jesbb wrote:

1. void MainWindow::forwardSlice()
2. {
3.     QPixmap nextPix = thumbnailPixmapList.at(s+1);
4.     s+=1;
5.     ui->horizontalSlider->setValue(s);
6.     newSlice2->setPixmap(nextPix);
7.     ui->sliceView->update();
8. }

That part does not look safe at all.
What is the contents of “s” ? You might want to make sure that it is within the bounds 0 <= s < thumbnailPixmapList.size().

jesbb wrote:

1. void MainWindow::createThumbnailPixmaps()
2. {
3.     for (int i=0; i<thumbnailPixmapList.size(); ++i)
4.     {
5.         QPixmap thumbnailPix(QPixmap::fromImage(extractSlice(0,i)));
6.         thumbnailPixmapList.insert(i, thumbnailPix);
7.     }
8. }

Was a little fast in answering. Here is another problem. How do you want to loop if nothing is in the list?

‘s’ is an integer; its default value is 50. If I wanted to make sure if 0 <= s < thumbnailPixmapList.size(), would I need to make an if loop to put inside the forwardSlice method?

I haven’t thought about how I would loop if nothing is in the list.

I do not know where you are getting the parts of your picture, but all looks a little suspicious to me.

Assuming that somehow extractSlice (0, j) makes sense for j up to 9. This would make more sense.

For reading you could use something similar to:

Disclaimer: This part of code is typed directly into the thread. Furthermore, I cannot know what you are up to. So, the snippet might need significant additions until it runs as you need.

I made some minor changes to the code you gave me and it works now.

Thank you very much for your help; I really appreciate it!

You
are
welcome :-)

To see what triggers this type of error you should run it in qt-creator in the debugger then click on each higher function in the call stack and inspect the local variables to see what causes the problem.