Using Counter() in Python to build histogram?

Question

I saw on another question that I could use Counter() to count the number of occurrences in a set of strings. So if I have ['A','B','A','C','A','A'] I get Counter({'A':3,'B':1,'C':1}). But now, how can I use that information to build a histogram for example?

Solution 1

For your data it is probably better to use a barchart instead of a histogram. Check out this code:

from collections import Counter
import numpy as np
import matplotlib.pyplot as plt


labels, values = zip(*Counter(['A','B','A','C','A','A']).items())

indexes = np.arange(len(labels))
width = 1

plt.bar(indexes, values, width)
plt.xticks(indexes + width * 0.5, labels)
plt.show()

Result:

Solution 2

You can write some really concise code to do this using pandas:

    import numpy as np

    from pandas import Series
    
    sample = np.random.choice(['a', 'b'], size=10)
    
    s = Series(sample)
    
    In [29]: s
    Out[29]:
    0    a
    1    b
    2    b
    3    b
    4    a
    5    b
    6    b
    7    b
    8    b
    9    a
    dtype: object
    
    vc = s.value_counts()
    
    In [31]: vc
    Out[31]:
    b    7
    a    3
    dtype: int64
    
    vc = vc.sort_index()
    
    In [33]: vc
    Out[33]:
    a    3
    b    7
    dtype: int64
    
    c.plot(kind='bar')

Resulting in:

Solution 3

Based on Igonato's answer I created a helper module 'plot' with a class 'Plot'.

It has two functions hist() and barchart() two show Igonato's approach as well as using the matplotlib hist functionality directly as might haven been originally intended in the question.

The approach allows adding a title and lables with a given fontsize and displays the y-axis with a tick frequency of 1. You can also change the mode so that it will save the chart with the given title. There are close and debug options for convenience.

python unit test test_Plot.py

'''
Created on 2020-07-05

@author: wf
'''
import unittest


from ptp.plot import Plot

class TestPlot(unittest.TestCase):


    def setUp(self):
        pass


    def tearDown(self):
        pass


    def testPlot(self):
        ''' test a plot based on a Counter '''
        valueList=['A','B','A','C','A','A'];
        plot=Plot(valueList,"barchart example",xlabel="Char",ylabel="frequency")
        plot.barchart(mode='save')
        plot.title="histogram example"
        plot.debug=True
        plot.hist(mode='save')        
        pass


if __name__ == "__main__":
    #import sys;sys.argv = ['', 'Test.testName']
    unittest.main()

Results:

debug output:

   value  list:  ['A', 'B', 'A', 'C', 'A', 'A']
counter  items:  dict_items([('A', 4), ('B', 1), ('C', 1)])
counter values:  dict_values([4, 1, 1])
counter   keys:  dict_keys(['A', 'B', 'C'])

plot.py

    '''
Created on 2020-07-05

@author: wf
'''
import matplotlib.pyplot as plt
from collections import Counter
import numpy as np
import os

class Plot(object):
    '''
    create Plot based on counters
    see https://stackoverflow.com/questions/19198920/using-counter-in-python-to-build-histogram
    '''
    def __init__(self, valueList,title,xlabel=None,ylabel=None,fontsize=12,plotdir=None,debug=False):
        '''
        Constructor
        '''
        self.counter=Counter(valueList)
        self.valueList=valueList
        self.title=title
        self.xlabel=xlabel
        self.ylabel=ylabel
        self.fontsize=fontsize
        self.debug=debug
        path=os.path.dirname(__file__)
        if plotdir is not None:
            self.plotdir=plotdir
        else:
            self.plotdir=path+"/../plots/"
            os.makedirs(self.plotdir,exist_ok=True)
            
    def titleMe(self):        
        plt.title(self.title, fontsize=self.fontsize)
        if self.xlabel is not None:
            plt.xlabel(self.xlabel)
        if self.ylabel is not None:    
            plt.ylabel(self.ylabel)
            
    def showMe(self,mode='show',close=True):
        ''' show me in the given mode '''
        if mode=="show":
            plt.show() 
        else:
            plt.savefig(self.plotdir+self.title+".jpg")
        if close:    
            plt.close()    
            
    def barchart(self,mode='show'):
        ''' barchart based histogram for the given counter '''
        labels, values = zip(*self.counter.items())
        indexes = np.arange(len(labels))
        width = 1
        self.titleMe()
        plt.bar(indexes, values, width)
        plt.xticks(indexes + width * 0.5, labels)
        plt.yticks(np.arange(1,max(values)+1,step=1))
        self.showMe(mode)
        
    def showDebug(self):    
        print("   value  list: ",self.valueList)
        print("counter  items: ",self.counter.items())
        print("counter values: ",self.counter.values())
        print("counter   keys: ",self.counter.keys())
        
    def hist(self,mode="show"):
        ''' create histogram for the given counter '''
        if self.debug:
            self.showDebug()
        self.titleMe()
        # see https://stackoverflow.com/a/2162045/1497139
        plt.hist(self.valueList,bins=len(self.counter.keys()))
        self.showMe(mode)
        pass
        
    

Comments

[Thomas Matthew]

What if OP's data was best suited to a histogram? I know it's late to the game, but I'd like to update your answer to apply to a histogram example (but I'm not sure how to do it yet). That would answer the title to the question.

[Igonato]

@ThomasMatthew technically, it is a histogram. "barchart instead of histogram" is more referring to the use of matplotlib.pyplot.bar instead of matplotlib.pyplot.hist. I believe bar goes better with Counter (which is what OP wanted), that's it

[Igonato]

Also this is a 3 year old question something might have changed

[Igonato]

@ThomasMatthew how did you get there, what were you searching for? Does it score high on google? What query? If it shows on something other than python Counter matplotlib maybe it makes sense to edit the answer

[Thomas Matthew]

I searched "build histogram from counter" and it was Google search result #1. It's also almost the same title of OP's question. It's worth an update for all the folks who search that query, which may be a high percentage views on this question. You might get more up votes and favorites you update (add) the histogram example

[Thomas Matthew]

To answer you're previous point, the counter could be use to hold bin memberships as {bin:freq...} for arrays too big to hold in memory.

[Igonato]

@ThomasMatthew ok, so what is your suggestion on how to change it?

[blckbird]

How can I add the height (count) to the chart?

[EntangledLoops]

But OP specifically asked about Counter. To answer the question, you should show how to go from Counter to Series, unless there's another Pandas type more suited to a dictionary. It's not clear that your solution really applies here--it's just an example of a bar plot.