python - Adding a DataFrame column with len() of another column's values

Question

I'm having a problem trying to get a character count column of the string values in another column, and haven't figured out how to do it efficiently.

for index in range(len(df)):
    df['char_length'][index] = len(df['string'][index]))

This apparently involves first creating a column of nulls and then rewriting it, and it takes a really long time on my data set. So what's the most effective way of getting something like

'string'     'char_length'
abcd          4
abcde         5

I've checked around quite a bit, but I haven't been able to figure it out.

Answer 1

Pandas has a vectorised string method for this: str.len(). To create the new column you can write:

df['char_length'] = df['string'].str.len()

For example:

>>> df
  string
0   abcd
1  abcde

>>> df['char_length'] = df['string'].str.len()
>>> df
  string  char_length
0   abcd            4
1  abcde            5

This should be considerably faster than looping over the DataFrame with a Python for loop.

Many other familiar string methods from Python have been introduced to Pandas. For example, lower (for converting to lowercase letters), count for counting occurrences of a particular substring, and replace for swapping one substring with another.