Fetching column values in Panda DataFrame using while loop

Question

I have created a panda DataFrame with columns as: "Radius", "Zenith", "Tem". Now I want to retrieve all the temperature values based on the zenith values in the DataFrame. Next thing is to fetch the max and min value of "Tem" for each value of the zenith angle.

I have written the below code block but it is throwing me ValueError: The truth value of a DataFrame is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

theta = np.around(np.arange(0,90,90/151),6)
a = np.ndarray(shape=(1,2), dtype=float, order='C')

for i in theta:
    while (final_data[final_data['zenith'] == i]):
        T = final_data[['Tem']].values
        T_max = np.max(T)
        T_min = np.min(T)
        T_range = np.row_stack((a,[[T_max,T_min]]))

Where is the dataframe? You should show an example of that and the expected output. I don't think you should be iterating at all here — roganjosh
– roganjosh, Commented May 14, 2020 at 11:55
@roganjosh You are right. I was stupid to iterate it. I figured it out. I directly used the index and it worked. for i in theta: final_data[final_data['zenith'] == i] T = final_data[['Tem']].values T_max = np.max(T) T_min = np.min(T) ` T_limit = np.row_stack((a,[[T_max,T_min]]))` Thanks a lot for the help. — Baba_Voss
– Baba_Voss, Commented May 14, 2020 at 12:14

Arne · Accepted Answer · 2020-05-14 12:14:01Z

1

These kinds of operations are already implemented in pandas:

import pandas as pd

# example data
df = pd.DataFrame({'Zenith': [70, 70, 70, 80, 80, 80], 
                   'Tem':    [20, 21, 22, 23, 24, 25]})

# get all temperatures for a given zenith value (e.g. 80)
df.Tem[df.Zenith == 80]

3    23
4    24
5    25
Name: Tem, dtype: int64

# get the minimum temperature for each zenith value
df.groupby('Zenith').min()

       Tem
Zenith  
70     20
80     23

# get the maximum temperature for each zenith value
df.groupby('Zenith').max()

       Tem
Zenith  
70     22
80     25

answered May 14, 2020 at 12:14

Arne

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1 Comment

Baba_Voss Over a year ago

Thanks a lot. It's a big help, you saved me lot of time and effort.

kait · Accepted Answer · 2020-05-14 15:35:00Z

1

You could use this to get the min, max, and range

import pandas as pd

df = pd.DataFrame({'Zenith': [70, 70, 70, 80, 80, 80],
                   'Tem':    [20, 21, 22, 23, 24, 25]})

df.groupby('Zenith')['Tem'].agg([
    ('T_min',  'min'),
    ('T_max',  'max'),
    ('T_range', lambda x: x.max() - x.min())
])

Returns

        T_min  T_max  T_range
Zenith
70         20     22        2
80         23     25        2

answered May 14, 2020 at 15:35

kait

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