Visualization of Merge sort using Matplotlib

Last Updated : 15 Jul, 2025

Prerequisites: Introduction to Matplotlib, Merge Sort

Visualizing algorithms makes it easier to understand them by analyzing and comparing the number of operations that took place to compare and swap the elements. For this we will use matplotlib, to plot bar graphs to represent the elements of the array,

Approach:

We will generate an array with random elements.
The algorithm will be called on that array and yield statement will be used instead of a return statement for visualization purposes.
We will yield the current states of the array after comparing and swapping. Hence the algorithm will return a generator object.
Matplotlib animation will be used to visualize the comparing and swapping of the array.
The array will be stored in a matplotlib bar container object ('bar_rects'), where the size of each bar will be equal to the corresponding value of the element in the array.
The inbuilt FuncAnimation method of matplotlib animation will pass the container and generator objects to the function used to create animation. Each frame of the animation corresponds to a single iteration of the generator.
The animation function is repeatedly called will set the height of the rectangle equal to the value of the elements.

Below is the implementation of the above approach.

Python3

# import all the modules
import matplotlib.pyplot as plt
from matplotlib.animation import FuncAnimation
from mpl_toolkits.mplot3d import axes3d
import matplotlib as mp
import numpy as np
import random


# function to recursively divide the arra
def mergesort(A, start, end):
    if end <= start:
        return

    mid = start + ((end - start + 1) // 2) - 1
    
    # yield from statements have been used to yield 
    # the array from the functions 
    yield from mergesort(A, start, mid)
    yield from mergesort(A, mid + 1, end)
    yield from merge(A, start, mid, end)

# function to merge the array
def merge(A, start, mid, end):
    merged = []
    leftIdx = start
    rightIdx = mid + 1

    while leftIdx <= mid and rightIdx <= end:
        if A[leftIdx] < A[rightIdx]:
            merged.append(A[leftIdx])
            leftIdx += 1
        else:
            merged.append(A[rightIdx])
            rightIdx += 1

    while leftIdx <= mid:
        merged.append(A[leftIdx])
        leftIdx += 1

    while rightIdx <= end:
        merged.append(A[rightIdx])
        rightIdx += 1

    for i in range(len(merged)):
        A[start + i] = merged[i]
        yield A

# function to plot bars
def showGraph():
    
    # for random unique values
    n=20
    a=[i for i in range(1, n+1)]
    random.shuffle(a)
    datasetName='Random'
    
    # generator object returned by the function
    generator = mergesort(a, 0, len(a)-1)
    algoName='Merge Sort'
    
    # style of the chart
    plt.style.use('fivethirtyeight')
    
    # set colors of the bars
    data_normalizer = mp.colors.Normalize()
    color_map = mp.colors.LinearSegmentedColormap(
        "my_map",
        {
            "red": [(0, 1.0, 1.0),
                    (1.0, .5, .5)],
            "green": [(0, 0.5, 0.5),
                      (1.0, 0, 0)],
            "blue": [(0, 0.50, 0.5),
                     (1.0, 0, 0)]
        }
    )

    fig, ax = plt.subplots()
    
    # bar container 
    bar_rects = ax.bar(range(len(a)), a, align="edge", 
                       color=color_map(data_normalizer(range(n))))
    
    # setting the limits of x and y axes
    ax.set_xlim(0, len(a))
    ax.set_ylim(0, int(1.1*len(a)))
    ax.set_title("ALGORITHM : "+algoName+"\n"+"DATA SET : "+datasetName, 
                 fontdict={'fontsize': 13, 'fontweight': 'medium', 
                           'color' : '#E4365D'})
    
    # the text to be shown on the upper left
    # indicating the number of iterations
    # transform indicates the position with 
    # relevance to the axes coordinates.
    text = ax.text(0.01, 0.95, "", transform=ax.transAxes, 
                   color="#E4365D")
    iteration = [0]

    def animate(A, rects, iteration):
        for rect, val in zip(rects, A):
            
            # setting the size of each bar equal 
            # to the value of the elements
            rect.set_height(val)
        iteration[0] += 1
        text.set_text("iterations : {}".format(iteration[0]))
    
    # call animate function repeatedly
    anim = FuncAnimation(fig, func=animate,
        fargs=(bar_rects, iteration), frames=generator, interval=50,
        repeat=False)
    plt.show()

showGraph()

Output:

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