finding values in pandas series - Python3

问题

i have this excruciatingly annoying problem (i'm quite new to python)

df=pd.DataFrame[{'col1':['1','2','3','4']}]

col1=df['col1']

Why does col1[1] in col1 return False?

回答1:

For check values use boolean indexing:

#get value where index is 1
print (col1[1])
2 
#more common with loc
print (col1.loc[1])
2

print (col1 == '2')
0    False
1     True
2    False
3    False
Name: col1, dtype: bool

And if need get rows:

print (col1[col1 == '2'])
1    2
Name: col1, dtype: object

For check multiple values with or:

print (col1.isin(['2', '4']))
0    False
1     True
2    False
3     True
Name: col1, dtype: bool 

print (col1[col1.isin(['2', '4'])])
1    2
3    4
Name: col1, dtype: object

And something about in for testing membership docs:

Using the Python in operator on a Series tests for membership in the index, not membership among the values.

If this behavior is surprising, keep in mind that using in on a Python dictionary tests keys, not values, and Series are dict-like. To test for membership in the values, use the method isin():

For DataFrames, likewise, in applies to the column axis, testing for membership in the list of column names.

#1 is in index
print (1 in col1)
True

#5 is not in index
print (5 in col1)
False

#string 2 is not in index
print ('2' in col1)
False

#number 2 is in index
print (2 in col1)
True

You try to find string 2 in index values:

print (col1[1])
2

print (type(col1[1]))
<class 'str'>

print (col1[1] in col1)
False

来源：https://stackoverflow.com/questions/42697905/finding-values-in-pandas-series-python3