IndexError: list index out of range Why?

Question

I know that this question has been answered so many times, but I can't figure out where my issue is. This is my code:

from random import*

def VerificationLongue():

    x=randint(0,1000)
    liste=CreerListe()
    Check=0
    i=0

    while i<=len(liste):
        if x==liste[i]:
            Check=Check+1
        i=i+1

    print("X est dans la liste",Check," fois")

def CreerListe(): 
    ListeAleatoire=[]
    for i in range (0,100):
        valeur=randint(0,1000)
        ListeAleatoire.append(valeur)
    return (ListeAleatoire)


VerificationLongue()

This is a simple algorithm to find if a number is in a list of random numbers. I know there is function such 'count', or 'in' but this is for school and they don't want us to use them. So there is the error I get:

line 11, in VerificationLongue
    if x==liste[i]:
IndexError: list index out of range

I don't know why there is this error because it is initialized to 0.

Answer 1

Here:

while i<=len(liste):

i can be equals to len(liste). And it shouldn't. You have to use i<len(liste):

>>> l = range(5)
>>> l
[0, 1, 2, 3, 4]
>>> len(l)
5
>>> l[5]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range
>>> l[4]
4

---

Side note: you could use comprehension to do your while loop:

>>> liste = [randrange(10) for _ in xrange(20)]
>>> liste
[4, 4, 0, 7, 6, 6, 2, 9, 8, 1, 4, 7, 2, 4, 1, 4, 7, 4, 0, 2]
>>> x=randint(0,10)
>>> x
4
>>> sum(x == i for i in liste)
6

But lists also have a count method:

>>> liste.count(x)
6

Answer 2

You've got i<=len(liste) but the last element of your list will occur at index len(liste)-1, meaning you'll get an IndexError.

You can fix this by replacing it with i < len(liste).

Answer 3

All you have to do is print i in your loop and you'll easily see why it's happening.

Your loop should be i < len(liste) rather than <=. Lists are indexed from zero, so if you have 100 items they are numbered 0-99. By uising <= you are going from 0-100, and liste[100] does not exist.