python if statement returns value error

Question

I'm sorry to ask this apparently simple question, but I am a python beginner and couln't find an answer anywhere. I want to run a simple if statement, but python returns just:

    ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all(). 

No matter which of the given alternatives I apply - it just doesn't work. Do you have any ideas?

    import pandas as pd

    df = pd.DataFrame({'a': [None] * 4, 'b': [2, 3, 10, 3]})

    df["c"] =0    
    if df.b == 2:
        df.c =1

Answer 1

In if statement you're trying to compare an array with scalar. You can use something like

if df.b[0] == 2:
    df.c =1

or

if 2 in df.b:
    df.c =1

if you want to check for a specific element in the array.

Answer 2

You can't compare a scalar with an array like that using if it becomes ambiguous as what if one of the values matches or all but one you want to do something like:

In [3]:

df['c'] = np.where(df['b'] == 2, 1, np.NaN)
df
Out[3]:
      a   b   c
0  None   2   1
1  None   3 NaN
2  None  10 NaN
3  None   3 NaN

using np.where looks at the whole array for that condition and if True returns 1, otherwise NaN

You can perform comparisons using syntax like:

In [4]:

df['b'] == 2
Out[4]:
0     True
1    False
2    False
3    False
Name: b, dtype: bool

but not using if unless you are comparing a single value:

In [8]:

for index, row in df.iterrows():
    if row['b'] == 2:
        print('equals 2')
    else:
        print('some other value')
equals 2
some other value
some other value
some other value

Answer 3

You set df.b to be a list containing four numbers. It can never equal a single number.

And Pandas cannot handle comparing a list to anything using == because it's not clear what answer you want, so it's prompting you to explain.

http://pandas.pydata.org/pandas-docs/stable/gotchas.html

I don't think you are trying to do that at all, but it's not clear what you are expecting to happen.