How would I make this loop recursive?-CodePudding

def main():
    n = int(input("How many lines to display? "))
    for i in range(n):
        print("*" * i)
    input("Press the <Enter> Key to quit")
main()

I somehow need to make this recursive. But this is suck a simple loop I'm not sure how I could make it recursive. Any advice is appreciated.

CodePudding user response：

Hoping you can learn by seeing:

def triangle(n):
    if n <= 1:
        return ['']
    return [*triangle(n - 1), '*' * n] # or triangle(n - 1)   ['*' * n]

n = 5
print('\n'.join(triangle(n)))

#
# **
# ***
# ****
# *****

The function returns a list of lines, not a single string (since concatenating strings at every step is not recommended). Hence you need to join them with '\n', the newline character, and print it.

I believe the above is a very standard recursive function; there is a "base case" (i.e., when the recursion ends; here, it is when n <= 1), and the body, where recursively calls itself (triangle(n - 1)), and then a return.

For a one-liner, you could use generator comprehension:

print('\n'.join('*' * x for x in range(n)))

CodePudding user response：

One way of doing it is to tell the function what iteration you're on and what the total iterations you want are. The function then prints a line, and then calls the function for the next iteration. So, for instance, if n=3, you first call the function for n=3 and i=0 to get the first iteration, then the function calls itself for n=3 and i =1 for the next iteration, which calls itself for n =3 and i=2.

def display_rows(n,i):
    if i == n:
        return
    print("*" * i)
    display_rows(n, i 1)

You can simplify this by combining the two variables n and i into one; define u = n-i, and when u gets to zero, stop.

def display_rows(u):
    if u == 0:
        return
    display_rows(u-1)
    print("*" * u)

I changed the order of the print("*" * u) and display_rows(u) lines because u is going "backwards" (from n to 0), so you need to do the recursion call before printing. This does make it a bit less tail-recursiony, but given that you asked this question in the first place, you probably aren't familiar with tail recursion anyway.

CodePudding user response：

you need to wrapper a recurisive loop out of for loop. And you need better to check input_line is numeric to exclude bad input. As input will return empty str to it, you can check if input_line is empty to exit while loop by break

def main():
    while True:
        input_line = input("How many lines to display? ")
        if input_line.isnumeric():
            for i in range(int(input_line)):
                print("*" * i)
        elif input_line == '':
            break
        else:
            print("wrong input")
main()

CodePudding user response：

It's not the prettiest way to do this but -> Here is an example with input and recursion:

def printy(n): return f"{'*'*n}\n{printy(n-1)}" if n > 0 else ''

def main():
    x = printy(int(n) if (n:=input("How many lines to display? ")).isdigit() else 0)
    if x:
        print(x)
        main()

print('Press the <Enter> Key to quit')
main()