[Python] Update a dataframe in pandas while iterating row by row

I have a pandas data frame that looks like this (its a pretty big one)

date exer exp ifor mat
1092 2014-03-17 American M 528.205 2014-04-19
1093 2014-03-17 American M 528.205 2014-04-19
1094 2014-03-17 American M 528.205 2014-04-19
1095 2014-03-17 American M 528.205 2014-04-19
1096 2014-03-17 American M 528.205 2014-05-17


now I would like to iterate row by row and as I go through each row, the value of ifor in each row can change depending on some conditions and I need to lookup another dataframe.

Now, how do I update this as I iterate. Tried a few things none of them worked.

for i, row in df.iterrows():
if <something>:
row['ifor'] = x
else:
row['ifor'] = y

df.ix['ifor'] = x


None of these approaches seem to work. I don't see the values updated in the dataframe.

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