Appending a value to a nested QList
I am trying to append values to a QList inside another QList but it doesn't seem to work?
Here is a MCVE of my problem where I try to append int values:
#include <QList>
#include <QDebug>
int main(int argc, char *argv) {
QList<QList<int>> my_list;
int result;
for (int i = 0; i < 2; ++i) {
for (int j = 0; j < 4; ++j) {
result = i * j;
my_list.value(i).push_back(result);
qDebug() << my_list.size() << "," << my_list.value(i).size() << " : " << my_list.value(i).value(j);
}
}
return 0;
}
This yields:
Starting C:Users ... buildreleasename_of_the_app.exe...
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
C:Users ... buildreleasename_of_the_app.exe exited with code 0
Can anyone please tell me what I'm doing wrong?
qt nested append push-back qlist
edited Nov 20 at 18:43
TrebuchetMS
1,9861618
asked Nov 20 at 17:52
Matt
106
add a comment |
I am trying to append values to a QList inside another QList but it doesn't seem to work?
Here is a MCVE of my problem where I try to append int values:
#include <QList>
#include <QDebug>
int main(int argc, char *argv) {
QList<QList<int>> my_list;
int result;
for (int i = 0; i < 2; ++i) {
for (int j = 0; j < 4; ++j) {
result = i * j;
my_list.value(i).push_back(result);
qDebug() << my_list.size() << "," << my_list.value(i).size() << " : " << my_list.value(i).value(j);
}
}
return 0;
}
This yields:
Starting C:Users ... buildreleasename_of_the_app.exe...
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
C:Users ... buildreleasename_of_the_app.exe exited with code 0
Can anyone please tell me what I'm doing wrong?
qt nested append push-back qlist
edited Nov 20 at 18:43
TrebuchetMS
1,9861618
asked Nov 20 at 17:52
Matt
106
add a comment |
I am trying to append values to a QList inside another QList but it doesn't seem to work?
Here is a MCVE of my problem where I try to append int values:
#include <QList>
#include <QDebug>
int main(int argc, char *argv) {
QList<QList<int>> my_list;
int result;
for (int i = 0; i < 2; ++i) {
for (int j = 0; j < 4; ++j) {
result = i * j;
my_list.value(i).push_back(result);
qDebug() << my_list.size() << "," << my_list.value(i).size() << " : " << my_list.value(i).value(j);
}
}
return 0;
}
This yields:
Starting C:Users ... buildreleasename_of_the_app.exe...
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
C:Users ... buildreleasename_of_the_app.exe exited with code 0
Can anyone please tell me what I'm doing wrong?
qt nested append push-back qlist
edited Nov 20 at 18:43
TrebuchetMS
1,9861618
asked Nov 20 at 17:52
Matt
106
I am trying to append values to a QList inside another QList but it doesn't seem to work?
Here is a MCVE of my problem where I try to append int values:
#include <QList>
#include <QDebug>
int main(int argc, char *argv) {
QList<QList<int>> my_list;
int result;
for (int i = 0; i < 2; ++i) {
for (int j = 0; j < 4; ++j) {
result = i * j;
my_list.value(i).push_back(result);
qDebug() << my_list.size() << "," << my_list.value(i).size() << " : " << my_list.value(i).value(j);
}
}
return 0;
}
This yields:
Starting C:Users ... buildreleasename_of_the_app.exe...
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
0 , 0 : 0
C:Users ... buildreleasename_of_the_app.exe exited with code 0
Can anyone please tell me what I'm doing wrong?
qt nested append push-back qlist
qt nested append push-back qlist
edited Nov 20 at 18:43
TrebuchetMS
1,9861618
asked Nov 20 at 17:52
Matt
106
edited Nov 20 at 18:43
TrebuchetMS
1,9861618
asked Nov 20 at 17:52
Matt
106
edited Nov 20 at 18:43
TrebuchetMS
1,9861618
edited Nov 20 at 18:43
TrebuchetMS
1,9861618
edited Nov 20 at 18:43
TrebuchetMS
1,9861618
1,9861618
asked Nov 20 at 17:52
Matt
106
asked Nov 20 at 17:52
Matt
106
asked Nov 20 at 17:52
Matt
106
106
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
There are two issues with this code sample:
First:
The container my_list hasn't been initialised yet. The line
my_list.value(i).push_back(result);
doesn't actually push values into the container (as we might have hoped).
