循环遍历字符串列表

Question

我有以下练习，其中我有一个列表 directions = ["N", "E", "S", "W"] 用于罗盘上的每个方向。我必须创建一个函数，如果你输入 "N" 它 return 是顺时针方向的下一个 "E"。当你输入 "W" 它应该回到开始和 return "N"。当输入不在列表中时，例如"F" 应该 return none。这是我想出的：

def turn_clockwise(direction):
    directions = ["N", "E", "S", "W"]
    for i in range(3):
        if direction == directions[i]:
            result = directions[i+1]
            print(result)
        else:
            return(None)

这仅在我输入 "N" 时有效。当我删除 else 时，它也适用于其他项目，但是当输入为 "W" 时，它不会循环回到开始。我想知道如何使代码适合作业，或者是否有更简单的方法来做到这一点。

Answer 1

def turn_clockwise(direction):
    directions = ["N", "E", "S", "W"]
    if direction in directions:
        return directions[(directions.index(direction) + 1) % len(directions)]
    else:
        return None

使用模 % 运算符环绕特定数字。

这里附上@AKX的建议：

def cycle(values, current): 
    try:
        return values[(values.index(current) + 1) % len(values)]
    except ValueError:
         print(f"{current} not in {values}")

def turn_clockwise(direction):
    directions = ["N", "E", "S", "W"]
    if direction in directions:
        return cycle(directions, direction]
    else:
        return None

Answer 2

您还可以使用 itertools.cycle 中的预制 cycle:

from itertools import cycle

directions = cycle(["N", "E", "S", "W"])

for _ in range(10):
    print(next(directions), end = " -> ")
print(next(directions))

输出：

N -> E -> S -> W -> N -> E -> S -> W -> N -> E -> S

或者简单地创建一个查找字典。

两个版本的可用方法：

from itertools import cycle

def next_dir(what):
    d = "NESW"
    directions = cycle(d)
    if what in d:   
        while next(directions) != what:
            pass
        return next(directions)
    else:
        raise ValueError(what + " not possible")


def next_lookup(what):
    d = {"N":"E", "E":"S", "S":"W", "W":"N"}
    r = d.get(what)
    if r: 
        return r
    raise ValueError(what+" not possible")

for l in "NESW":
    print(l, next_dir(l)) 
    print(l, next_lookup(l)) 

try:
    print(next_dir("q"))
except Exception as e:
    print(e)

try:
    print(next_lookup("q"))
except Exception as e:
    print(e)

输出：

N E  # next_dir
N E  # next_lookup  .. etc ..
E S
E S
S W
S W
W N
W N
q not possible
q not possible

循环遍历字符串列表

Cycling through a list of strings

python

python-3.x

cycle