QGIS style polylines with alternate colours

Question

I have a bunch of shapefiles with multiple polylines. Each consistst of 5 up to 30 polylines. I would like to define a style which colours them alternating two colours.

When I use the Categorized style and define a colour ramp of two colours I get the first 10 polylines in blue and the next 10 in red. Is there an easy way to do this or should I manually create a rule-based style in which all odd numbers are red and all even numbers are blue?

Answer 1

I prefer using the following expressions, which contain the modulus operator, for rule-based styling to determine if a value is even or odd:

• Even:

  "id" % 2 = 0
  

• Odd

  "id" % 2 != 0
  

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If you want to create a categorised style, you could use a little bit of Python by using the following in the Python Console:

from PyQt4.QtGui import QColor

layer = iface.activeLayer()
idx = layer.fieldNameIndex('id')
style_dict = {}

for feat in layer.getFeatures():
    if feat[idx] % 2 == 0:
        style_dict[(feat[idx])] = ('#ff0000', str(feat[idx]))
    else:
        style_dict[(feat[idx])] = ('#46add4', str(feat[idx]))

categories = []
for name, (color, label) in style_dict.items():
    symbol = QgsSymbolV2.defaultSymbol(layer.geometryType())
    symbol.setColor(QColor(color))
    category = QgsRendererCategoryV2(name, symbol, label)
    categories.append(category)

field = 'id' 
renderer = QgsCategorizedSymbolRendererV2(field, categories)
layer.setRendererV2(renderer)
layer.triggerRepaint()

Example:

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In my opinion, depending on how many values you have, a rule-based style as you have used may not only be easier to create but also more easy on the eyes ;)

Answer 2

The only thing I have come up with so far is to make a rule-based style with 1 selecting odd and other selecting even id's: