A For loop with embedded if statement to update a dataframe

Question

I am a newcomer to python. I want to implement a "For" loop on the elements of a dataframe, with an embedded "if" statement.

Code:

import numpy as np
import pandas as pd

#Dataframes
x = pd.DataFrame([1,-2,3])
y = pd.DataFrame()

for i in x.iterrows():
    for j in x.iteritems():
        if x>0:
            y = x*2
        else:
            y = 0

With the previous loop, I want to go through each item in the x dataframe and generate a new dataframe y based on the condition in the "if" statement. When I run the code, I get the following error message.

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

Any help would be much appreciated.

If your data is one dimensional, as in your example, consider using a series instead. — 9769953
– 9769953, Commented Sep 28, 2018 at 10:58
@DanielRoseman when I do as you suggested, I get the error TypeError: '>' not supported between instances of 'tuple' and 'int' — iouraich
– iouraich, Commented Sep 28, 2018 at 10:58
@zipa The expected output would be a populated "y" dataframe with the same dimension as the "x" dataframe, but with values 2, 0, 6. — iouraich
– iouraich, Commented Sep 28, 2018 at 11:00

jezrael · Accepted Answer · 2018-09-28 10:58:47Z

3

In pandas is best avoid loops if exist vectorized solution:

x = pd.DataFrame([1,-2,3], columns=['a'])

y = pd.DataFrame(np.where(x['a'] > 0, x['a'] * 2, 0), columns=['b'])
print (y)
   b
0  2
1  0
2  6

Explanation:

First compare column by value for boolean mask:

print (x['a'] > 0)
0     True
1    False
2     True
Name: a, dtype: bool

Then use numpy.where for set values by conditions:

print (np.where(x['a'] > 0, x['a'] * 2, 0))
[2 0 6]

And last use DataFrame constructor or create new column:

x['new'] = np.where(x['a'] > 0, x['a'] * 2, 0)
print (x)
   a  new
0  1    2
1 -2    0
2  3    6

answered Sep 28, 2018 at 10:58

jezrael

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Comments

gyx-hh · Accepted Answer · 2018-09-28 11:01:36Z

0

You can try this:

y = (x[(x > 0)]*2).fillna(0)

answered Sep 28, 2018 at 11:01

gyx-hh

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