feat: tag loop-remove

bot/resources/tags/loop-remove.md

@@ -0,0 +1,31 @@
+---
+embed:
+ title: "Removing items inside a for loop"
+---
+Avoid removing items from a collection, such as a list, as you iterate that collection in a `for` loop:
+```py
+data = [1, 2, 3, 4]
+for item in data:
+ data.remove(item)
+print(data) # [2, 4] <-- every OTHER item was removed!
+```
+`for` loops track the index of the current item with a kind of pointer. Removing an element causes all other elements to shift, but the pointer is not changed:
+```py
+[1, 2, 3, 4] # First iteration: point to the first element
+ ^
+[2, 3, 4] # Remove current: all elements shift
+ ^
+[2, 3, 4] # Next iteration: move the pointer
+ ^ # and so on...
+```
+You can avoid this pitfall by:
+- using a **list comprehension** to produce a new list (as a way of filtering items):
+ ```py
+ data = [x for x in data if x % 2 == 0]
+ ```
+- using a `while` loop and `.pop()` (treating the list as a stack):
+ ```py
+ while data:
+ item = data.pop()
+ ```
+- considering whether you need to remove items in the first place!