使用公式在python中绘图

Question

My teacher asked us to write a python program that will graph stars. He gave us this equation "y=x2+3" for the range x = 0 to x = 6. So Python will ask for the value of x and the program should automatically output the stars in the right places by doing the math.

It's an example of how the graph should look like:

Here, the formula is y=x^2:

  y

16|            *     

  |

14|

  |

12|

  |

10|

  |         *

 8|

  |

 6|

  |

 4|      *

  |

 2|

  |   *

   ------------- X

   0  1  2  3  4

So far i could write a program for the axis:

print ("{0:>4}".format ("y"))
print ()
for counter in range (40,-1,-2):
    print ("{0:>4}".format ("|"))
    print ("{0:>2}".format (counter))

for counter in range (1,2):
    print ("{0:>32}".format("------------------------- X"*counter))
    print ("{0:>6}{1:>4}{2:>4}{3:>4}{4:>4}{5:>4}{6:>4}" .format("0", "1", "2", "3", "4", "5", "6",))

but i don't know how to write a program that will output the "*"s based on the equation! I'm a beginner and we haven't learned "Matplotlib" yet.

He also gave us this to help us:

for count in range(1,60,3):
    myWidth = count
    myCharacter = '*'
    print('{0:>{width}}'.format(myCharacter, width=myWidth))

It'd be great if anyone can help. Thank you!

Answer 1

I agree with the comment of Phuc Tran , you could create an array and then you can plot the array.

Another non orthodox way for doing it is to calculate the inverse of the given function. Due that you're iterating in the y direction, calculate the value for x and, if it is an integer, plot it.

The following code is a modified version of your work that will do that, it iterates from the top value of y , calculates the inverse of the function and plots the x if it is an integer:

#!/usr/bin/env python

import math

print ("{0:>4}\n".format ("y"))

for counter in range (40,-1,-1):
    # Calculate the inverse of the equation, i.e, given a Y value, 
    # obtain the X 
    y = counter
    x = math.sqrt(y)

    # Now, if x is an integer value, we should output it (floating values won't
    # fit in the ascii graph)
    # To know if x is integer, you can either use the following trick 
    # 
    #    x == int(x)    
    # 
    # Or you can use the is_integer function (Here we'll be using this one)
    if x.is_integer(): 
        # Here we calculate the number of spaces to add before outputing the '*'
        #
        # (there is a small issue with the space count, they are not
        # properly aligned, but I hope you can come up with solution for it ;) )
        spaces = "   " * int(x)
        print ("{0:>2}{1:>4}{2}*".format(counter if counter % 2 == 0 else "", "|", spaces))
    else:
        # This will fix your y axis so it will look like the asked format. It 
        # just ignores the counter if it is odd and keeps the space to add the '|'
        print ("{0:>2}{1:>4}".format(counter if counter % 2 == 0 else "", "|"))         

for counter in range (1,2):
    print ("{0:>32}".format("------------------------- X" * counter))
    print ("{0:>6}{1:>4}{2:>4}{3:>4}{4:>4}{5:>4}{6:>4}" .format("0", "1", "2", "3", "4", "5", "6",))

You can use this to think in another simpler solution or as a base for Phuc Tran suggestion.

Answer 2

Here's one way to get the desired plot.

First fill a 2D array with " " and * :

x = # whatever value
y = x*x + 3
coord = []
y_ax = y

for y_ax in range(y, -1, -1):
    pts = []
    for x_ax in range(1, x+1):
        eq = x_ax*x_ax + 3
        if y_ax == eq:      
            pts.append("*")
        else:
            pts.append(" ")
    coord.append(pts)

Then simply print out the 2D array:

label = y
for row in coord:
    if "*" in row:
        print('{:<2}|'.format(label), end=" ")
    else:
        print('{:<2}|'.format(""), end=" ")
    for col in row:
        print(col, end="")
    print(end="\n")
    label-=1