Packages like Numpy, Pandas etc

pandas tricks

• pd.cut() helps you cut a series. There are two ways to do so
  • By specifying how many categories we need like so pd.cut(dataFrame, NO_OF_CATEGORIES)
  • By specifying the exact bins of categories like so pd.cut(dataFrame, bins=[0., 1.5, 3.0, 4.5, np.inf], labels= [1,2,3,4,5])
  • Labelling is done to give a name (in this case ordinal) to each of the categories

pd.DataFrame

• #concept axis refers to the row, column or a higher dimensional point (e.g. axis=0 refers to rows, axis=1 refers to columns. axis=3 can refer to time, if we have data across years and each row corresponds to the same entity.)

• loc and iloc have this commonly known distinction that the first is used to index by label but the latter by index.

  • But also know that when we take a subset of the dataframe (say, main) (lets say while test-train splitting of the data), the original index values of main df is preserved as the labels of the subsets.
  • e.g. main = [1,2,3,4,5,6,7] are the default labels (for an unlabelled df main), and upon splitting it we get subsets A = [1,3,4,5] and B = [2,6,7]
  • So now note that we cannot do A.loc[range(1,5)] since we don’t have label 2 in our A. But A.iloc[range(1,5)] will definitely work since indices are always in sequence.

Tricks

• list(df) gives us a list of all the columns of the pd.DataFrame. Instead of using df.columns which gives us an pd.Index of column names.

pd.Series

• pd.Series.apply() applies a function to each element of the series and returns another series with the output of each of those function call in the same order.

• if you want the “label” to each of the items in a pandas series, its actually called index

pd.plotting

python
housing.plot(kind="scatter",x='longitude',y='latitude',alpha=.4,
             s=housing['population']/100, label="population",
             c="median_house_value", colorbar=True, cmap=plt.get_cmap('jet'))

• s is the radius of the circle.
• alpha opacity.
• c is the color (always give the name of the property (e.g. "population" instead of housing['population'] whenever possible, here we needed to make an adjustment to the size).
• cmap gives us the color map.

scatter_matrix is used to plot correlation maps.

numpy tricks

np.ndarray

• N-dimensional array
• Use iloc to reference by index
• Use loc to reference by label (==check this==: It is also possible to pass in an index, if label for the data is not available)

Intuition

Indices

• What does X[:,:] mean?
• How did you say all rows? How did you say all columns?
• So, what is X[a:b,:], (where a and b are integers)?
  • Rows/Records are lower dimensional, so the first index (here a:b) always refer to rows. This is what we call axis=0
  • Columns/Headers/Attributes are higher dimensional, so the second index (here :) always refer to column. This is what we call axis=1
  • Thus, X[a:b,:] means, “select all columns from the records whose indices start from a and do not equal or exceed b”
  • Similarly, X[:,:c] means “select those columns with index c from all records”

General Tricks

Typecasting

• pandas.core.series.Series.astype(np.int8) to convert string to integers (or float to integers)