understanding math errors in pandas dataframes

Question

I'm trying to generate a new column in a pandas dataframe from other columns and am getting some math errors that I don't understand. Here is a snapshot of the problem and some simplifying diagnostics...

I can generate a data frame that looks pretty good:

import pandas
import math as m

data = {'loc':['1','2','3','4','5'],
        'lat':[61.3850,32.7990,34.9513,14.2417,33.7712],
        'lng':[-152.2683,-86.8073,-92.3809,-170.7197,-111.3877]}
frame = pandas.DataFrame(data)

frame

Out[15]:
lat lng loc
0    61.3850    -152.2683    1
1    32.7990     -86.8073    2
2    34.9513     -92.3809    3
3    14.2417    -170.7197    4
4    33.7712    -111.3877    5
5 rows × 3 columns

I can do simple math (i.e. degrees to radians):

In [32]:
m.pi*frame.lat/180.

Out[32]:
0    1.071370
1    0.572451
2    0.610015
3    0.248565
4    0.589419
Name: lat, dtype: float64

But I can't convert from degrees to radians using the python math library:

 In [33]:
 m.radians(frame.lat)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-33-99a986252f80> in <module>()
----> 1 m.radians(frame.lat)

/Users/user/anaconda/lib/python2.7/site-packages/pandas/core/series.pyc in wrapper(self)
     72             return converter(self.iloc[0])
     73         raise TypeError(
---> 74             "cannot convert the series to {0}".format(str(converter)))
     75     return wrapper
     76 

TypeError: cannot convert the series to <type 'float'>

And can't even convert the values to floats to try to force it to work:

In [34]:

float(frame.lat)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-34-3311aee92f31> in <module>()
----> 1 float(frame.lat)

/Users/user/anaconda/lib/python2.7/site-packages/pandas/core/series.pyc in wrapper(self)
     72             return converter(self.iloc[0])
     73         raise TypeError(
---> 74             "cannot convert the series to {0}".format(str(converter)))
     75     return wrapper
     76 

TypeError: cannot convert the series to <type 'float'>

I'm sure there must be a simple explanation and would appreciate your help in finding it. Thanks!

Solution 1

math functions such as math.radians expect a numeric value such as a float, not a sequence such as a pandas.Series.

Instead, you could use numpy.radians, since numpy.radians can accept an array as input:

In [95]: np.radians(frame['lat'])
Out[95]: 
0    1.071370
1    0.572451
2    0.610015
3    0.248565
4    0.589419
Name: lat, dtype: float64

---

Only Series of length 1 can be converted to a float. So while this works,

In [103]: math.radians(pd.Series([1]))
Out[103]: 0.017453292519943295

in general it does not:

In [104]: math.radians(pd.Series([1,2]))
TypeError: cannot convert the series to <type 'float'>

---

math.radians is calling float on its argument. Note that you get the same error calling float on pd.Series([1,2]):

In [105]: float(pd.Series([1,2]))
TypeError: cannot convert the series to <type 'float'>

Solution 2

I had a similar issue but was using a custom function. The solution was to use the apply function:

def monthdiff(x):
    z = (int(x/100) * 12) + (x - int(x/100) * 100)
    return z

series['age'].apply(monthdiff)

Now, I have a new column with my simple (yet beautiful) calculation applied to every line in the data frame!