Write a recursive function called draw_triangle() that outputs lines of *'s to form a right side up isosceles triangle. Function draw_triangle() has one parameter, an integer representing the base length of the triangle. Assume the base length is always odd and less than 20. Output 9 spaces before the first '*' on the first line for correct formatting.

Hint: The number of '*' increases by 2 for every line drawn.

Ex: If the input of the program is: 3,

Then the function draw_triangle outputs:

         *
        ***

If the input of the program is 19,

Then the function outputs

         *
        ***
       *****
      *******
     *********
    ***********
   *************
  ***************
 *****************
*******************

No space is output before the first asterisk on the last line when the base length is 19.

I was able to code this non-recursively as follows:

def draw_triangle(n):
    lines_to_print = int(((n // 2)   1))
    spaces_to_print = 9
    asts_to_print = 1
    for i in range(lines_to_print):
        print(spaces_to_print * ' ', end='')
        print(asts_to_print * '*', end='')
        print()
        spaces_to_print -= 1
        asts_to_print  = 2

base_length = int(input())
draw_triangle(base_length)

But I cannot, for my life, figure out how to do it recursively, let alone with only one argument. What am I missing about recursion?

CodePudding user response：

One method is is to define functions to print the symbole you want to use and the spaces between them:

def print_ecart(ecart):
     
    if (ecart == 0):
        return;
    print(" ", end = "")
    print_ecart(ecart - 1)

def print_etoile(etoile):
    if(etoile == 0):
        return
    print("* ", end = "")
 
    print_etoile(etoile - 1)
    
def pattern(n, num):  
    if (n == 0):
        return
    print_ecart(n - 1)
    print_etoile(num - n   1)
    print("");
    pattern(n - 1, num)

In this case print_ecrat prints spaces and print_etoile prints the stars:

pattern(10,10)
 

returns:

         * 
        * * 
       * * * 
      * * * * 
     * * * * * 
    * * * * * * 
   * * * * * * * 
  * * * * * * * * 
 * * * * * * * * * 
* * * * * * * * * * 

CodePudding user response：

By printing after the recursion itself happens, you can print smaller lines over the bigger lines:

def draw_triangle(n):
    spaces = 9 - n // 2  # Ensures correct spacing for each line
    if n == 1:
        print(spaces * ' '   '*')
    else:
        draw_triangle(n-2)
        print(spaces * ' '   n * '*')


draw_triangle(19)

CodePudding user response：

Well, it is a quite interesting question. The main obstacle is that you have to preserve initial base length of triangle. You can't save it via assigning input value to another parameter inside a recursive function because:

1. only one argument is allowed
2. assigning will work every function call.

Not sure if the following code is convenient with the conditions of the task but it may be useful to illustrate an idea of recursive functions for you:

base_length = 20
def draw_triangle(base=base_length):
    if base % 2 == 0:
        base = base - 1
        draw_triangle(base)
    else:
        if base > 0:
            # print(base)
            # print(int(base/2 - 1))
            print(' '*int((base - 2)/2 1)   '*'*(base_length - base)   ' '*int((base - 2)/2))
            base = base - 2
            draw_triangle(base)

        elif base == 0:
            return 0

draw_triangle(base=base_length)

Formally there is only one argument in this recursive function, but you have to define base_length paramater outside of the function and the function uses base_length inside.

CodePudding user response：

The simple trick here is to use the str.center() method.

def draw_triangle(n):
    if n == 1:
        print('*'.center(19))
    else:
        draw_triangle(n-2)
        print((n*'*').center(19))

Test it:

draw_triangle(19)

         *         
        ***        
       *****       
      *******      
     *********     
    ***********    
   *************   
  ***************  
 ***************** 
*******************