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

kvorobiev · Accepted Answer · 2015-06-04 08:26:22Z

3

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.

answered Jun 4, 2015 at 8:26

kvorobiev

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Comments

EdChum · Accepted Answer · 2015-06-04 08:26:17Z

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

TessellatingHeckler · Accepted Answer · 2015-06-04 08:28:35Z

1

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.

answered Jun 4, 2015 at 8:28

TessellatingHeckler

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Comments

CoMartel · Accepted Answer · 2015-06-04 08:27:52Z

0

you are trying to check if df.b, a List, is equal to an int.

Try this:

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

answered Jun 4, 2015 at 8:27

CoMartel

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Comments

Matthew Runchey · Accepted Answer · 2015-06-04 08:28:48Z

0

Well, first off. you're trying to use a conditional on an entire dictionary!

You're basically saying "if this dicitonary == 2", which simply is an ambiguous statement because that dictionary (df.b) is a set of values, which doesn't necessarily evaluate to "true"...

df.b is

Honestly, based on your question, I'm not entirely sure what you're trying to evaluate! You should try printing df before you evaluate in the sub-ccomposition of df.b.

answered Jun 4, 2015 at 8:28

Matthew Runchey

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