Why is QString::remove not working as expected?

HFT_developer

Hi, I am trying to remove certain chars form a QString however the remove function is doing something funny:

    // remove all instances of power, wheather negative positive
    for (int i{};i<_numerator.size();++i){
        if (_numerator[i] == coeff){
            if (_numerator[i+2] == '-'){
                _numerator.remove(i+1, 3);
            }
            else{
                _numerator.remove(i+1, 2);
            }
        }
    }

For given a string of "15x^-2+10x^-3" I want to modify it to "15x+10x". Can someone help me as to why this doesn't work?

mrjj

Hi and welcome to the forums

It is a classic mistake.
when you loop over a container using its size and in that loop, remove elements, fun things are bound to happen.

Try to loop it reversed and see if that cures it.

If not, you need to copy on the fly the parts you want and then skip the unwanted ones.

JonB

@HFT_developer
Hello and welcome.

Assuming coeff is the ^ character you know that is at _numerator[i]. Yet you look for any - symbol at i+2 when it would come immediately after the ^ which would be i+1. And you only start deleting from i+1 which is after the ^ so that will be retained, which you do not want.

Untested, but don't you want:

            if (_numerator[i+1] == '-'){
                _numerator.remove(i, 3);
            }
            else{
                _numerator.remove(i, 2);
            }

?

Note that you are making assumptions here like (a) once you meet a ^ or ^- there will be at least one character after it and (b) only one character after it, e.g. 15x^-20+10x^333 would not be replaced correctly at either ^... sequence.

HFT_developer

@JonB yeah your right. Could you recommend a better algorithm?

JonB

@HFT_developer
If I were writing this I would probably use the overload QString &QString::remove(const QRegularExpression &re) which uses a regular expression to do all of the removals in one call:

Removes every occurrence of the regular expression re in the string

There is a bit of an issue specifying the ^ character because that can mean "beginning of string` in a regular expression. Again untested, but how does this behave:

_numerator.remove(QRegularExpression("\\^-?\\d+"));

? That is intended to remove (a) a literal ^ (b) optionally immediately followed by a single - and (c) immediately followed by one or more digits.

HFT_developer

@JonB I checked and that works well. I have another question about converting a list to another. In STL you have std::transform to do that. What is the equivalent for this in qt?

For example, I want to convert a QList<QString> to QList<double>, in this case I can guarantee that is will contain something like {"15.5", "19.9", "25.5"} there wont be any non numbers. Right now I am using a simple loop:

    QList<QString> _splitter = data.split(coeff);
    QList<double> ret;

    // transform into double from QString
    for (int i{};i<_splitter.size();++i){
        ret.push_back(_splitter[i].toDouble());
    }

what are your thoughts on this?

SGaist

Hi,

This is valid. You could also use std::transform.