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UnKnown X · Dec 25th, 2005, 7:03 PM

When I try to do something like this:

y = ['1', '2', '3', '4', '5']    # example list
x = []                           # example list 2
i = 0
while i < len(y):
    x.append(y[i])
    i += 1
print x

I get an error message about trying to access an out-of-bounds element of y. This gets fixed if I do this:

(Toggle Plain Text)

y = ['1', '2', '3', '4', '5']    # example list
x = []                           # example list 2
i = 0
while i < len(y) - 1:            # fix is here
    x.append(y[i])
    i += 1
print x

But then, since i only goes to the second last item in y, the programme will only print:
['1', '2', '3', '4']

This is easily fixed by appending an extra item (such as 0) to the end of y, but I'm wondering why this happens.

Edit: wait a second. This list did work. I must've changed something when simplifying from the code from which I took it. Here's where it was originally:

(Toggle Plain Text)

def f(y):
    y.sort()
    y.append('0')
    i, j, k, flag = 0, 0, 0, 0
    x = []
    while j <= 9:
        try:
            while str(y[i]) == str(j) and i < len(y)-1:    # Here's the problem
                flag = 1
                i += 1
                k += 1
        except:
            print "Error!"
            return []
        if flag == 1:
            if k == 1:
                s = str(k) + ' time'
            else:
                s = str(k) + ' times'
            x.append([str(j),s])
        flag, k = 0, 0
        j += 1
    return x

Edit again: Oh, the problem must be in the str(y[i]) part of str(y[i]) == str(j), but still: why?

UnKnown X · Dec 25th, 2005, 7:14 PM

Ah, problem found.

In str(y[i]) == str(j) and i < len(y)-1, it would check whether str(y[i]) == str(j) after i had been incremented, but before it could check if i < len(y), thus it would go beyond the list's boundary.

Putting the i < len(y) test before the str(y[i]) == str(j) test fixed it.

coldDeath · Dec 26th, 2005, 2:40 AM

Nice job :-)

Arevos · Dec 26th, 2005, 3:59 AM

You might want to think about using a dictionary to hold the counts of your items:

(Toggle Plain Text)

d = {}
for x in y:
    d[x] = d.get(x, 1) + 1

And perhaps giving your variables and functions longer and more descriptive names. It's all too easy to forget what does what several months down the line:

(Toggle Plain Text)

bag = {}
for item in list:
    bag[item] = bag.get(item, 1) + 1

(A bag being the technical name of the data struct your function creates.)

UnKnown X · Dec 26th, 2005, 6:04 AM

Thanks for the replies!

I was simply doing this as a test. In slightly more serious projects, I do use more descriptive names.

I'll try the dictionaries, too. Thanks!

Dec 25th, 2005, 7:03 PM	#1
UnKnown X Hobbyist Programmer Join Date: Dec 2005 Location: Sandvika, Norway Posts: 114 Rep Power: 0	Question about list error When I try to do something like this: (Toggle Plain Text) y = ['1', '2', '3', '4', '5'] # example list x = [] # example list 2 i = 0 while i < len(y): x.append(y[i]) i += 1 print x y = ['1', '2', '3', '4', '5'] # example list x = [] # example list 2 i = 0 while i < len(y): x.append(y[i]) i += 1 print x I get an error message about trying to access an out-of-bounds element of y. This gets fixed if I do this: (Toggle Plain Text) y = ['1', '2', '3', '4', '5'] # example list x = [] # example list 2 i = 0 while i < len(y) - 1: # fix is here x.append(y[i]) i += 1 print x y = ['1', '2', '3', '4', '5'] # example list x = [] # example list 2 i = 0 while i < len(y) - 1: # fix is here x.append(y[i]) i += 1 print x But then, since i only goes to the second last item in y, the programme will only print: ['1', '2', '3', '4'] This is easily fixed by appending an extra item (such as 0) to the end of y, but I'm wondering why this happens. Edit: wait a second. This list did work. I must've changed something when simplifying from the code from which I took it. Here's where it was originally: (Toggle Plain Text) def f(y): y.sort() y.append('0') i, j, k, flag = 0, 0, 0, 0 x = [] while j <= 9: try: while str(y[i]) == str(j) and i < len(y)-1: # Here's the problem flag = 1 i += 1 k += 1 except: print "Error!" return [] if flag == 1: if k == 1: s = str(k) + ' time' else: s = str(k) + ' times' x.append([str(j),s]) flag, k = 0, 0 j += 1 return x def f(y): y.sort() y.append('0') i, j, k, flag = 0, 0, 0, 0 x = [] while j <= 9: try: while str(y[i]) == str(j) and i < len(y)-1: # Here's the problem flag = 1 i += 1 k += 1 except: print "Error!" return [] if flag == 1: if k == 1: s = str(k) + ' time' else: s = str(k) + ' times' x.append([str(j),s]) flag, k = 0, 0 j += 1 return x Edit again: Oh, the problem must be in the str(y[i]) part of str(y[i]) == str(j), but still: why?

Dec 26th, 2005, 2:40 AM	#3
coldDeath Expert Programmer Join Date: Aug 2005 Location: UK Posts: 862 Rep Power: 3	Nice job :-) __________________ Programming Tutorials - Linux Installation Guide - Learn Python - Learn C++ Got Slack? Join us at #programmingforums @ irc.freenode.net! My software never has bugs. It just develops random features.

Dec 26th, 2005, 3:59 AM	#4
Arevos Programming Guru Join Date: Aug 2005 Location: England Posts: 1,499 Rep Power: 4	You might want to think about using a dictionary to hold the counts of your items: (Toggle Plain Text) d = {} for x in y: d[x] = d.get(x, 1) + 1 d = {} for x in y: d[x] = d.get(x, 1) + 1 And perhaps giving your variables and functions longer and more descriptive names. It's all too easy to forget what does what several months down the line: (Toggle Plain Text) bag = {} for item in list: bag[item] = bag.get(item, 1) + 1 bag = {} for item in list: bag[item] = bag.get(item, 1) + 1 (A bag being the technical name of the data struct your function creates.)

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Dec 25th, 2005, 7:14 PM	#2
UnKnown X Hobbyist Programmer Join Date: Dec 2005 Location: Sandvika, Norway Posts: 114 Rep Power: 0	Ah, problem found. In str(y[i]) == str(j) and i < len(y)-1, it would check whether str(y[i]) == str(j) after i had been incremented, but before it could check if i < len(y), thus it would go beyond the list's boundary. Putting the i < len(y) test before the str(y[i]) == str(j) test fixed it.

Dec 26th, 2005, 6:04 AM	#5
UnKnown X Hobbyist Programmer Join Date: Dec 2005 Location: Sandvika, Norway Posts: 114 Rep Power: 0	Thanks for the replies! I was simply doing this as a test. In slightly more serious projects, I do use more descriptive names. I'll try the dictionaries, too. Thanks!

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Programming Forums > Script Programming > Python

Question about list error