Pandas pct change from initial value

Question

I want to find pct_change Dew_P Temp (C) from an initial value of -3.9. I want pct_change in a new column.

Source here:

 weather = pd.read_csv('https://raw.githubusercontent.com/jvns/pandas-cookbook/master/data/weather_2012.csv') weather[weather.columns[:4]].head() Date/Time Temp (C) Dew_P Temp (C) Rel Hum (%) 0 2012-01-01 -1.8 -3.9 86 1 2012-01-01 -1.8 -3.7 87 2 2012-01-01 -1.8 -3.4 89 3 2012-01-01 -1.5 -3.2 88 4 2012-01-01 -1.5 -3.3 88 

I tried variations of this loop (even if I add the index shown here), but to no avail:

 for index, dew_point in weather['Dew_P Temp (C)'].iteritems(): new = weather['Dew_P Temp (C)'][index] old = weather['Dew_P Temp (C)'][0] pct_diff = (new-old)/old*100 weather['pct_diff'] = pct_diff 

I think the problem is weather['pct_diff'] , it does not accept new , it takes the last value of the data frame and subtracts it from old

Therefore, its always (2.1-3.9) /3.9*100, so my percentage change is always -46%.

As a result, I want:

 Date/Time Temp (C) Dew_P Temp (C) Rel Hum (%) pct_diff 0 2012-01-01 -1.8 -3.9 86 0.00% 1 2012-01-01 -1.8 -3.7 87 5.12% 2 2012-01-01 -1.8 -3.4 89 12.82% 

Any ideas? Thanks!

Answer 1

You can use iat to access a scalar value (for example, iat[0] accesses the first value in the series).

 df = weather df['pct_diff'] = df['Dew_P Temp (C)'] / df['Dew_P Temp (C)'].iat[0] - 1 

Answer 2

IIUC you can do it like this:

 In [88]: ((weather['Dew Point Temp (C)'] - weather.ix[0, 'Dew Point Temp (C)']).abs() / weather.ix[0, 'Dew Point Temp (C)']).abs() * 100 Out[88]: 0 0.000000 1 5.128205 2 12.820513 3 17.948718 4 15.384615 5 15.384615 6 20.512821 7 7.692308 8 7.692308 9 20.512821 

Answer 3

I find it more elegant

 weather['Dew_P Temp (C)'].pct_change().fillna(0).add(1).cumprod().sub(1) 0 0.000000 1 -0.051282 2 -0.128205 3 -0.179487 4 -0.153846 Name: Dew_P Temp (C), dtype: float64 

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To get the expected result with absolute values

 weather['pct_diff'] = weather['Dew_P Temp (C)'].pct_change().fillna(0).add(1).cumprod().sub(1).abs() weather