Python for loop

In the Python programming language, the for loop is often used to repeat code. Another word for “repeat code” is iteration.

Other than iterating over code, a for loop can iterate any sequence of items, such as a list or a string.

grammar:

for loop syntax is as follows:

<span>   </span><span>for</span> <span>iterating_var</span> <span>in</span> <span>sequence</span><span>:</span>
<span>       </span><span>statements</span> <span>(</span><span>s</span><span>)</span>
<span>   </span><span>for</span> <span>iterating_var</span> <span>in</span> <span>sequence</span><span>:</span>
<span>       </span><span>statements</span> <span>(</span><span>s</span><span>)</span>
   for iterating_var in sequence:
       statements (s)

flow chart:

The control flow graph shows the execution of a for loop

Example:

<span>#!/usr/bin/python #-*- coding: UTF-8 -*- </span>
<span>for</span> <span>letter</span> <span>in</span> <span>'Python'</span><span>:</span> <span># first instance </span>   <span>print</span><span>(</span><span>'current letter:'</span><span>,</span> <span>letter</span><span>)</span>
<span>fruits</span> <span>=</span> <span>[</span> <span>'banana'</span><span>,</span> <span>'apple'</span><span>,</span> <span>'mango'</span><span>]</span>
<span>for</span> <span>fruit</span> <span>in</span> <span>fruits</span><span>:</span> <span># second instance </span>   <span>print</span><span>(</span><span>'current letter:'</span><span>,</span> <span>fruit</span><span>)</span>
<span>print</span><span>(</span><span>"Good bye!"</span><span>)</span>
<span>#!/usr/bin/python #-*- coding: UTF-8 -*- </span>
<span>for</span> <span>letter</span> <span>in</span> <span>'Python'</span><span>:</span> <span># first instance </span>   <span>print</span><span>(</span><span>'current letter:'</span><span>,</span> <span>letter</span><span>)</span>

<span>fruits</span> <span>=</span> <span>[</span> <span>'banana'</span><span>,</span> <span>'apple'</span><span>,</span> <span>'mango'</span><span>]</span>
<span>for</span> <span>fruit</span> <span>in</span> <span>fruits</span><span>:</span> <span># second instance </span>   <span>print</span><span>(</span><span>'current letter:'</span><span>,</span> <span>fruit</span><span>)</span>

<span>print</span><span>(</span><span>"Good bye!"</span><span>)</span>
#!/usr/bin/python #-*- coding: UTF-8 -*- 
for letter in 'Python': # first instance    print('current letter:', letter)

fruits = [ 'banana', 'apple', 'mango']
for fruit in fruits: # second instance    print('current letter:', fruit)

print("Good bye!")

Examples of the above output:

current letter: P
current letter: y
current letter: t
current letter: h
current letter: o
current letter: n
current letter: banana
current letter: apple
current letter: mango
current letter: P
current letter: y
current letter: t
current letter: h
current letter: o
current letter: n
current letter: banana
current letter: apple
current letter: mango
current letter: P
current letter: y
current letter: t
current letter: h
current letter: o
current letter: n
current letter: banana
current letter: apple
current letter: mango

by an iterative sequence index

You can traverse through the execution cycle by index, the following examples:

<span>#!/usr/bin/python #-*- coding: UTF-8 -*- </span>
<span>fruits</span> <span>=</span> <span>[</span> <span>'banana'</span><span>,</span> <span>'apple'</span><span>,</span> <span>'mango'</span><span>]</span>
<span>for</span> <span>index</span> <span>in</span> <span>range</span><span>(</span><span>len</span><span>(</span><span>fruits</span><span>)):</span>
    <span>print</span><span>(</span><span>"Current fruit:"</span><span>,</span> <span>fruits</span><span>[</span><span>index</span><span>])</span>
<span>print</span><span>(</span><span>"Good bye!"</span><span>)</span>
<span>#!/usr/bin/python #-*- coding: UTF-8 -*- </span>
<span>fruits</span> <span>=</span> <span>[</span> <span>'banana'</span><span>,</span> <span>'apple'</span><span>,</span> <span>'mango'</span><span>]</span>
<span>for</span> <span>index</span> <span>in</span> <span>range</span><span>(</span><span>len</span><span>(</span><span>fruits</span><span>)):</span>
    <span>print</span><span>(</span><span>"Current fruit:"</span><span>,</span> <span>fruits</span><span>[</span><span>index</span><span>])</span>

<span>print</span><span>(</span><span>"Good bye!"</span><span>)</span>
#!/usr/bin/python #-*- coding: UTF-8 -*- 
fruits = [ 'banana', 'apple', 'mango']
for index in range(len(fruits)):
    print("Current fruit:", fruits[index])

print("Good bye!")

