Looping every features pair with PyQGIS

Question

I need code that loops between each pair of features in sequence (feature 1 with feature 2, feature 2 with feature 3...) but using only a for, I searched the forum and found similar code but with two "for" and one "if", having to go through the entire list again every time the loop happens, would there be a way for me to do this with just one for? I tried using something like "next feature" but it failed.

The original code:

iter1 = layer.getFeatures()
for feature1 in iter1:
    p1 = feature1.geometry().asPoint()
    # loop all points again:
    iter2 = layer.getFeatures()
    for feature2 in iter2:
        # following line creates lines from one feature to previous feature:
        if feature2.id() == feature1.id() + 1:
            # create new line feature:
            p2 = feature2.geometry().asPoint()
            l = QgsGeometry.fromPolylineXY([p1,p2])
            linha = QgsFeature()
            linha.setGeometry(l)
            linha.setAttributes([l.length()])
            pr.addFeature(linha)

My code attempt:

features = layer.getFeatures()
for index, actualFeature in enumerate(features):
    p1 = actualFeature.geometry().asPoint()
    # loop all points again:
    nextFeature = features.index()+1
    
    # create new line feature:
    p2 = nextFeature.geometry().asPoint()
    l = QgsGeometry.fromPolylineXY([p1,p2])
    linha = QgsFeature()
    linha.setGeometry(l)
    linha.setAttributes([l.length()])
    pr.addFeature(linha)


vl.updateFields()
QgsProject.instance().addMapLayer(vl)

But one of the problems is that QgsFeatureIterator object has no attribute 'index', and I think I didn't use PyQGIS correctly. I'm still new to this language, could anyone correct the code to my idea?

Answer 1

You can use zip and indexing:

layer = iface.activeLayer()
featurelist = [f for f in layer.getFeatures()] #List all features

for featurepair in zip(featurelist[::2], featurelist[1::2]):
    f1, f2 = featurepair
    print(f1["id"], f2["id"])

Answer 2

In most latest QGIS versions like QGIS 3.34.1 Prizren​, it should come with the pairwise() function from the itertool module. Since Python 3.10.

# imports
from itertools import pairwise
from qgis.core import QgsProject

# getting a layer by its name
layer = QgsProject.instance().mapLayersByName("YOUR_LAYER_NAME")[0]
# a list of all features in the layer
features = [feat for feat in layer.getFeatures()]

# looping over pairs of features
for pair in pairwise(features):
    feat1, feat2 = pair[0], pair[1]
    print(feat1["id"], feat2["id"])

For earlier version of QGIS, one can use the following approach:

# imports
from qgis.core import QgsProject

# getting a layer by its name
layer = QgsProject.instance().mapLayersByName("YOUR_LAYER_NAME")[0]
# a list of all features in the layer
features = [feat for feat in layer.getFeatures()]

# looping over pairs of features
for i in range(len(features) - 1):
    feat1, feat2 = features[i], features[i + 1]
    print(feat1["id"], feat2["id"])

Keep in mind, that features in the may not come sorted, therefore one may find this thread useful: Sorting features by attribute using PyQGIS

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References:

• How can I iterate over overlapping (current, next) pairs of values from a list?
• Iterate over all pairs of consecutive items in a list

Answer 3

I solved the problem, here's the new code:

from qgis.core import QgsProject, QgsGeometry

layer = QgsProject.instance().mapLayersByName("YOUR_LAYER_NAME")[0]
features = [f for f in layer.getFeatures()]

for index, actualFeature in enumerate(features[:-1]):
    p1 = actualFeature.geometry().asPoint()
    # loop all points again:
    nextFeature = features[index+1]
    # create new line feature:
    p2 = nextFeature.geometry().asPoint()
    l = QgsGeometry.fromPolylineXY([p1,p2])
    linha = QgsFeature()
    linha.setGeometry(l)
    linha.setAttributes([l.length()])
    pr.addFeature(linha)


vl.updateFields()
QgsProject.instance().addMapLayer(vl)