Video If-else if-else in list comprehension Chi tiết

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Given a list comprehension you can append one or more if conditions to filter values.

Tóm lược đại ý quan trọng trong bài

• Example list comprehension if elif else in Python
• Python example elif in the list comprehension
• Python if … else List Comprehension
• Related Article – Python List

[ for in if ]

For each in ; if evaluates to True, add (usually a function of ) to the returned list.

For example, this can be used to extract only even numbers from a sequence of integers:

[x for x in range(10) if x % 2 == 0]
# Out: [0, 2, 4, 6, 8]

Live demo

The above code is equivalent to:

even_numbers = []
for x in range(10):
if x % 2 == 0:
even_numbers.append(x)

print(even_numbers)
# Out: [0, 2, 4, 6, 8]

Also, a conditional list comprehension of the form [e for x in y if c] (where e and c are expressions in terms of x) is equivalent to list(filter(lambda x: c, map(lambda x: e, y))).

Despite providing the same result, pay attention to the fact that the former example is almost 2x faster than the latter one. For those who are curious, this is a nice explanation of the reason why.

Note that this is quite different from the … if … else … conditional expression (sometimes known as a ternary expression) that you can use for the part of the list comprehension. Consider the following example:

[x if x % 2 == 0 else None for x in range(10)]
# Out: [0, None, 2, None, 4, None, 6, None, 8, None]

Live demo

Here the conditional expression isn’t a filter, but rather an operator determining the value to be used for the list items:

if else

This becomes more obvious if you combine it with other operators:

[2 * (x if x % 2 == 0 else -1) + 1 for x in range(10)]
# Out: [1, -1, 5, -1, 9, -1, 13, -1, 17, -1]

Live demo

If you are using Python 2.7, xrange may be better than range for several reasons as described in the xrange documentation.

[2 * (x if x % 2 == 0 else -1) + 1 for x in xrange(10)]
# Out: [1, -1, 5, -1, 9, -1, 13, -1, 17, -1]

The above code is equivalent to:

numbers = []
for x in range(10):
if x % 2 == 0:
temp = x
else:
temp = -1
numbers.append(2 * temp + 1)
print(numbers)
# Out: [1, -1, 5, -1, 9, -1, 13, -1, 17, -1]

One can combine ternary expressions and if conditions. The ternary operator works on the filtered result:

[x if x > 2 else ‘*’ for x in range(10) if x % 2 == 0]
# Out: [‘*’, ‘*’, 4, 6, 8]

The same couldn’t have been achieved just by ternary operator only:

[x if (x > 2 and x % 2 == 0) else ‘*’ for x in range(10)]
# Out:[‘*’, ‘*’, ‘*’, ‘*’, 4, ‘*’, 6, ‘*’, 8, ‘*’]

See also: Filters, which often provide a sufficient alternative to conditional list comprehensions.

PDF – Download Python Language for không lấy phí

You can’t use elif in list comprehension because it’s not part of the if-else short-expression syntax in Python.

Get the same logic with chaining:

if b1:
a
elif b2:
b
else:
c

Becomes

a if b1 else b if b2 else c

Example list comprehension if elif else in Python

Simple example code.

[print(‘Hi’) if num == 2 and num % 2 == 0 else print(‘Bye’) if num % 2 == 0 else print(
‘buzz’) if num == 5 else print(num) for num in range(1, 6)]

Output:

Note: it is totally discouraged to use such unreadable list comprehensions in real-life projects!

Source: stackoverflow

Another Example

Python’s conditional expressions were designed exactly for this sort of use-case:

l = [1, 2, 3, 4, 5]
res = [‘Y’ if v == 1 else ‘N’ if v == 2 else ‘Idle’ for v in l]

print(res)

Output: [‘Y’, ‘N’, ‘Idle’, ‘Idle’, ‘Idle’]

Do comment if you have any doubts and suggestions on this Python list tutorial.

Note: IDE: PyCharm 2021.3 (Community Edition)

Windows 10

Python 3.10.1

All Python Examples are in Python 3, so Maybe its different from python 2 or upgraded versions.

Degree in Computer Science and Engineer: App Developer and has multiple Programming languages experience. Enthusiasm for technology & like learning technical.

There’s no direct use “elif” construct ist comprehension conditionals, but it can be simulated with nested if/else statements.

Common if-else syntax

[‘Yes’ if v == 1 else ‘No’ for v in l]

The ternary form of the if/else operator doesn’t have an ‘elif’ built-in, but you can simulate it in the ‘else’ condition:

[‘Yes’ if v == 1 else ‘No’ if v == 2 else ‘0’ for v in l]

Python example elif in the list comprehension

Simple example code use list comprehension is you are going to create another list from the original.

l = [1, 2, 3, 4, 5]
res = [‘Yes’ if v == 1 else ‘No’ if v == 2 else ‘0’ for v in l]

print(res)

Output:

Another Example code

Creating product reviews that take values from 1 to 5 and create a list with three categories:

• Good >= greater or equal to 4
• Neutral = if the review is 3
• Negative < if the review is less than 3

x = [5, 2, 1, 4, 5, 2]

res = [“Good” if i >= 4 else “Neutral” if i == 3 else “Bad” for i in x]

print(res)

Output: [‘Good’, ‘Bad’, ‘Bad’, ‘Good’, ‘Good’, ‘Bad’]

Do comment if you have any doubts and suggestions on this Python List topic.

Note: IDE: PyCharm 2021.3 (Community Edition)

Windows 10

Python 3.10.1

All Python Examples are in Python 3, so Maybe its different from python 2 or upgraded versions.

Degree in Computer Science and Engineer: App Developer and has multiple Programming languages experience. Enthusiasm for technology & like learning technical.

This tutorial will explain multiple ways to perform list comprehension with the if … else statement in Python. A list comprehension is a short and syntactic way to create a list based on an existing list. It is usually used to create a new filtered or changed list from a current list.

For example, we have a python list [‘Ali’,’Mark’, None, ‘Sara’, None, ‘Rahul’] and we want to create a new list [‘Ali’,’Mark’, ”, ‘Sara’, ”, ‘Rahul’], we can do it by using list comprehension.

Python if … else List Comprehension

The below example code demonstrates how we can create a changed list from the existing list using list comprehension with the if … else statement:

my_list = [‘Ali’,’Mark’, None, ‘Sara’, None, ‘Rahul’]

new_list = [str(x.strip()) if x is not None else ” for x in my_list]
print(new_list)

Output:

[‘Ali’, ‘Mark’, ”, ‘Sara’, ”, ‘Rahul’]

The general syntax of list comprehension in Python with if … else is:

[f(x) if condition else g(x) for x in list]

If condition is true for the list element x, f(x), any applicable function, is applied to the element; otherwise, g(x) will be applied.

Example code:

my_list = [‘Ali’,’Mark’, None, ‘Sara’, None, ‘Rahul’]

new_list = [x.upper() if x is not None else ” for x in my_list]
print(new_list)

Output:

[‘ALI’, ‘MARK’, ”, ‘SARA’, ”, ‘RAHUL’]

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Related Article – Python List

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