Categories in Python with Pandas (special case)

Question

I have some data and want to build some categories.

Now, the data looks like this:

Var     Category
a         cat1
a         cat1
b         cat2
a         cat1
b         cat2
a         cat1

But it should look like this:

Var     Category
  a         cat1
  a         cat1
  b         cat2
  a         cat2
  b         cat3
  a         cat3

So, whenever 'Var' != 'a' 'Category' should move on to the next category and so on. How could I do this?

Answer 1

You can compare for not equal and then add cumulative sum by Series.cumsum, add 1 if necessary, convert to strings and add to cat:

df['Category'] = 'cat' + df.Var.ne('a').cumsum().add(1).astype(str)

Alternative:

df['Category'] = 'cat' + (df.Var != 'a').cumsum().add(1).astype(str)
print (df)
  Var Category
0   a     cat1
1   a     cat1
2   b     cat2
3   a     cat2
4   b     cat3
5   a     cat3