In-depth understanding of the for statement in Python

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Hello, I’m Peter

Year round, cycle round: it’s all about cycle in the end

The for statement actually solves the loop problem. There are for loops in many high-level languages.

The for statement is a programming language statement for iterables that allows code to be executed repeatedly. Take a look at this introduction from Wikipedia:

In computer science, a for-loop (or simply for loop) is a control flow statement for specifying iteration, Which allows code to be executed repeatedly. (What is a for loop?)

A for-loop has two parts: A header specifying the iteration, and a body which is executed once per iteration.

What is it: in computing, a control-flow statement for a particular iterator that can be executed repeatedly
How to form: a header (which is an iterable) + the body of each object

iterable

What is an iterable

An Iteratable Object is an Object that can return one of its members ata time, such as strings, lists, tuples, collections, dictionaries, etc., all of which are iterable objects that can be retrieved using a for loop.

To put it simply, any object that you can loop over is an iterable.

How to judge

How do I determine if a Python object is iterable? The isinstance() function is usually used to determine whether an object is iterable

from collections import可迭代Copy the code

Summary: Of Python’s common data objects, only numbers are non-iterable; strings, tuples, lists, dictionaries, and so on are all iterable

String for loop

Iterate over each element in the printed string once

for i in "python": 
    print(i)    
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p
y
t
h
o
n
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Here’s another example:

for i in "abcdefg":
    print(i)
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a
b
c
d
e
f
g
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List for loop

Whether it’s a single-layer list or a multi-layer nested list, we can iterate over and print it out:

# Single layer list

a = ["Xiao Ming"."Little red"."Zhang"."Wang"]

for i in a:
    print(i)  Print each element in the list
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Xiao Ming, Xiao Hong, Xiao Zhang, Xiao WangCopy the code

# nested list

b = ["Xiao Ming"."Little red"."Zhang"."Wang"[19.20.18.23]]

for i in b:
    print(i)
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Xiao Ming, Xiao Hong, Xiao Zhang, Xiao Wang [19, 20, 18, 23]Copy the code

The last element in the above example is printed as a whole. What if you want to print it separately?

def qiantao(x) :   Define a function
    for each in x:  Iterate over every element in each original list
        if isinstance(each, list) :Isintance = isintance
            qiantao(each)  # if it is a list, the recursive executive function qiantao()
        else:
            print(each)  If it is not a list, print it directly
            
b = ["Xiao Ming"."Little red"."Zhang"."Wang"[19.20.18.23]]

Call the function, passing in listing B
qiantao(b)  
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Xiao Ming, Xiao Hong, Xiao Zhang, Xiao Wang 19, 20, 18, 23Copy the code

For loops for tuples

The loop and list of tuples are similar:

t = ("Xiao Ming"."Little red"."Wang")

for i in t:
    print(i)
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Xiao Ming, Xiao Hong, Xiao WangCopy the code

Dictionary for loop

Using keys(), values(), items(), we can walk through the dictionary’s keys, values, and key-value pairs, respectively. The default value for traversing a dictionary is the key of the dictionary.

d = {"name":"Peter"."age":20."sex":"male"."address":"china"}
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keys

Iterate over the keys of the dictionary;

for i in d.keys():  Walk through the keys of the dictionary
    print(i)
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name
age
sex
address
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for i in d:  The default is to iterate over key values
    print(i)
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name
age
sex
address
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values

Iterating through the dictionary values:

for i in d.values():  Walk through the dictionary values
    print(i)
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Peter
20
male
china
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items()

Iterate over the dictionary’s keys and values at the same time

for i in d.items():  Walk through the dictionary values
    print(i)
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('name', 'Peter')
('age', 20)
('sex', 'male')
('address', 'china')
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Retrieve the keys and values of the dictionary separately:

for k,v in d.items():
    print(k + "- >" + str(v))
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name--->Peter
age--->20
sex--->male
address--->china
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For loop for range function

The range function is a Python built-in function that generates a series of consecutive integers, mostly used in for loops.

range(start,stop,step)
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Start: contains start. The default value is 0
Stop: does not contain stop and must be written
The step size can be positive or negative. The default value is 1, not 0

