Fill empty slot with value by checking the line before

Question

I have a DataFrame like this:

day  value  
1 HSE  
2 HSE   
3  
4  
5  
6 LSE  
7 LSE  
8  
9  
10  

Now, I want to fill the empty slots with values by checking the value before. So, I want 3,4,5 set to "fromHSE"and 8,9,10 "fromLSE".

I tried it like this:

e = "HSE"

for line in df:
    if df['value'] == "":
        if e == "HSE":
            df['value'] = "fromHSE"
        elif e == "LSE":
            df['value'] = "fromLSE"
    elif df['value'] == "HSE":
        e = "HSE"
    elif df['value'] == "LSE":
        e = "LSE"

But then i get the error:

ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

I hope you can help me.

Answer 1

You can first replace empty string to NaN, create mask with isnull and create new Series with ffill. Last add string from with mask:

df.value.replace('',np.NaN, inplace=True)
mask = df.value.isnull()
new = df.value.ffill()
print (new.mask(mask, 'from' + new))
0        HSE
1        HSE
2    fromHSE
3    fromHSE
4    fromHSE
5        LSE
6        LSE
7    fromLSE
8    fromLSE
9    fromLSE
Name: value, dtype: object