Bài trước: Python căn bản (11): Python If ... Else
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12. Python Loops
Python has two primitive loop commands:
- while loops
- for loops
12.1 The while Loop
With the while
loop we can execute a set of statements as long as a condition is true.
Example
Print i as
long as i is less than 6:
i = 1
while i < 6:
print(i)
i += 1
Note: remember to
increment i, or else the loop will continue forever.
The while loop
requires relevant variables to be ready, in this example we need to define an
indexing variable i, which we set to 1.
The break Statement
With the break statement
we can stop the loop even if the while condition is true.
Example
Exit the loop when i is
3:
i = 1
while i < 6:
print(i)
if i == 3:
break
i += 1
The continue Statement
With the continue
statement we can stop the current iteration, and continue with the next.
Example
Continue to the next
iteration if i is 3:
i = 0
while i < 6:
i += 1
if i == 3:
continue
print(i)
The else Statement
With the else
statement we can run a block of code once when the condition no longer is true.
Example
Print a message once when
the condition is false:
i = 1
while i < 6:
print(i)
i += 1
else:
print("i is no longer less
than 6")
12.2 The for Loop
A for loop is used for iterating over a sequence (that
is either a list, a tuple, a dictionary, a set, or a string).
This is less like the for keyword in other programming languages, and works
more like an iterator method as found in other object-oriented programming
languages.
With the for loop we can execute a set of statements, once for
each item in a list, tuple, set etc.
Example
Print each fruit in a fruit list:
fruits = ["apple", "banana", "cherry"]
for x in fruits:
print(x)
The for loop does not require an indexing variable to set
beforehand.
Looping Through a String
Even strings are iterable objects, they contain a sequence of
characters.
Example
Loop through the letters in the word "banana":
for x in "banana":
print(x)
The break Statement
With the break statement we can stop the loop before it has looped
through all the items.
Example
Exit the loop when x is "banana":
fruits = ["apple", "banana", "cherry"]
for x in fruits:
print(x)
if x == "banana":
break
The continue Statement
With the continue statement we can stop the current iteration of
the loop, and continue with the next.
Example
Do not print banana:
fruits = ["apple", "banana", "cherry"]
for x in fruits:
if x == "banana":
continue
print(x)
The range() Function
To loop through a set of code specified number of times, we can
use the range() function.
The range() function returns a sequence of numbers, starting from
0 by default, and increments by 1 (by default), and ends at a specified number.
Example
Using the range() function:
for x in range(6):
print(x)
Note that range(6) is not the values of 0 to 6, but the values 0
to 5.
The range() function defaults to 0 as a starting value, however it
is possible to specify the starting value by adding a parameter: range(2, 6),
which means values from 2 to 6 (but not including 6).
Example
Using the start parameter:
for x in range(2, 6):
print(x)
The range() function defaults to increment the sequence by 1,
however it is possible to specify the increment value by adding a third
parameter: range(2, 30, 3):
Example
Increment the sequence with 3 (default is 1):
for x in range(2, 30, 3):
print(x)
Else in For Loop
The else keyword in a for loop specifies a block of code
to be executed when the loop is finished.
Example
Print all numbers from 0 to 5, and print a message when the loop
has ended:
for x in range(6):
print(x)
else:
print("Finally finished!")
Note: The else block will NOT be executed if the loop is stopped
by a break statement.
Example
Break the loop when x is 3, and see what happens with the else
block:
for x in range(6):
if x == 3: break
print(x)
else:
print("Finally finished!")
Nested Loops
A nested loop is a loop inside a loop.
The "inner loop" will be executed one time for each
iteration of the "outer loop".
Example
Print each adjective for every fruit:
adj = ["red", "big", "tasty"]
fruits = ["apple", "banana", "cherry"]
for x in adj:
for y in fruits:
print(x, y)
The pass Statement
for loops cannot be empty, but
if you for some reason have a for loop with no content, put in the pass
statement to avoid getting an error.
for x in [0, 1, 2]:
pass
12.3 Exercise
1. Which statement is the
correct syntax to break out of a loop?
A. end
B. return
C. break
D. stop
2. What will be the result of the following code:
for x in range(3):
print(x)
A. 0 1 2
B. 0 1 2 3
C. 1 2 3
D. x x x
3. Write the output of the
following code.
x = 2
while (x < 10):
print(x**2)
x += 2
4. Write the output of the
following code.
i = 9
while True:
if i % 3 == 0:
break
print("A")
5. Write a Python code to print the following number pattern using
the while loop.
1
1 2
1 2 3
1 2 3 4
1 2 3 4 5
6. Write a Python program to print the first 10 even numbers.
Using the for loop.
7. Write a Python program to print the following number pattern
using the for loop.
5 4 3 2 1
4 3 2 1
3 2 1
2 1
1
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The answer hints:
1(C), 2(A),
3. Write the output of the
following code.
x = 2
while (x < 10):
print(x**2)
x += 2
[4, 16, 36, 64]
4. Write the output of the
following code.
i = 9
while True:
if i % 3 == 0:
break
print("A")
[nothing]
5. Write a Python code to print the following number pattern using
the while loop.
1
1 2
1 2 3
1 2 3 4
1 2 3 4 5
i = 1
while i <= 5:
j = 1
while j <= i:
print(j, end=" ")
j += 1
print()
i += 1
6. Write a Python program to print the first 10 even numbers.
Using the for loop.
for i in range(2, 22, 2):
print(i)
7. Write a Python program to print the following number pattern using
the for loop.
5 4 3 2 1
4 3 2 1
3 2 1
2 1
1
n = 5
k = 5
for i in range(0,n+1):
for j in range(k-i,0,-1):
print(j,end=' ')
print()
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Cập nhật: 30/10/2024
Bài sau: Python căn bản (13): Python Lists
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