I got a question about if-else block in Python, here is an example:
a = [1,2,3]
b = [i if i%2 else i+1 for i in a]
# following sentence is wrong
c = [i if i%2 for i in a else i+1]
But in some cases, if can be wrriten after for, just like this:
d = [i for i in a if a%2]
I wonder the reason about this, Thank you all!
This question has already been solved, Thank you again!!!
There are multiple ways when you can actually use if else in Python. The very basic usage is of course the execution of block of code conditionally:
if <condition>:
<body>
else:
<body>
The if block will be executed when the condition is true, otherwise else block comes into execution.
Another use case is when you want to assign values conditionally:
x = y if <condition> else z
In above conditional assignment, the variable x is assigned value of y if the condition is true, other wise it is assigned the value of z. It is widely used in comprehension and return statements as well: [i if i%2==0 else i+1 for i in <iterable>]
Another use case is using if only, this is used in comprehension to filter out some values:
x = [i for i in <iterable> if <condition>]
The above expression will include only the items from iterable for which condition holds true.
Another use case is for else, it is quite unique to Python which allows using of else block after a loop:
for i in <iterable>:
<loop body>
else:
<else body>
The else part will be executed if loop exits normally without a break statement that means the else part is executed even if the code doesn't enter the loop body.
This type of if-else expression is officially (in the Python docs) called a conditional expression, also known as a ternary conditional operator.
In your second and third examples, you combine this with a list comprehension - a for loop in list-brackets.
In your second example, you place the conditional expression (correctly) before the for loop of the list comprehension. This is a valid syntax. For each item that is looped over, the if-else expression is evaluated.
In your thrid example, you place the for loop between the if and the else. This is invalid syntax and to be honest, I don't know what you expect of the output.
In end of your question you state that in some cases the for can be put before the else (and after the if). In the example you give, there is no for.
foranywhere.forstatement in the second part of the example. The syntax is pretty consistent to yourb =statementi if i%2 else i+1work as ternary operator in python