This is how I did it:

_maxValue = _dataValues[0];
_minValue = _dataValues[0];


for(int i = 0; i<200; i++){
_maxValue = qMax(_maxValue, _dataValues[i]);
_minValue = qMin(_minValue, _dataValues[i]);
}

How can I know when max or min value is found, so that I can use index.