Python Basics-fifth-5.2 recursion

Is another sunny day, small white look at just from the East Rising Sun, feel the Sun Grandpa also greet her, small white on the smile up! Thought: What interesting things will you learn today? Some small expectations, but also some small excitement!

Xiao Liu came earlier than the small white, two laugh played a greeting!

Xiao Liu: Xiao Bai! You come over here and let you think about how this code is executed? Why don't you just pick up the morning and wake up?

Xiao Liu: I want to go to the early meeting! You think about it and tell me when I get back!

Small white point nod, small bai look this piece of code:

Def f1 ():    return ' F1 ' def f2 ():    r = F1 ()    return Rdef F3 ():    r = F2 ()    return Rren = F3 ()

Small white put this picture of the colorful, small white point head, Mouth said a little meaning

Xiao Liu came back and asked her how she was thinking.

Small White replied: The result of the Ren variable is ' F1 ', explained from the top down to explain, when the definition function does not execute, just brush to memory, execute F3 up and down ran a back and forth

Xiao Liu looked at a little white painting of the picture, instantly understand, hurriedly praised small white: basic science is good ah, yes, you said

Xiao Liu: Then I'll show you an example, see if you can realize it?

def f4 (A1,A2):    If A1 > 3:        return    print (a1)    a3 = A1 + A2    F4 (a2,a3) R = F4 (0,1) print (R)

Small white looked, and drew up again

　　

Little White will come out quickly. The 0,1,1,2,3 function returns the default value of None

Xiao Liu laughs: little white! You go back! No, I'll teach you.

Small white smiled: stupid people have stupid way, good time long, still need you this big God guidance!

Xiao Liu asked: "Do you know what this algorithm is called?"

Small white: It seems to be recursive, have learned a little before

Xiao Liu: Do you know the advantages and disadvantages of it and its definition?

Little White shook his head!

Xiao Liu: The Little White Mind

1. Calling itself in a procedure or function

2. Recursive algorithm is simple and easy to read, but inefficient

3. When using a recursive strategy, there must be an end condition, which is called a recursive exit with a maximum recursion depth of 999 layers

Small white exclamation way: a little one recursion so much learning AH

Xiao Liu: Don't you know that the series you just came up with is called the Fibonacci sequence?

Little white: I don't know.

Xiao Liu: That is, a number equals the first two, and I have a function to set the recursion depth to you here.

def f5 (DEPTH,A1,A2):    if depth = = Ten:        return a1    a3 = A1 + A2    r = F5 (DEPTH+1,A2,A3)    return rret = F5 (1,0 , 1) print (ret)

Xiao Liu: Xiao Bai! Does this make sense?

Little white dot nod

Xiao Liu: So let's look at a more complicated example, called binary search, which uses the principle of recursion.

Binary Method Search

Binary search is also called semi-search, the premise is to find the ordered series, take the number to find and in the middle of the data source to compare, every comparison to exclude half

#二分法查找lis = List (range (2,11,4)) print (LIS) #[2, 6, 10]def Semi_find (find_n,data_resources):    i = int (len (data_ Resources)/2)  #求得中间项的索引位置    If Len (data_resources) >= 1:    #递归终止条件        if data_resources[i] > find_n:  #要查找的数处在前半部分            semi_find (find_n,data_resources[:i])  #排除后半部分        elif data_resources[i] < find_n:  #要查找的数处在后半部分            If find_n < Data_resources[i + 1]:  #因为int对小数是向下取整, this excludes the value between the corresponding number of index positions I to i+1                print (' That's not the count! ')               #一旦要查找的数判定处于上面范围, that is, the output does not have this number                return            else:                semi_find (find_n,data_resources[i:])  # Excluding the first half of the        else:            print ('%s in Lis '%find_n)  #如果等于中间数, we found the    else:        print (' No!! ') #加等号semi_find (2,lis)  #能找到2semi_find (7,lis)  #7, 8,9 more than the maximum number of recursive layers also resolves # no equals # Semi_find (2,lis)  #错误提示找不到2 # Semi_find (7,lis)  #能正确提示7, 8,9 does not exist

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Author: Pat Province Mr.

　　

Python Basics-fifth-5.2 recursion