I have a dataframe with 3 columns: day, morning and evening, where morning and evening are measurements taken.

I need to convert it into a dataframe with only day and measurement so that:

```
Day, Morning, Evening
[0, 2, 3]

```

becomes

```
Day,value
[0,2]
[0,3]

```

I can do it with a loop, but I was wondering if there is a more pythonic way to do it outside a loop? Especially because I will be repeating this over large(ish) datasets.

This is what I have put together:

```
import numpy as np
import pandas as pd

mor = np.linspace(80,85,10)
ev=np.linspace(82,87,10)

df=pd.DataFrame()
df['mor']=mor
df['ev']=ev
df.index.name='day'

x=np.zeros(mor.shape[0] * 2)*np.nan

for i in np.arange(0,len(mor)):
    x[2*i]=mor[i]
    x[2*i+1]=ev[i]

df_output =pd.DataFrame()
df_output['day']=np.repeat(df.index,2)
df_output['value']=x

```