Python下Pandas的14个最佳特色功能.doc

Python下Pandas的14个最佳特色功能14 Best Python Pandas FeaturesPandas is the most widely used tool for data munging. It contains high-level data structures and manipulation tools designed to make data analysis fast and easy.In this post, I am going to discuss the most frequently used pandas features. I will be using olive oil data set for this tutorial, you can download the data set from thispage(scroll down todatasection). Apart from serving as a quick reference, I hope this post will help you to quickly start extracting value from Pandas. So lets get started!1) Loading Data“The Olive Oils data set has eight explanatory variables (levels of fatty acids in the oils) and nine classes(areas of Italy)”. For more information you can check myIpython notebook.I am importing numpy,pandasandmatplotlibmodules.1234%matplotlib inlineimport numpy as npimport matplotlib.pyplot as pltimport pandas as pdI am using pd.read_csv to load olive oil data set. Function head returns the first n rows of olive.csv. Here I am returning the first 5 rows.2) Rename FunctionI am going to rename the first column (Unnamed: 0) to area_Idili.Rename functionas an argument it takes a dictionary of column names that should be renamed as keys(olive_oil.columns0) and the new title(area_Idili) to be the value. Olive_oil.columns will return the column names.inplace = Trueis used in case you want to modify the existing DataFrame.3) MapOne thing that I want to do is to clean the area_Idli column and remove the numbers. I am usingmapobject to perform this operation.Mapproperty applies changes to every element of a column. I am applying split function to columnarea_idili.Split function returns a list, and -1 returns the last element of the list. A detailed explanation of lambda is givenhere.See how split function works:4) Apply and Apply MapI have a list of acids called acidlist. Apply is a pretty flexible function, it applies a function along any axis of the DataFrame. I will be usingapplyfunction to divide each value of the acid by 100.list_of_acids =palmitic, palmitoleic, stearic, oleic, linoleic, linolenic, arachidic, eicosenoic12df = olive_oillist_of_acids.apply (lambda x: x/100.00)df.head (5)Similar toapply,apply mapfunction works element-wise on a DataFrame.Summing up,applyworks on a row/column basis of a DataFrame,applymapworks element-wise on a DataFrame, andmapworks element-wise on a Series.5)Shape and ColumnsShapeproperty will return a tuple of the shape of the data frame.olive_oil.columns will give you the column values.6) Unique functionOlive_oil.region.unique()will return unique entries in region column, there are three unique regions (1,2,3). I am applying the sameuniqueproperty toareacolumn, there are 9 unique areas.7) Cross TabCross Tab computes the simplecross tabulationof two factors. Here I am applying cross tabulation to area and region columns.8)Accessing Sub data framesThe syntax for indexing multiple columns is given below.To index a single column you can useolive_oilpalmiticorolive_oil.palmitic.9) Plottingplt.hist(olive_oil.palmitic). You can plot histogram usingplt.histfunction.You can also generate subplots of pandas data frame. Here I am generating 4 different subplots for palmitic and linolenic columns. You can set the size of the figure usingfigsizeobject, nrows and ncols are nothing but the number of columns and rows.10) Groupby and StatisticsGroupbygroups the data into 3 parts(region 1, 2 and 3). The functiongroupbygives dictionary like object. Here I am grouping by regions olive_oil.groupby(region).I am applyingdescribeon the group, describe takes any data frame and compute statistics on it. This is the quick way of getting statistics by group of any data frame.You can also calculate standard deviation of theregion_groupby usingolive_oil.groupby(region).std()11) Aggregate functionAggregate function takes a function as an argument and applies the function to columns in thegroupbysub dataframe. I am applying np.mean(computes mean) on all three regions.12) JoinI am renaming ol mean and olstd columns.In 34: list_of_acids =palmitic, palmitoleic, stearic, oleic, linoleic, linolenic, arachidic, eicosenoicPandas can do general merges. When we do that along an index, its called ajoin. Here I make two sub-data frames and join them on the common region index.13) MaskingYou can also mask a particular part of the data frame.olive_oil.eicosenoic 0.05 will check if each value in column eicosenoic is less than 0.05, if the value is less than 0.05 then it will return true, else it will return false.In 29: eico=(olive_oil.eicosenoic 0.05)14) Handling Missing ValuesMissing data is common in most data analysis applications. I find drop na and fill na function very useful while handlingmissing data.I am creating a new data frame.Thedropnacan used to drop rows or columns with missing data (None). By default, it drops all rows with any missing entry.fillnacan be used to fill missing data (None). First, I am creating a data frame with a single column.I am usingfillnareplaces the missing values with the mean of DataFrame(data).ConclusionThese are some of the important functions I use frequently while cleaning data. I highly recommend Wes MicknneysPython for Data Analysisbookfor learning pandas. Is there any other