In your code, i is always out of bounds. (Since my_list will always have size 0.) As a result, and according to the documentation,
If the index i is out of bounds, the function returns a default-constructed value.
Since this default-constructed value isn't assigned anywhere, it will most probably be allocated on the heap and be left sitting there until destructed.
Further accesses into my_list such as qDebug() << my_list.value(i).size() will default-construct another QList (since again, i is out of bounds).
Make sure you already have QList<int> ready to push values into.
Second:
The value() method returns a const reference to the QList which does not allow modification (same for at()). If you want to push values to a QList you should use the instead of the value() method.
The following code does what you want:
for (int i = 0; i < 2; ++i) {
my_list.push_back(QList<int>()); // first appends a new QList at index `i`
for (int j = 0; j < 4; ++j) {
result = i * j;
my_list[i].push_back(result); // safely retrieves QList at index `i` and adds an element
}
}
edited Nov 21 at 0:19
Matt
106
answered Nov 20 at 18:04
TrebuchetMS
1,9861618
There where two issues with my code. Your answer addressed the first (major) issue (thanks!) and allowed me to further correct my code. I added it to your answer as an edit.
– Matt
Nov 20 at 18:42
I've approved the edit. 👍 Didn't realise.value()didn't provide a reference. I usually only useand.at(). :-)
– TrebuchetMS
Nov 20 at 18:44
add a comment |
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1 Answer
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1 Answer
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active
oldest
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oldest
votes
active
oldest
votes
There are two issues with this code sample:
First:
The container my_list hasn't been initialised yet. The line
my_list.value(i).push_back(result);
doesn't actually push values into the container (as we might have hoped).
In your code, i is always out of bounds. (Since my_list will always have size 0.) As a result, and according to the documentation,
If the index i is out of bounds, the function returns a default-constructed value.
Since this default-constructed value isn't assigned anywhere, it will most probably be allocated on the heap and be left sitting there until destructed.
Further accesses into my_list such as qDebug() << my_list.value(i).size() will default-construct another QList (since again, i is out of bounds).
Make sure you already have QList<int> ready to push values into.
Second:
The value() method returns a const reference to the QList which does not allow modification (same for at()). If you want to push values to a QList you should use the instead of the value() method.
The following code does what you want:
for (int i = 0; i < 2; ++i) {
my_list.push_back(QList<int>()); // first appends a new QList at index `i`
for (int j = 0; j < 4; ++j) {
result = i * j;
my_list[i].push_back(result); // safely retrieves QList at index `i` and adds an element
}
}
edited Nov 21 at 0:19
Matt
106
answered Nov 20 at 18:04
TrebuchetMS
1,9861618
There where two issues with my code. Your answer addressed the first (major) issue (thanks!) and allowed me to further correct my code. I added it to your answer as an edit.
– Matt
Nov 20 at 18:42
I've approved the edit. 👍 Didn't realise.value()didn't provide a reference. I usually only useand.at(). :-)
– TrebuchetMS
Nov 20 at 18:44
add a comment |
There are two issues with this code sample:
First:
The container my_list hasn't been initialised yet. The line
my_list.value(i).push_back(result);
doesn't actually push values into the container (as we might have hoped).
In your code, i is always out of bounds. (Since my_list will always have size 0.) As a result, and according to the documentation,
If the index i is out of bounds, the function returns a default-constructed value.
Since this default-constructed value isn't assigned anywhere, it will most probably be allocated on the heap and be left sitting there until destructed.
Further accesses into my_list such as qDebug() << my_list.value(i).size() will default-construct another QList (since again, i is out of bounds).
Make sure you already have QList<int> ready to push values into.
Second:
The value() method returns a const reference to the QList which does not allow modification (same for at()). If you want to push values to a QList you should use the instead of the value() method.
The following code does what you want:
for (int i = 0; i < 2; ++i) {
my_list.push_back(QList<int>()); // first appends a new QList at index `i`
for (int j = 0; j < 4; ++j) {
result = i * j;
my_list[i].push_back(result); // safely retrieves QList at index `i` and adds an element
}
}
edited Nov 21 at 0:19
Matt
106
answered Nov 20 at 18:04
TrebuchetMS
1,9861618
There where two issues with my code. Your answer addressed the first (major) issue (thanks!) and allowed me to further correct my code. I added it to your answer as an edit.
