Question

I THINK DUPLICATE FROM

Remove items from a list while iterating

where best solution is:

somelist[:] = [tup for tup in somelist if determine(tup)]

I don’t understand in deepth how it works, but seems to work !!!

NOW I understand how it works !! (2017/02/05)

I’m a python beginner and I find something maybe ot strange in list iteration and deletion. Here is my code:

# -*- coding: utf-8 -*-s
#
# Fichier: testques.py
#

liste = [0,1,2,3,4,5,6,7,8,9]

cpt1=0
cpt2=0
cpt3=len(liste)

for i in liste:
    if i != 2:
        cpt1 += 1
    else:
        cpt2 += 1
        del liste[i]

print cpt1
print cpt2
print cpt3

>python test.py
8
1
10

As you can see, 8+1 is not 10.

My thing is that “i” is only a reference to a list index. When I delete the element list, then all elements below the one I delete go up one level and then just next one is not parse by for … in.

Then can somebody help me to achieve the goal of having at the end the following result:
9
1
10

EDIT AFTER 3 ANSWERS
My goal is not counting number of elements equal to 2.

In fact, each element of my list is a dictionnary, and I want to delete all list items where the value for the xxx key of each dictionnary is no exactly 7 digits. And after I’ll do others operations on the now clean list. This is what i try to achieve.

Sorry for not havjng been enough clear in first question.

Here is my real code:

cpt = 0
liste_longueur_totale = len(i)
liste_longueur_clean = 0
liste_delete = 0
numero = re.compile('^[0-9]{7}$')
for row in i:
    if re.match(numero, row['j_numero']):
        liste_longueur_clean += 1
        cpt += 1
    else:
        liste_delete += 1
        print liste_delete, '-', i[cpt]['j_id'], '-', i[cpt]['j_numero']
        del i[cpt]
        cpt += 1
print "%s : %s + %s" % (liste_longueur_totale,liste_longueur_clean,liste_delete )
print len(i)

“i” is a list of 703 items. Each one is a dictionnary.

When I run it, I’ve the following result:

703: 673 + 15
688

When I run the same code without the line:

del i[cpt]

I’ve the following result:

703: 681 + 21
703

An in fact I know that 21 items are not responding to the re.match()

Then, I’m a little perplex !!!

Answer 1

Forget doing it the hard way. Especially if your list isn’t so long that iterating over it multiple times is a big deal.

cpt1 = sum(1 for x in liste if x != 2)
cpt2 = sum(1 for x in liste if x == 2)