1. Basic cases

range(10)  The iterable is generated
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range(0, 10) 
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The default is 0

range(0.10)
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range(0, 10)
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Specify a start of 1

range(1.10)
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range(1, 10)
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Here’s how to expand the results into a list:

list(range(10))  # not including 10 (tail)
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[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
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Specify step size 2:

list(range(0.10.2))  # does not contain 10, the step size is 2
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[0, 2, 4, 6, 8]
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Conclusion: The range function contains the head and not the tail

for i in range(10) :print(i)
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0
1
2
3
4
5
6
7
8
9
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2, Find the number within 100 that is divisible by 5:

for i in range(101) :# does not contain 101, 0-100
    if i % 5= =0:  # % denotes remainder: remainder 0 denotes divisor
        print(i,end="、")
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0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100,Copy the code

3. Gaussian summation

Find the sum of all the numbers from 1 to 100

sum = 0

for i in range(1.101) :sum = sum + i  # For each loop, sum is this number
    
sum
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5050
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Find the sum of odd numbers up to 100:

sum = 0

1, 3, 5, 7...
for i in range(1.101.2) :sum = sum + i
    
sum
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2500
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Find the sum of even numbers up to 100:

sum = 0

2,4,6,8...
for i in range(2.101.2) :sum = sum + i
    
sum
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2550
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Multiple for statements

We can also use the for statement again:

for i in ["python"."java"."html"] :for j in i:
        print(i.upper(),j)    # upper() : Uppercase letters
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PYTHON p
PYTHON y
PYTHON t
PYTHON h
PYTHON o
PYTHON n
JAVA j
JAVA a
JAVA v
JAVA a
HTML h
HTML t
HTML m
HTML l
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for i in [4.5.6] :for j in [1.2.3] :print(i*j)  # multiply any two numbers
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4  # 4 * 1
8  # 4 * 2
12 # 4 * 3
5  # 5 * 1
10 # 5 * 2
15 # 5 * 3
6  # 6 * 1
12 # 6 * 2
18 # 6 * 3
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The derived type

(1) Above we mentioned numbers divisible by 5: use the for loop and if to solve this problem

five = []  Define an empty list

for i in range(101) :# does not contain 101, 0-100
    if i % 5= =0:  # % denotes remainder: remainder 0 denotes divisor
        five.append(i)  # Add to the list
        
five
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[0,
 5,
 10,
 15,
 20,
 25,
 30,
 35,
 40,
 45,
 50,
 55,
 60,
 65,
 70,
 75,
 80,
 85,
 90,
 95,
 100]
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(2) How to use list derivation?

[x for x in range(101) if x % 5= =0]
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[0,
 5,
 10,
 15,
 20,
 25,
 30,
 35,
 40,
 45,
 50,
 55,
 60,
 65,
 70,
 75,
 80,
 85,
 90,
 95,
 100]
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for-else

Maybe you’ve heard if-else, but have you heard for-else? This is also a bit of a mystery in Python

for i in range(5) :print(i)
else:
    print("The end")
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0, 1, 2, 3, 4 EndCopy the code

In other words: the for statement will still execute the else statement

for i in[] :print(i)
    
else:
    print("The end")
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The end of theCopy the code

In the following example, when the remainder of I divided by 3 is 2, the loopback terminates the entire for loop and the else does not execute

for i in range(10) :print(i)
    if i % 3= =2:
        break
else:
    print("The end")
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0
1
2
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Implementing a triangular array

for i in range(1.11) :for k in range(1,i):
        print(k, end="")
    print("\n")
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1 2 3 4 5 6 1 2 3 4 5 6 7 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 9Copy the code

If we wanted to do the reverse, how would we do it?

for i in range(10.0, -1) :for k in range(1,i):
        print(k, end="")
    print("\n")
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1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 1 2 3 1 2 3 1 2 2 3 4 1 2 3 1 2 3 1 2 3 1 2 1 1 2 3 4Copy the code

99 times table

Here is an example to illustrate how to implement the 99 times table, how to play the 99 times table refer to the previous article

for i in range(1.10) :for j in range(1,i+1) :# In order to make sure that there's 4 times 4, which is the same number multiplied by the same number, we need to make I appear, and use I +1
        print('{}x{}={}'.format(j, i, i*j), end="")  The end of # end is marked with a space
    print("\n")
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