Printing a list giving error that says Iterator is not decrementable

Question

I am trying to create a member function that is supposed to replicate a stack by using a list. So I have to print the elements of the list in reverse order. I have a private list declared in my header file as:

class miniStackLT {
private:
    list<T> content;

I have the following code for that function:

template <typename T>
void miniStackLT<T>::PrintStack(){
    list<T>::iterator iter;
    iter = content.end();
    iter--;
    while (iter != content.end()) {
    cout << *iter << " | ";
    iter--;
    }
}

When I run the program I check the size of the so list and it tells me the correct number but when I print it, it prints the correct elements then I get an error that says the list iterator is not decrementable.

What is causing this error? I have tried messing around with the starting and ending points of the loop and the iterator, but I keep getting the error.

Edit: I have also used

template <typename T>
void miniStackLT<T>::PrintStack(){
    list<T>::iterator rit;
    for (auto rit = content.rbegin(); rit != content.rend(); ++rit){
        cout << *iter << " | ";
    }
}

But I still get the error.

`for (auto i = list.rbegin(); i != list.rend(); ++i)` Done in reverse... — DeiDei, Mar 28 '17 at 15:22

score 4 · Accepted Answer · edited May 23 '17 at 12:10

4

You have two issues with your code. First while (iter != content.end()) will always be true since you are walking backwards away from content.end(). Second, even if you change it to while (iter != content.begin()) you will miss the first iterator in the list

The way to walk a list backwards is to use a reverse iterator. std::list has rbegin() and rend() which give you begin and end iterators but they traverse the list backwards so rbegin() gives you the last element in the list. to walk the list you would just use something like

for (auto rit = content.rbegin(); rit != content.rend(); ++rit)

You could also one up that and use a ranged based for loop. Using the code from here by Prikso NAI you would be able to have

for (const auto& e : reverse(content))
    std::cout << e << " | ";

edited May 23 '17 at 12:10

Community

1
1

answered Mar 28 '17 at 15:25

NathanOliver

171,901
28
288
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Still getting the same error when I try this method – Zack Sloan Mar 28 '17 at 15:28
@ZackSloan You don't change the iterator before you do `for (auto rit = content.rbegin(); rit != content.rend(); ++rit)` do you? – NathanOliver Mar 28 '17 at 15:30
No, I just declare it as `list::iterator rit;` – Zack Sloan Mar 28 '17 at 15:33
@ZackSloan Can you add the updated code to your question? – NathanOliver Mar 28 '17 at 15:34
@ZackSloan I saw you edit and I can't see why it would not work. You would need to provide a full [mcve] to figure it out. – NathanOliver Mar 28 '17 at 15:42
Figured it out, was using `cout << *iter << " | ";` instead of `cout << *rit << " | ";` – Zack Sloan Mar 28 '17 at 15:44
@ZackSloan Oh yeah. I missed that as well. Glad you got it – NathanOliver Mar 28 '17 at 15:44

score 0 · Answer 2 · answered Mar 28 '17 at 15:21

0

At some point you reach content.begin(). After that, it is not decrementable.

Changing your condition to while (iter != content.begin()) should fix this.

But you should consider using rbegin() and rend() and a reverse iterator.

answered Mar 28 '17 at 15:21

Rosme

1,049
9
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Printing a list giving error that says Iterator is not decrementable

2 Answers2