substring of an entire column in pandas dataframe

I have a pandas dataframe "df". In this dataframe I have multiple columns, one of which I have to substring. Lets say the column name is "col". I can run a "for" loop like below and substring the column:

for i in range(0,len(df)):
  df.iloc[i].col = df.iloc[i].col[:9]

But I wanted to know, if there is an option where I don't have to use a "for" loop, and do it directly using an attribute.I have huge amount of data, and if I do this, the data will take a very long time process.

Use the str accessor with square brackets:

df['col'] = df['col'].str[:9]

Or str.slice:

df['col'] = df['col'].str.slice(0, 9)

In case the column isn't a string, use astype to convert it:

df['col'] = df['col'].astype(str).str[:9]

I needed to convert a single column of strings of form nn.n% to float. I needed to remove the % from the element in each row. The attend data frame has two columns.

attend.iloc[:,1:2]=attend.iloc[:,1:2].applymap(lambda x: float(x[:-1]))

Its an extenstion to the original answer. In my case it takes a dataframe and applies a function to each value in a specific column. The function removes the last character and converts the remaining string to float.