Selecting first items of same attribute value using PyQGIS?

Question

I've been getting into PyQGIS, and as a start I want to select the first instance of pair values of a point layer (start and end of a street), identified by the same integer, I got this, but it doesn't work:

("int_id" is the value that is the same for the start and end points)

cap = iface.activeLayer()
feat = cap.getFeatures()

frm = []

for f in feat:
    if f["int_id"] not in frm:
        frm.append(f.id())

cap.select(frm)

I need to create lines from these points (start and end), I haven't found a way to create them in PyQGIS, but I found point connector plugin that could at least create the streets that are a straight line, but first I need to separate the start points from the end points.

Answer 1

You can use collections.Counter to find which ID:s occur twice:

from collections import Counter
cap = iface.activeLayer()
feat = cap.getFeatures()
fieldname = 'int_id'

cnt = Counter([f[fieldname] for f in feat]) #Create dictionary of int_id and their counts
twice = [k for k,v in cnt.items() if v==2] #cnt.items() if python3/qgis3, cnt.iteritems() py2/qgis2

expr = '"{0}"={1}'.format(fieldname, twice[0])
cap.selectByExpression(expr)

Answer 2

The below assumes that only the init_id of the first and last points are equal

from collections import Counter
from itertools import groupby
from qgis.core import *

cap = iface.activeLayer()
feat = cap.getFeatures()

#create a list conatining all features
frm = []

#assuming field int_id is in the first column=0
for f in feat:
    frm.append(f[0])


print (frm)

#now creating new list containing the 2 points
list2=[]
for k,v in Counter(frm).items():
    if v>1:
        list2.append(k) 

print ("list2 =", list2)
#finally selecting the 2 points with same id
for m in cap.getFeatures():
    if m[0] in list2:
        cap.select(m.id())

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