How to implement a wrap around index?-CodePudding

I want a wrap around index like 1232123...., and the frame size is 3. How to implement it? Does it has a term?

for i in 1..100 {
   let idx = loop_index(i);
   print!("{} ", idx);
}

Expected output for frame 3:

1 2 3 2 1 2 3 2 1...

Expected output for frame 4:

1 2 3 4 3 2 1 2 3 4 3 2 1...

CodePudding user response：

For a size of 3, notice that the sequence 1232 has length 4, and then it repeats. In general, for size n, the length is 2*(n-1). If we take the modulo i % (2*(n-1)), the task becomes simpler: turn a sequence 0123..(2*(n-1)-1) into 123..(n-1)n(n-1)..321. And this can be done using abs and basic arithmetic:

n = 3
r = 2 * (n - 1)
for i in range(20):
    print(n - abs(n - (i % r)) - 1))

CodePudding user response：

When you reach the top number, you start decreasing; when you reach the bottom number, you start increasing.

Don't change direction until you reach the top or bottom number.

def count_up_and_down(bottom, top, length):
    assert(bottom < top)
    direction =  1
    x = bottom
    for _ in range(length):
        yield x
        direction = -1 if x == top else  1 if x == bottom else direction
        x = x   direction

for i in count_up_and_down(1, 4, 10):
    print(i, end=' ')
# 1 2 3 4 3 2 1 2 3 4

Alternatively, combining two ranges with itertools:

from itertools import chain, cycle, islice

def count_up_and_down(bottom, top, length):
    return islice(cycle(chain(range(bottom, top), range(top, bottom, -1))), length)

for i in count_up_and_down(1, 4, 10):
    print(i, end=' ')
# 1 2 3 4 3 2 1 2 3 4