How to iterate over a pandas dataframe while referencing previous rows?

Question

I am iterating over a Python dataframe and finding it to be extremely slow. I understand that in Pandas you try to vectorize everything, but in this case I specifically need to iterate (or if it is possible to vectorize, I'm unclear how to do it).

The logic is simple: you have two columns "A" and "B" and a result column "signal." If A equals 1, then you set signal to 1. If B equals 1, then you set signal to 0. Otherwise, signals is whatever it was previously. In other words, column A is an "on" signal, column B is an "off" signal, and "signal" represents the state.

Here is my code:

def signals(indata):
    numrows = len(indata)
    data = pd.DataFrame(index= range(0,numrows))
    data['A'] = indata['A']
    data['B'] = indata['B']
    data['signal'] = 0


    for i in range(1,numrows):
        if data['A'].iloc[i] == 1:
            data['signal'].iloc[i] = 1
        elif data['B'].iloc[i] == 1:
            data['signal'].iloc[i] = 0
        else:
            data['signal'].iloc[i] = data['signal'].iloc[i-1]
    return data

Example input/output:

indata = pd.DataFrame(index = range(0,10))
indata['A'] = [0, 1, 0, 0, 0, 0, 1, 0, 0, 0]
indata['B'] = [1, 0, 0, 0, 1, 0, 0, 0, 1, 1]

signals(indata)

Output:

    A   B   signal
0   0   1   0
1   1   0   1
2   0   0   1
3   0   0   1
4   0   1   0
5   0   0   0
6   1   0   1
7   0   0   1
8   0   1   0
9   0   1   0

This simple logic takes my computer 46 seconds to run on a dataframe of 2000 rows with randomly generated data.

Answer 1

df['signal'] = df.A.groupby((df.A != df.B).cumsum()).transform('head', 1)

df
   A  B  signal
0  0  1       0
1  1  0       1
2  0  0       1
3  0  0       1
4  0  1       0
5  0  0       0
6  1  0       1
7  0  0       1
8  0  1       0
9  0  1       0

The logic here involves dividing your series into groups based on the inequality between A and B , and every group's value is determined by A .

Answer 2

You dont need to iterate at all you can do some Boolean indexing

#set condition for A
indata.loc[indata.A == 1,'signal'] = 1
#set condition for B
indata.loc[indata.B == 1,'signal'] = 0
#forward fill NaN values
indata.signal.fillna(method='ffill',inplace=True)

Answer 3

The simplest answer to my problem was to not write to the dataframe while iterating through it. I created an array of zeros in numpy, then did my iterative logic in the array. Then I wrote the array to the column in my dataframe.

def signals3(indata):
    numrows = len(indata)
    data = pd.DataFrame(index= range(0,numrows))

    data['A'] = indata['A'] 
    data['B'] = indata['B']
    out_signal = np.zeros(numrows)

    for i in range(1,numrows):
        if data['A'].iloc[i] == 1:
            out_signal[i] = 1
        elif data['B'].iloc[i] == 1:
            out_signal[i] = 0
        else:
            out_signal[i] = out_signal[i-1]


    data['signal'] = out_signal

    return data

On a dataframe of 2000 rows of random data, this takes only 43 milliseconds as opposed to 46 seconds (~1,000x faster).

I also tried a variant where I assigned the dataframe columns A and B to series, and then iterated through the series. This was a bit faster (27 milliseconds). But it appears most of the slowness is in writing to a dataframe.

Both coldspeed and djk's answers were faster than my solution (about 4.5ms) but in practice I'll probably just iterate through series even though that is not optimal.