I am new in using python for data science.
What is the difference between selecting a a column with: df['name'].values and df.iloc[:,1].values and df.iloc[:,1:2].values they return differnt types of numpy vectors. why?
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3$\begingroup$ assuming 'name' is the second column. they should be identical. please includes some example codes, so we can help you. $\endgroup$– Louis TCommented Feb 10, 2019 at 2:03
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1 Answer
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Not entirely sure what you mean by "numpy vectors" but am assuming the question is why each of these methods return essentially (almost but not quite) the same output...
Reference: pandas docs.
df['name'].values is a "Series corresponding to colname". In other words, you're just calling the data from that column and putting the in an array by calling .values.
.iloc is a "Purely integer-location based indexing for selection by position". Same as above but you're calling the indexed location of the column where df.iloc[:, 1] is df.iloc[all rows, col 2]. Probably an easier method to call multiple consecutive columns in a DataFrame then writing out each individual column name.
df.iloc[:,1:2].values <-- creates an array of arrays where the main array is the column that you called (col2) and each row values is contained in a subarray. This is--I think-- because you're slicing the dataframe between column index locations 1 and 2 (rather than just calling loc 1 like above). This would mean that each row is being called individually so that a new array is created for each row that exists between column index locations 1 and 2 (which is the 'parent' array).