Examples of the above output:

Current fruit: banana
Current fruit: apple
Current fruit: mango
Good bye!
Current fruit: banana
Current fruit: apple
Current fruit: mango
Good bye!
Current fruit: banana
Current fruit: apple
Current fruit: mango
Good bye!

The above examples we use the built len() function and range(), len() function returns the length of the list, i.e. the number of elements. range returns a sequence number.

Other examples

Print a triangular array 1-9:

<span>#!/usr/bin/python # - * - coding: UTF-8 - * - </span>
<span>for</span> <span>i</span> <span>in</span> <span>range</span> <span>(</span><span>1</span><span>,</span><span>11</span><span>):</span>
    <span>for</span> <span>k</span> <span>in</span> <span>range</span> <span>(</span><span>1</span><span>,</span> <span>i</span><span>):</span>
        <span>print</span><span>(</span><span>k</span><span>,</span> <span>end</span><span>=</span><span>''</span><span>),</span>
    <span>print</span><span>(</span><span>"</span><span>\n</span><span>"</span><span>,</span> <span>end</span><span>=</span><span>''</span><span>)</span>
<span>#!/usr/bin/python # - * - coding: UTF-8 - * - </span>
<span>for</span> <span>i</span> <span>in</span> <span>range</span> <span>(</span><span>1</span><span>,</span><span>11</span><span>):</span>
    <span>for</span> <span>k</span> <span>in</span> <span>range</span> <span>(</span><span>1</span><span>,</span> <span>i</span><span>):</span>
        <span>print</span><span>(</span><span>k</span><span>,</span> <span>end</span><span>=</span><span>''</span><span>),</span>
    <span>print</span><span>(</span><span>"</span><span>\n</span><span>"</span><span>,</span> <span>end</span><span>=</span><span>''</span><span>)</span>
#!/usr/bin/python # - * - coding: UTF-8 - * - 
for i in range (1,11):
    for k in range (1, i):
        print(k, end=''),
    print("\n", end='')

Example of the above output:

1
12
123
1234
12345
123456
1234567
12345678
123456789
1
12
123
1234
12345
123456
1234567
12345678
123456789

1
12
123
1234
12345
123456
1234567
12345678
123456789

You can also use a for loop on a dictionary, but this works a bit differently:

<span>d</span> <span>=</span> <span>{</span><span>'x'</span><span>:</span> <span>1</span><span>,</span> <span>'y'</span><span>:</span> <span>2</span><span>,</span> <span>'z'</span><span>:</span> <span>3</span><span>}</span> 
<span>for</span> <span>k</span><span>,</span><span>v</span> <span>in</span> <span>d</span><span>.</span><span>items</span><span>():</span>
    <span>print</span><span>(</span><span>k</span><span>,</span> <span>'corresponds to'</span><span>,</span> <span>v</span><span>)</span>
<span>d</span> <span>=</span> <span>{</span><span>'x'</span><span>:</span> <span>1</span><span>,</span> <span>'y'</span><span>:</span> <span>2</span><span>,</span> <span>'z'</span><span>:</span> <span>3</span><span>}</span> 
<span>for</span> <span>k</span><span>,</span><span>v</span> <span>in</span> <span>d</span><span>.</span><span>items</span><span>():</span>
    <span>print</span><span>(</span><span>k</span><span>,</span> <span>'corresponds to'</span><span>,</span> <span>v</span><span>)</span>
d = {'x': 1, 'y': 2, 'z': 3} 
for k,v in d.items():
    print(k, 'corresponds to', v)

原文链接：Python for loop