How to fix a IndexError: string index out of range

Question

I'm still working on my script for vineyards. I need to count row and vine in row. So I code this :

#Nombre de rang
nbr = QgsExpression( "ID LIKE '%-1'" )
itnbr = layer.getFeatures( QgsFeatureRequest( nbr ) )
litnbr = list(itnbr)
print 'nombre de rang : ' + str(len(litnbr))
#Nombre de pied dans le rang
for i in range(len(litnbr)):
 rang = str(i+1)
 print 'rang : '+ rang
 itp = layer.getFeatures(QgsFeatureRequest().setFilterExpression("ID LIKE '{0}-%'".format(rang[i])))
 litp = list(itp)
 print 'nombre de pieds dans le rang ' + rang + ' : ' + str(len(litp))

I runned it and it works for first loop, first part of second loop and then it crash :

nombre de rang : 60
rang : 1
nombre de pieds dans le rang 1 : 6
rang : 2
exec((script), ns)
 File "<string>", line 34, in <module>
 IndexError: string index out of range

How do I fix this?

Answer 1

rang contains a single value, but you later use it as if it was an array. The line

 itp = layer.getFeatures(QgsFeatureRequest().setFilterExpression("ID LIKE '{0}-%'".format(rang[i])))

should be changed to

 itp = layer.getFeatures(QgsFeatureRequest().setFilterExpression("ID LIKE '{0}-%'".format(rang))

(if your ID = rang, else use i)

Let's note that it works in the 1st iteration as the 1st element of a single value is valid (rang[0]==rang)