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These two uses of if are different The if at the end of a list comprehension syntax: [num for num in hand if num != 11] is a filter; its purpose is to decide whether or not the resulting list ...
Answer
#2: Post edited
by
Karl Knechtel
·
2023-12-01T23:16:54Z (about 1 year ago)
Update link to refer to new canonical I wrote for the syntax in question
- ## These two uses of `if` are different
- The `if` at the end of a list comprehension syntax:
- ```python
- [num for num in hand if num != 11]
- ```
- is a *filter*; its purpose is to decide *whether or not* the resulting list should contain a value that corresponds to any given `num` (`for` each one found `in hand`).
- This is part of the list comprehension syntax. It **cannot** have a corresponding `else`, because that doesn't make logical sense: the purpose is to exclude elements when the condition isn't met, so specifying an alternate value wouldn't be meaningful.
- The `if` in the working if-else example:
- ```python
- [num if num != 11 else 22 for num in hand]
- ```
is part of a *conditional [expression](https://software.codidact.com/posts/289228)* `num if num != 11 else 22`; it is **not** part of the list comprehension syntax, and it *independent* meaning. This expression evaluates to `num` when `num != 11`, and to `22` otherwise (i.e., when `num == 11`).- Such an expression **must** contain an `else` part, because it's part of the syntax. It's a conditional *expression* - not a condition that controls execution of the prior code. Python doesn't have a "void type", and Python expressions must evaluate to some value (or raise an exception) regardless of the input; so a resulting value needs to be specified for both cases of the conditional.
- ## These two uses of `if` are different
- The `if` at the end of a list comprehension syntax:
- ```python
- [num for num in hand if num != 11]
- ```
- is a *filter*; its purpose is to decide *whether or not* the resulting list should contain a value that corresponds to any given `num` (`for` each one found `in hand`).
- This is part of the list comprehension syntax. It **cannot** have a corresponding `else`, because that doesn't make logical sense: the purpose is to exclude elements when the condition isn't met, so specifying an alternate value wouldn't be meaningful.
- The `if` in the working if-else example:
- ```python
- [num if num != 11 else 22 for num in hand]
- ```
- is part of a *[conditional expression](https://software.codidact.com/posts/290335)* `num if num != 11 else 22`; it is **not** part of the list comprehension syntax, and it *independent* meaning. This expression evaluates to `num` when `num != 11`, and to `22` otherwise (i.e., when `num == 11`).
- Such an expression **must** contain an `else` part, because it's part of the syntax. It's a conditional *expression* - not a condition that controls execution of the prior code. Python doesn't have a "void type", and Python expressions must evaluate to some value (or raise an exception) regardless of the input; so a resulting value needs to be specified for both cases of the conditional.
#1: Initial revision
by
Karl Knechtel
·
2023-10-19T21:09:40Z (about 1 year ago)
## These two uses of `if` are different The `if` at the end of a list comprehension syntax: ```python [num for num in hand if num != 11] ``` is a *filter*; its purpose is to decide *whether or not* the resulting list should contain a value that corresponds to any given `num` (`for` each one found `in hand`). This is part of the list comprehension syntax. It **cannot** have a corresponding `else`, because that doesn't make logical sense: the purpose is to exclude elements when the condition isn't met, so specifying an alternate value wouldn't be meaningful. The `if` in the working if-else example: ```python [num if num != 11 else 22 for num in hand] ``` is part of a *conditional [expression](https://software.codidact.com/posts/289228)* `num if num != 11 else 22`; it is **not** part of the list comprehension syntax, and it *independent* meaning. This expression evaluates to `num` when `num != 11`, and to `22` otherwise (i.e., when `num == 11`). Such an expression **must** contain an `else` part, because it's part of the syntax. It's a conditional *expression* - not a condition that controls execution of the prior code. Python doesn't have a "void type", and Python expressions must evaluate to some value (or raise an exception) regardless of the input; so a resulting value needs to be specified for both cases of the conditional.