Use map instead of applymap (#330)

* Use map instead of applymap

* Update lectures/inflation_history.md

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Co-authored-by: mmcky <mamckay@gmail.com>
Co-authored-by: Matt McKay <mmcky@users.noreply.github.com>

lectures/inflation_history.md

@@ -259,7 +259,7 @@ def process_df(df):
     df = df[~df.index.duplicated(keep='first')]
     
     # Convert attribute values to numeric
-    df = df.applymap(lambda x: float(x) \
+    df = df.map(lambda x: float(x) \
                 if x != '—' else np.nan)
     
     # Finally, we only focus on data between 1919 and 1925
@@ -367,7 +367,7 @@ for i in range(4):
     # Apply process_entry on the selected sheet
     sheet_list = [
         pd.read_excel(xls, 'Table3.' + str(ind), 
-            header=1).iloc[:row].applymap(process_entry)
+            header=1).iloc[:row].map(process_entry)
         for ind, row in zip(indices, rows)]
     
     sheet_list = [process_df(df) for df in sheet_list]