Sorting Algorithm Visualization : Merge Sort

The human brain can easily process visuals instead of long codes to understand the algorithms. In this article, a program that program visualizes the Merge sort Algorithm has been implemented.

The GUI(Graphical User Interface) is implemented using pygame package in python.

Approach:

An array of random values is generated and are drawn as lines(bars) in the window .

Different colors are used to indicate which elements being compared, sorted and unsorted.

Since the algorithm performs the operation very fast, pygame.time.delay() has been used to slow down the process.

New array can be generated by pressing ‘r’ key.

The actions are performed using ‘pygame.event.get()’ method, which stores all the events which user performs.

Examples:

Input :
Press “Enter” key to Perform Visualization.
Press “r” key to generate new array.
Output :
Initial:

Visualizing:

Final:

Below is the program to visualize the Merge Sort algorithm:

# Python implementation for visualizing merge sort.
import pygame
import random
pygame.font.init()
# Total window
screen = pygame.display.set_mode((900, 650))

# Title and Icon
pygame.display.set_caption("SORTING VISUALISER")
# Place any custom png file in same folder as the source code
# and mention it below and uncomment below two lines.
# img = pygame.image.load
# ('E:/Projects / Sorting Visualiser / sorticon.png')
# pygame.display.set_icon(img)

# Boolean variable to run the program in while loop
run = True

# Window size
width = 900
length = 600
array =[0]*151
arr_clr =[(0, 204, 102)]*151
clr_ind = 0
clr =[(0, 204, 102), (255, 0, 0),
(0, 0, 153), (255, 102, 0)]
fnt = pygame.font.SysFont("comicsans", 30)
fnt1 = pygame.font.SysFont("comicsans", 20)
# Generate new Array
def generate_arr():
    for i in range(1, 151):
        arr_clr[i]= clr[0]
        array[i]= random.randrange(1, 100)
generate_arr()
def refill():
    screen.fill((255, 255, 255))
    draw()
    pygame.display.update()
    pygame.time.delay(20)

# Sorting Algo:Merge sort
def mergesort(array, l, r):
    mid =(l + r)//2
    if l<r:
        mergesort(array, l, mid)
        mergesort(array, mid + 1, r)
        merge(array, l, mid,
            mid + 1, r)
def merge(array, x1, y1, x2, y2):
    i = x1
    j = x2
    temp =[]
    pygame.event.pump()
    while i<= y1 and j<= y2:
        arr_clr[i]= clr[1]
        arr_clr[j]= clr[1]
        refill()
        arr_clr[i]= clr[0]
        arr_clr[j]= clr[0]
        if array[i]<array[j]:
                temp.append(array[i])
                i+= 1
        else:
                temp.append(array[j])
                j+= 1
    while i<= y1:
        arr_clr[i]= clr[1]
        refill()
        arr_clr[i]= clr[0]
        temp.append(array[i])
        i+= 1
    while j<= y2:
        arr_clr[j]= clr[1]
        refill()
        arr_clr[j]= clr[0]
        temp.append(array[j])
        j+= 1
    j = 0
    for i in range(x1, y2 + 1):
        pygame.event.pump()
        array[i]= temp[j]
        j+= 1
        arr_clr[i]= clr[2]
        refill()
        if y2-x1 == len(array)-2:
            arr_clr[i]= clr[3]
        else:
            arr_clr[i]= clr[0]

# Draw the array values
def draw():
    # Text should be rendered
    txt = fnt.render("PRESS"\
    " 'ENTER' TO PERFORM SORTING.", 1, (0, 0, 0))
    # Position where text is placed
    screen.blit(txt, (20, 20))
    txt1 = fnt.render("PRESS 'R' FOR NEW ARRAY.",
                    1, (0, 0, 0))
    screen.blit(txt1, (20, 40))
    txt2 = fnt1.render("ALGORITHM USED: "\
                    "MERGE SORT", 1, (0, 0, 0))
    screen.blit(txt2, (600, 60))
    element_width =(width-150)//150
    boundry_arr = 900 / 150
    boundry_grp = 550 / 100
    pygame.draw.line(screen, (0, 0, 0),
                    (0, 95), (900, 95), 6)
    for i in range(1, 100):
        pygame.draw.line(screen,
                        (224, 224, 224),
                        (0, boundry_grp * i + 100),
                        (900, boundry_grp * i + 100), 1)

    # Drawing the array values as lines
    for i in range(1, 151):
        pygame.draw.line(screen, arr_clr[i],\
            (boundry_arr * i-3, 100),\
            (boundry_arr * i-3, array[i]*boundry_grp + 100),\
            element_width)

# Infinite loop to keep the window open
while run:
    # background
    screen.fill((255, 255, 255))
    # Event handler stores all event
    for event in pygame.event.get():
        # If we click Close button in window
        if event.type == pygame.QUIT:
            run = False
        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_r:
                generate_arr()
            if event.key == pygame.K_RETURN:
                mergesort(array, 1, len(array)-1)
    draw()
    pygame.display.update()

pygame.quit()

Output: