          0.000000
7   Question 2      0  Neither agree nor disagree    0.000000        0.000000          0.000000
15  Question 4      0           Strongly disagree    0.000000      -15.625000        -15.625000
20  Question 5      0           Strongly disagree    0.000000      -10.416667        -10.416667
11  Question 3      0                    Disagree    0.000000      -10.000000        -10.000000
17  Question 4      1  Neither agree nor disagree    6.250000        3.125000         -3.125000
32  Question 7      1  Neither agree nor disagree   20.000000       10.000000        -10.000000
25  Question 6      1           Strongly disagree    6.250000      -12.500000        -18.750000
26  Question 6      1                    Disagree    6.250000       -6.250000        -12.500000
21  Question 5      1                    Disagree    4.166667       -6.250000        -10.416667
39  Question 8      2              Strongly agree  100.000000      100.000000          0.000000
14  Question 3      2              Strongly agree   20.000000       70.000000         50.000000
27  Question 6      2  Neither agree nor disagree   12.500000        6.250000         -6.250000
12  Question 3      2  Neither agree nor disagree   20.000000       10.000000        -10.000000
10  Question 3      2           Strongly disagree   20.000000      -10.000000        -30.000000
6   Question 2      2                    Disagree    9.090909        0.000000         -9.090909
5   Question 2      2           Strongly disagree    9.090909       -9.090909        -18.181818
16  Question 4      2                    Disagree   12.500000       -3.125000        -15.625000
22  Question 5      3  Neither agree nor disagree   12.500000        6.250000         -6.250000
29  Question 6      3              Strongly agree   18.750000       81.250000         62.500000
13  Question 3      4                       Agree   40.000000       50.000000         10.000000
33  Question 7      4                       Agree   80.000000       90.000000         10.000000
24  Question 5      4              Strongly agree   16.666667       89.583333         72.916667
19  Question 4      6              Strongly agree   37.500000       84.375000         46.875000
18  Question 4      7                       Agree   43.750000       46.875000          3.125000
8   Question 2      7                       Agree   31.818182       31.818182          0.000000
28  Question 6      9                       Agree   56.250000       62.500000          6.250000
9   Question 2     11              Strongly agree   50.000000       81.818182         31.818182
23  Question 5     16                       Agree   66.666667       72.916667          6.250000
0   Question 1     24           Strongly disagree    0.746501      -18.382582        -19.129082
1   Question 1    294                    Disagree    9.144635       -9.237947        -18.382582
4   Question 1    376              Strongly agree   11.695179       80.870918         69.175739
2   Question 1    594  Neither agree nor disagree   18.475894        9.237947         -9.237947
3   Question 1   1927                       Agree   59.937792       69.175739          9.237947

Now 30 is the index of the first row and 3 is the index of last row. If I print values[3] it prints the last value, not the first

values = source["value"]
print(values[3])

Output:
1927

Notice that it prints the value from the last row, not the 4th row.