Sort a list of numerical strings in ascending order
Question:
I have created a sqlite database which has a table which stores temperature values. The temperature values are written to the database in ascending order for the first time. Then I read the temperature values from the database into a list and then add that list to a combo box to select temperatures – works fine.
The resulting list is, say:
templist = ['25', '50', '100', '150', '200', '250', '300'].
Then I add a new temperature value, say, ’33’ to the database.
It gets appended to the end of the table. If I read the temperatures now, the list will become:
['25', '50', '100', '150', '200', '250', '300', '33'].
If I do templist.sort() or sorted(templist), the end result is
['150', '200', '25', '250', '300', '33', '50']
Is there any simple way to sort the list in ascending order so that I get:
['25', '33', '50', '100', '150', '200', '250', '300']
Answers:
in python sorted works like you want with integers:
>>> sorted([10,3,2])
[2, 3, 10]
it looks like you have a problem because you are using strings:
>>> sorted(['10','3','2'])
['10', '2', '3']
(because string ordering starts with the first character, and “1” comes before “2”, no matter what characters follow) which can be fixed with key=int
>>> sorted(['10','3','2'], key=int)
['2', '3', '10']
which converts the values to integers during the sort (it is called as a function – int('10') returns the integer 10)
and as suggested in the comments, you can also sort the list itself, rather than generating a new one:
>>> l = ['10','3','2']
>>> l.sort(key=int)
>>> l
['2', '3', '10']
but i would look into why you have strings at all. you should be able to save and retrieve integers. it looks like you are saving a string when you should be saving an int? (sqlite is unusual amongst databases, in that it kind-of stores data in the same type as it is given, even if the table column type is different).
and once you start saving integers, you can also get the list back sorted from sqlite by adding order by ... to the sql command:
select temperature from temperatures order by temperature;
The recommended approach in this case is to sort the data in the database, adding an ORDER BY at the end of the query that fetches the results, something like this:
SELECT temperature FROM temperatures ORDER BY temperature ASC; -- ascending order
SELECT temperature FROM temperatures ORDER BY temperature DESC; -- descending order
If for some reason that is not an option, you can change the sorting order like this in Python:
templist = [25, 50, 100, 150, 200, 250, 300, 33]
sorted(templist, key=int) # ascending order
> [25, 33, 50, 100, 150, 200, 250, 300]
sorted(templist, key=int, reverse=True) # descending order
> [300, 250, 200, 150, 100, 50, 33, 25]
As has been pointed in the comments, the int key (or float if values with decimals are being stored) is required for correctly sorting the data if the data received is of type string, but it’d be very strange to store temperature values as strings, if that is the case, go back and fix the problem at the root, and make sure that the temperatures being stored are numbers.
I have created a sqlite database which has a table which stores temperature values. The temperature values are written to the database in ascending order for the first time. Then I read the temperature values from the database into a list and then add that list to a combo box to select temperatures – works fine.
The resulting list is, say:
templist = ['25', '50', '100', '150', '200', '250', '300'].
Then I add a new temperature value, say, ’33’ to the database.
It gets appended to the end of the table. If I read the temperatures now, the list will become:
['25', '50', '100', '150', '200', '250', '300', '33'].
If I do templist.sort() or sorted(templist), the end result is
['150', '200', '25', '250', '300', '33', '50']
Is there any simple way to sort the list in ascending order so that I get:
['25', '33', '50', '100', '150', '200', '250', '300']
in python sorted works like you want with integers:
>>> sorted([10,3,2])
[2, 3, 10]
it looks like you have a problem because you are using strings:
>>> sorted(['10','3','2'])
['10', '2', '3']
(because string ordering starts with the first character, and “1” comes before “2”, no matter what characters follow) which can be fixed with key=int
>>> sorted(['10','3','2'], key=int)
['2', '3', '10']
which converts the values to integers during the sort (it is called as a function – int('10') returns the integer 10)
and as suggested in the comments, you can also sort the list itself, rather than generating a new one:
>>> l = ['10','3','2']
>>> l.sort(key=int)
>>> l
['2', '3', '10']
but i would look into why you have strings at all. you should be able to save and retrieve integers. it looks like you are saving a string when you should be saving an int? (sqlite is unusual amongst databases, in that it kind-of stores data in the same type as it is given, even if the table column type is different).
and once you start saving integers, you can also get the list back sorted from sqlite by adding order by ... to the sql command:
select temperature from temperatures order by temperature;
The recommended approach in this case is to sort the data in the database, adding an ORDER BY at the end of the query that fetches the results, something like this:
SELECT temperature FROM temperatures ORDER BY temperature ASC; -- ascending order
SELECT temperature FROM temperatures ORDER BY temperature DESC; -- descending order
If for some reason that is not an option, you can change the sorting order like this in Python:
templist = [25, 50, 100, 150, 200, 250, 300, 33]
sorted(templist, key=int) # ascending order
> [25, 33, 50, 100, 150, 200, 250, 300]
sorted(templist, key=int, reverse=True) # descending order
> [300, 250, 200, 150, 100, 50, 33, 25]
As has been pointed in the comments, the int key (or float if values with decimals are being stored) is required for correctly sorting the data if the data received is of type string, but it’d be very strange to store temperature values as strings, if that is the case, go back and fix the problem at the root, and make sure that the temperatures being stored are numbers.