错误的数组变量被更改

Question

In the following function I clearly change the variable rightArray through the statement: rightArray[i] = dataValues[i];

However the behaviour I'm getting from the function is that the variable leftArray is changing in place of rightArray

//Returns the interquartile range
float StatisticalAnalyser::getInterquartileRange()
{
    float interquartileRange = 0;

    int numberOfDatums = dataFile.findNumberOfDatums();
    float numberOfDatumsFloat = dataFile.findNumberOfDatums();
    float dataValues[dataFile.findNumberOfDatums()];

    dataFile.initialiseArrayToFileData(dataValues);

    //If even number of datums
    if (numberOfDatums % 2 == 0)
    {
        //Arrays for for each side of the median
        int arraySize = numberOfDatumsFloat/2;
        float leftArray[numberOfDatums/2];
        float rightArray[numberOfDatums/2];

        //Initialise arrays for each side of the median
        for (int i = 0; i < numberOfDatums; i++)
        {
            if (i < numberOfDatums/2)
            {
                leftArray[i] = dataValues[i];
            }
            if (i >= numberOfDatums/2)
            {
                //leftArray SOMEHOW GETS CHANGED INSTEAD OF RIGHT ARRAY
                rightArray[i] = dataValues[i];

leftArray gets changed instead of rightArray here : rightArray[i] = dataValues[i];

            }
        }
    }

    if (numberOfDatums % 2 == 0.5)
    {
        //Not relevant, isn't triggered when problem occurs
    }

    return interquartileRange;
}

I am running Xcode 10.1, and want to know how to fix this error so that rightArray is changed by the function instead of leftArray .

Answer 1

Having

    float leftArray[numberOfDatums/2];
    float rightArray[numberOfDatums/2];

in

       if (i >= numberOfDatums/2)
        {
            //leftArray SOMEHOW GETS CHANGED INSTEAD OF RIGHT ARRAY
            rightArray[i] = dataValues[i];

you go out of rightArray and randomly write in leftArray (of course this is an undefined behavior)

must be

       if (i >= numberOfDatums/2)
        {
            rightArray[i - numberOfDatums/2] = dataValues[i];

---

Furthermore, to have

       if (i < numberOfDatums/2)
        {
            ...
        }
        if (i >= numberOfDatums/2)

with i unchanged is useless, the second if can be an else , but it is also better to do two for to not have to do numberOfDatums/2 all the times just to decide which array to use.