recursive (calculation)
The concept of recursion: a function contains a call to itself, then it is recursive
Scenarios for use: If you realize that what you're about to do is what you're doing now, then use recursion
Recursion is similar to a loop; when writing or reading about recursion, the first thing we focus on is the termination condition of the recursion
Recursive Summation
Before touching on recursion, let's do this: what is the best way to, say, sum a list of numbers (or any other sequence), other than the built-in sum function that we can use?
while loop
L = [1,2,3,4,5] mysum = 0 # Variables that hold sums while L: #Let the list be the most recurring condition mysum += L[0] # Add the value in the first position of the list to the sum each time L = L[1：] # Remove the first element of the list
for loop
L = [1,2,3,4,5] mysum = 0 for var in L: mysum += var
Recursive Summation
def mysum(L):
if not L:
print ('L is empty')
return 0
else:
return L[0]+mysum(L[1:])
# In the return value, we return a function call and pass the parameter as a new list with the first element of the current list removed
Recursive handling of nonlinear loops
Recursion can also handle non-linear loops that ordinary loops can't; for example, a list like this sums over it:
L = [1,[2,[3,4],5],6,[7,8]]
Since this list is not a linear iteration, and contains complex nesting of elements, it will be very difficult to control the processing of ordinary looping statements
L = [1,[2,[3,4],5],6,[7,8]]
sum = 0
def mysum(L):
global sum
for var in L:
if not isinstance(var,list):
# If one of the elements is not of list type, it is a definite value
sum += var
else:
mysum(var)
return
spend money recursively
Think: If you have $10,000 and you spend half of it every day, with the gross money just rounded up, how many days can you spend that money?
Recursive solution:
def cost(money,day=0):
if money > 0:
money = money // 2 # Spend half at a time
day += 1 # of days spent +1
cost(money,day) # Turn on the spend recursion
else:
print('It can take a total of %d days' % day)
return # There must be a termination condition
Recursive Considerations
In Python, the maximum number of recursions is almost 998, and a recursion without a termination condition will raise an error (similar to a dead loop)
This is because each function execution of recursion generates a new copy of the function in memory, and the memory consumption of recursion is greater than that of a normal loop
>>> def func(): ... return func() ... >>> func() Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 2, in func File "<stdin>", line 2, in func File "<stdin>", line 2, in func [Previous line repeated 995 more times] RecursionError: maximum recursion depth exceeded # Here we reach the top line after 995 recursions, thus reporting the error
We can also manually intervene in the upper limit of the recursion, but this is risky and has to be considered in the context of the computer's own memory
>>> import sys >>> (num) # num is the maximum number of recursion caps for control modifications
Implementing the Tree command
The core idea is that the depth and breadth of the directory structure is intricate, and making a determination by simply looping through it is a very difficult thing to do.
Recursion is suitable for such non-linear loops, but of course there are some drawbacks, as the directory structure becomes more and more complex, the program execution efficiency will become worse and worse.
import os
def getdir(path, level=0):
if path == '':
path = () # Get the current working directory
level += 4
num = level // 4
abs_path = (path)
for name in (path): # The return is a list
format_str = ''
if ((abs_path, name)):
for var in range(num): # The range function is used to control the number of loops
format_str += '_' * 4 + '▕'
format_str = format_str[0:-1]
format_str += name
mystr = format_str.replace('_', ' ', level-4) # Replace level-4 _
else:
for var in range(num): # The range function is used to control the number of loops
format_str += '_' * 4 + '▕' # Output style constructs
format_str += name
mystr = format_str.replace('_',' ',level-4) # Replace level-4 _
print(mystr) # Output format string
name = (abs_path,name)
if (name): # Absolute path, determine if it is a folder
getdir(name,level)
path = input('Please enter the directory you want to traverse:')
getdir(path)
summarize
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