为什么我只能在 python 中调用这个 .CSV 文件的第一列？

Question

I am trying to call the second column from this .CSV file in python so that I can carry out some data analysis with it, but I am only able to call the first column from this file ("Time_abs/ps"). I have tried calling both the second and third columns ("Signal/nA" and "Ref.signal/nA") individually in the same way as when I successfully call the first column, but I am receiving the error below whenever I try to do so. Does anyone know why this is happening and know how to work around this?

Input:

import pandas as pd
import io
 
df = pd.read_csv(io.BytesIO(uploaded['wf air.csv']))

print(df)
print(df['Time_abs/ps'])
print(df['Signal/nA'])

Output:

      Time_abs/ps   Signal/nA   Ref.signal/nA
0         1150.00    0.000865        0.001763
1         1150.05   -0.000280        0.002449
2         1150.10   -0.002557       -0.003643
3         1150.15   -0.002189       -0.003354
4         1150.20    0.004161        0.002784
...           ...         ...             ...
1396      1219.80    1.437238        1.439107
1397      1219.85    1.284854        1.286108
1398      1219.90    0.933574        0.934550
1399      1219.95    0.524213        0.528238
1400      1220.00    0.117463        0.120416

[1401 rows x 3 columns]
0       1150.00
1       1150.05
2       1150.10
3       1150.15
4       1150.20
         ...   
1396    1219.80
1397    1219.85
1398    1219.90
1399    1219.95
1400    1220.00
Name: Time_abs/ps, Length: 1401, dtype: float64
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
/usr/local/lib/python3.7/dist-packages/pandas/core/indexes/base.py in get_loc(self, key, method, tolerance)
   3360             try:
-> 3361                 return self._engine.get_loc(casted_key)
   3362             except KeyError as err:

4 frames
/usr/local/lib/python3.7/dist-packages/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()

/usr/local/lib/python3.7/dist-packages/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()

pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item()

pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item()

KeyError: 'Signal/nA'

The above exception was the direct cause of the following exception:

KeyError                                  Traceback (most recent call last)
<ipython-input-63-04bb76cf1fc2> in <module>()
      6 print(df)
      7 print(df['Time_abs/ps'])
----> 8 print(df['Signal/nA'])

/usr/local/lib/python3.7/dist-packages/pandas/core/frame.py in __getitem__(self, key)
   3456             if self.columns.nlevels > 1:
   3457                 return self._getitem_multilevel(key)
-> 3458             indexer = self.columns.get_loc(key)
   3459             if is_integer(indexer):
   3460                 indexer = [indexer]

/usr/local/lib/python3.7/dist-packages/pandas/core/indexes/base.py in get_loc(self, key, method, tolerance)
   3361                 return self._engine.get_loc(casted_key)
   3362             except KeyError as err:
-> 3363                 raise KeyError(key) from err
   3364 
   3365         if is_scalar(key) and isna(key) and not self.hasnans:

KeyError: 'Signal/nA'

Answer 1

You can access the first column by index using df.iloc() function.

Syntax to select column with index at position 0.

df.iloc[:, 0]

The columns in a data frame can be referenced by name (eg df['Time_abs/ps']), so possible that the CSV contents have whitespace before or after the column name in the first row. Dump the column names to verify they are as you expect them to be.

import pandas as pd
from io import StringIO

data = """Time_abs/ps,Signal/nA,Ref.signal/nA
1150.00,0.000865,0.001763
1150.05,-0.000280,0.002449
1150.10,-0.002557,-0.003643
1150.15,-0.002189,-0.003354
1150.20,0.004161,0.00278"""
 
df = pd.read_csv(StringIO(data))

# print(df)

# select column using name which returns a series
# print(df['Time_abs/ps'])
# print(df['Signal/nA'])

# select column with index position 0
col = df.iloc[:, 0]
print(col)

Output:

0    1150.00
1    1150.05
2    1150.10
3    1150.15
4    1150.20
Name: Time_abs/ps, dtype: float64