Reference previous row when iterating through dataframe

Question

Is there a simple way to reference the previous row when iterating through a dataframe? In the following dataframe I would like column B to change to 1 when A > 1

Answer 1

This is what you are trying to do?

In [38]: df = DataFrame(randn(10,2),columns=list('AB'))

In [39]: df['B'] = np.nan

In [40]: df.loc[df.A<-1,'B'] = -1

In [41]: df.loc[df.A>1,'B'] = 1

In [42]: df.ffill()
Out[42]: 
          A  B
0 -1.186808 -1
1 -0.095587 -1
2 -1.921372 -1
3 -0.772836 -1
4  0.016883 -1
5  0.350778 -1
6  0.165055 -1
7  1.101561  1
8 -0.346786  1
9 -0.186263  1