Python while loop not working properly

Question

I am a complete beginner in python (2.7), and this program is supposed to take some students names and their marks in 3 subjects, and return the average of specified name. I did get the answer correctly(through another method), but this program isn't working properly and I want to know why. The main problem is that the list elements with odd indices aren't being deleted. The code is

num=int(raw_input("enter the numbeer of students:"))
d=dict()
marks=[]
for i in range(num):
    name=raw_input("enter student name")
    j=0
    try:
        while (j<3):
            del marks[j]
            j+=1
    except:
        print "okay!"
    print marks
    for i in range(3):
        marks.append(int(raw_input("enther makrs:")))
    print marks,"after"
    d[name]=tuple(marks)
req=raw_input("enter the name you want to check")
s=0
for key in d.keys():
    if req==key:
        n=d[key]
        l=list(n)
        ave=sum(l)/3
        print ave
    else:
        print "boo"

The output for the above program is:

vamshi@vamshi-HP-Notebook:~/python$ python u.py
enter the numbeer of students:2
enter student namev
okay!
[]
enther makrs:1
enther makrs:2
enther makrs:3
[1, 2, 3] after
enter student namek
okay!
[2] #why isn't this 2 deleted?
enther makrs:5
enther makrs:6
enther makrs:7
[2, 5, 6, 7] after
enter the name you want to check

Thanks in advance

Never, ever use a blank "except" statement, especially when it just prints "okay". As MisterMiyagi points out, this hides any problems your script encounters. — Daniel Roseman
– Daniel Roseman, Commented Jul 2, 2016 at 10:07

MisterMiyagi · Accepted Answer · 2016-07-02 09:59:31Z

1

The loop is working properly, but your logic is flawed. Think about which elements you delete during iteration:

[1, 2, 3]
# delete 1st element, i.e. 1
[2, 3]
# delete 2nd element, i.e. 3!!!
[2]
# delete 3rd element, which doesn't exist

Part of your problem is also that your try: ... except: is masking the problem. It is inappropriate for what you want to do.

If you want to clear a list, you can just overwrite it with a new, empty one.

marks[:] = []

answered Jul 2, 2016 at 9:59

MisterMiyagi

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3 Comments

varun aaruru Over a year ago

One small doubt though. how to write this program without the use of try and except statement? I want a clean code. TIA

Moon Cheesez Over a year ago

@varunaaruru since it looks like you are trying to delete the first three items, you can check if the len of marks is >= 3 else delete everything.

MisterMiyagi Over a year ago

@varunaaruru The cleanest approach is to create a new list for every student. In general, reusing mutable data structures (lists, dicts, ...) is a bad idea, since you may get unwanted side effects. If you wouldn't make an explicit copy via tuple, cleaning the list for a student would remove the previous student's entries as well (since it's the same list).

score 0 · Accepted Answer · 2016-07-02 10:09:23Z

0

The del operator deletes only the sepcified index but moves the left objects to the front. That means you are skipping parts of the list.

Try that code:

j=0
try:
    while (j<3):
        del marks[0]
        j+=1
except:
    print "okay!"

Another way to clean the list is:

del marks[:]

Which also makes very clear (good for documentation) what you are trying to achieve.

edited Jul 2, 2016 at 10:09

answered Jul 2, 2016 at 9:58

user5653854

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