How to make a new column with a specific condition?

I want to make a new column with this condition:

• If the the value on Case Number column in current row equals with the previous row, then the value should be taken from column ‘diff’
• If the current row is not equal with the previous row, then the value should be taken from ‘lastmod-start’ column.

This is the code that I’ve tried :

df['new'] = [None] * len(df)
for i in range(1,len(df)):
    if df['Case Number'][i] == df['Case Number'][i-1]:
        df['new'][i] = df['diff']
    else:
        df['new'][i] = df['lastmod-start']

However the code above is resulting an error.
Is the anyone can help me? Thank you.

This is the screenshot The result

>Solution :

First, you can assign a new column as a single value and pandas will broadcast it to the entire column. Since the 'new' column will be string, you can just initialize it to an empty string.

df['new'] = ''

Next, if you want to compare each row to the row before it, you can use the .shift() method create a boolean index of which rows match. Then use that index to assign the values.

ix = df['Case Number'] == df['Case Number'].shift()
df.loc[ix, 'new'] = 'diff'
df.loc[~ix, 'new'] = 'lastmod-start'