Question

I have to prompt a user to input a value n >= 3 such that my python 3.2 program can print a n by n pattern.

Example, n=4 ==> Pattern:

x..x
.xx.
.xx.
x..x

Another example, n=5 ==> Pattern:

x...x
.x.x.
..x..
.x.x.
x...x

Notice that the x get closer to each other after each row. After the middle row the get further from each other and finally reach their respective ends.

Please help me with this problem. I know I have to use branching and looping but I just can’t figure the logic behind the pattern.

I’ve managed to print out a n by n pattern but it looks like this:
Example n=4 ==> Pattern:

xx..  
xx..
xx..
xx..

My code looks like this:

    pattern_size = int(input("Please enter pattern size:"))
    while pattern_size < 3:
    print("Pattern size should be at least 3")
    pattern_size = int(input("Please enter pattern size:"))

    for level in range(1,pattern_size+1):
        for num in range(2):
            print("x", end="")

        num_dot = pattern_size - 2
        for num in range(num_dot):
            print(".", end="")

        print()

Can someone advice me on how to print the “.” and “x” alternately?

Answer 1

Here is the code that works for python2.7. The trick here is one of the “x” always shifting left, while the other one is always shifting right.

def pattern_writer(pattern_size):
    first_x = 0
    last_x = pattern_size - 1
    while first_x < pattern_size and last_x > -1:
        for t in range(pattern_size):
            if t == first_x or t == last_x:
                print "x",
            else:
                print ".",
        print "\n"
        first_x += 1
        last_x -= 1

Here is another solution using lists:

def pattern_writer2(pattern_size):
    for t in range(pattern_size):
        pattern = list("." * pattern_size)
        pattern[t] = "x"
        pattern[-(t+1)] = "x"
        print "".join(pattern)