– Matt
Nov 20 at 18:42
I've approved the edit. 👍 Didn't realise.value()didn't provide a reference. I usually only useand.at(). :-)
– TrebuchetMS
Nov 20 at 18:44
add a comment |
There are two issues with this code sample:
First:
The container my_list hasn't been initialised yet. The line
my_list.value(i).push_back(result);
doesn't actually push values into the container (as we might have hoped).
In your code, i is always out of bounds. (Since my_list will always have size 0.) As a result, and according to the documentation,
If the index i is out of bounds, the function returns a default-constructed value.
Since this default-constructed value isn't assigned anywhere, it will most probably be allocated on the heap and be left sitting there until destructed.
Further accesses into my_list such as qDebug() << my_list.value(i).size() will default-construct another QList (since again, i is out of bounds).
Make sure you already have QList<int> ready to push values into.
Second:
The value() method returns a const reference to the QList which does not allow modification (same for at()). If you want to push values to a QList you should use the instead of the value() method.
The following code does what you want:
for (int i = 0; i < 2; ++i) {
my_list.push_back(QList<int>()); // first appends a new QList at index `i`
for (int j = 0; j < 4; ++j) {
result = i * j;
my_list[i].push_back(result); // safely retrieves QList at index `i` and adds an element
}
}
edited Nov 21 at 0:19
Matt
106
answered Nov 20 at 18:04
TrebuchetMS
1,9861618
There are two issues with this code sample:
First:
The container my_list hasn't been initialised yet. The line
my_list.value(i).push_back(result);
doesn't actually push values into the container (as we might have hoped).
In your code, i is always out of bounds. (Since my_list will always have size 0.) As a result, and according to the documentation,
If the index i is out of bounds, the function returns a default-constructed value.
Since this default-constructed value isn't assigned anywhere, it will most probably be allocated on the heap and be left sitting there until destructed.
Further accesses into my_list such as qDebug() << my_list.value(i).size() will default-construct another QList (since again, i is out of bounds).
Make sure you already have QList<int> ready to push values into.
Second:
The value() method returns a const reference to the QList which does not allow modification (same for at()). If you want to push values to a QList you should use the instead of the value() method.
The following code does what you want:
for (int i = 0; i < 2; ++i) {
my_list.push_back(QList<int>()); // first appends a new QList at index `i`
for (int j = 0; j < 4; ++j) {
result = i * j;
my_list[i].push_back(result); // safely retrieves QList at index `i` and adds an element
}
}
edited Nov 21 at 0:19
Matt
106
answered Nov 20 at 18:04
TrebuchetMS
1,9861618
edited Nov 21 at 0:19
Matt
106
edited Nov 21 at 0:19
Matt
106
edited Nov 21 at 0:19
Matt
106
106
answered Nov 20 at 18:04
TrebuchetMS
1,9861618
answered Nov 20 at 18:04
TrebuchetMS
1,9861618
answered Nov 20 at 18:04
TrebuchetMS
1,9861618
1,9861618
There where two issues with my code. Your answer addressed the first (major) issue (thanks!) and allowed me to further correct my code. I added it to your answer as an edit.
– Matt
Nov 20 at 18:42
I've approved the edit. 👍 Didn't realise.value()didn't provide a reference. I usually only useand.at(). :-)
– TrebuchetMS
Nov 20 at 18:44
add a comment |
There where two issues with my code. Your answer addressed the first (major) issue (thanks!) and allowed me to further correct my code. I added it to your answer as an edit.
– Matt
Nov 20 at 18:42
I've approved the edit. 👍 Didn't realise.value()didn't provide a reference. I usually only useand.at(). :-)
– TrebuchetMS
Nov 20 at 18:44
There where two issues with my code. Your answer addressed the first (major) issue (thanks!) and allowed me to further correct my code. I added it to your answer as an edit.
– Matt
Nov 20 at 18:42
There where two issues with my code. Your answer addressed the first (major) issue (thanks!) and allowed me to further correct my code. I added it to your answer as an edit.
– Matt
Nov 20 at 18:42
I've approved the edit. 👍 Didn't realise
.value() didn't provide a reference. I usually only use .at(). :-)– TrebuchetMS
Nov 20 at 18:44
I've approved the edit. 👍 Didn't realise
.value() didn't provide a reference. I usually only use .at(). :-)– TrebuchetMS
Nov 20 at 18:44
add a comment |
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