I'm creating a fictive map and so I need to create lots of points, lines and for sure polygons. Later on I export my data as geojson. But before that I aways have to go and give every element an unique ID.
I don't need a special sorting, like the biggest polygon gets the smallest ID or so. I just need all polygons with an ID at the end, without doing it manually like I have to do now.
Would be great if someone knows how to do that.
Using the field calculator is the way to go:
Hallelujah! Or Eureka. Or whatever. This can be done. With a shapefile.
maximum("FID") + 1.By unchecking Editable, you can't enter another value or delete what's there. Note that if there are values without an ID, these values won't be updated. At some point I'll experiment with checking Apply default value on update and revising my formula to check for a zero or NULL value to update only those records when they are edited, not any record with a value greater than 1. (Earlier in this post it was discussed how to update the FID field with unique values, which you will need to do if you added the field after there were already features in the shapefile.)
Note that this is saved with the current map file, not the shapefile, so adding that shapefile multiple times will require you to copy that part of the layer style to the newly added layer. To do this, right-click on the layer, choose Styles > Copy Style > Fields, and right-click on another layer, choose Styles > Paste Style > All Style Categories (or continue to Fields). You can also copy that part of the style to any other layer based on a shapefile, but the ID field must have the same name as the layer you're copying from.
if (maximum("id") is NULL, 1 ,maximum("id") + 1)
IF ("id" is NULL,maximum("id") + 1, "id")
I would like to add to vinayan's post and briefly mention the rownum function, as it is very similar and in some cases might be a little more convenient.
id returns the Feature ID, meaning that it always starts at zero.
rownum returns the number of the row, meaning that it starts at one.
So, basically, if you want the auto-increment to start at 0 go for $id, and if you want it to start at 1 then go for $rownum.
Update for QGIS 3
I know I am quite late to this but always good to give any updates:
In QGIS 3 there is now a native tool that can be used to do this exact job and it is called the "Add autoincremental field".
No need to use an expression in the field calculator or do any coding but nevertheless these are all still very useful and good to know.
This topic has come up here: Create Shapefile with auto increment primary key in QGIS
My suggestions would be:
1) SQLITE / SpatialLite databases support auto-incrementing on a field set to INTEGER PRIMARY KEY:
On an INSERT, if the ROWID or INTEGER PRIMARY KEY column is not explicitly given a value, then it will be filled automatically with an unused integer, usually the one more than the largest ROWID currently in use. This is true regardless of whether or not the AUTOINCREMENT keyword is used.
Each time you edit/create polygons, you can fill out their attributes, and SQLITE will give it an incremental unique value in the field you have set to INTEGER PRIMARY KEY type.
When you're ready to export to GEOJSON, you're all set with your UNIQUE ID's.
2) If using Shapefiles, create an OBJECTID field of INTEGER type and use a field calculator expression to populate that field each time you edit/create polygons and need to export them. You will lose the original ID a polygon once had, but you this is the only way to achieve this using .SHP. (I will have to find the field calculator expression).
PostGIS is another data source you might want to explore, though more of a heavy lift than SQLITE, you might find value in such a system as you move forward...
Old post but for anyone else looking for a speedy solution mine was to create a field with $ID + 1 and it will automatically generate starting with 1!
If you don't need something humanly digestlible, there is now an easy fix: In the field propreties, select "UUID Generator", and leave everything blank.
This will automatically create a UUID in the field. Not as friendly as a simple number (as per $id or $rownum), but generates the UUID right from the start, so no successive steps.
you can just delete the first colmn (id) and create new one "As virtual field"
Simply copy the files and create a CSV copy and display again creating OIDs and export as new shapefile or feature class.
these solutions no longer worked for me in QGIS 2.01 Dufour. Typing $id on a new or existing field named 'id' in the expression input field gave me an error "Expression is invalid"
What did work was to type the function $rownum and then click "OK"
The easiest way to do this would be probably with a python script or maybe it's possible with the field calculator. Sorry I don't have one for you, maybe someone else will. In the meantime, I'd search for a python script for it. I've seen a lot about this for ArcGIS, but I'm sure there's something out there for QGIS.
