Python Pandas, using the previous value in Dataframe

Question

I have a function that use b(t-1) variable like:

def test_b(a,b_1):
     return a + b_1

Assume the following dataframe:

df = pd.DataFrame({'a':[1,2,3],'b':np.nan})

I am assigning the b_1 initial value:

df['b'].ix[0]=0

and then (using my Matlab experience), i use the loop:

for i in range(1,len(df)):
    df['b'].ix[i] = test_b(df['a'].ix[i],df['b'].ix[i-1])

output:

  a|b
0|1|0
1|2|2
2|3|5

Is it a more elegant way to do the same?

jeff, i getting the output i want. I just want to see if another way exist with pandas , like apply or shift. I tried but i couldn't figure out — Stavros Anastasiadis
– Stavros Anastasiadis, Commented Jun 2, 2014 at 14:08
no, what I mean is best to actually show the output in the question — Jeff
– Jeff, Commented Jun 2, 2014 at 14:09

Community · Accepted Answer · 2017-05-23 12:20:55Z

1

You never want to do assignments like this, as this is chained indexing

This is a recurrent relation, so not easy way ATM to do this in a super performant manner, though see here.

here is an open issue about this with a pointer to this which uses ifilter to solve the relation.

edited May 23, 2017 at 12:20

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answered Jun 2, 2014 at 14:33

Jeff

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3 Comments

Woody Pride Over a year ago

for x in df.index[1:]: df.loc[x, 'b'] = df.loc[x, 'a'] + df.loc[x - 1, 'b']..... Is this also bad? I've never quite got to the bottom of this issue...

Woody Pride Over a year ago

but I am terrified of it!

Jeff Over a year ago

that's ok, but would be really slow on a big frame (better to create a list then assign it all at once

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