sorting an array of doubles [solved]

[Cremers]

Hi,

I was asking about sorting doubles but i seem to have found the answer. Thanks.

//-------------------------------------------------------------------------------
void sortdoubles(double *d, int num)
{
QList<double> tmp;
for (int i = 0; i < num; i++)
  tmp << d[i];
qSort(tmp);
for (int i = 0; i < num; i++)
  d[i] = tmp.at(i);
}

//-------------------------------------------------------------------------------
void sortdoubles(double *d, int num)
{
QList<double> tmp;
for (int i = 0; i < num; i++)
  tmp << d[i];
qSort(tmp);
for (int i = 0; i < num; i++)
  d[i] = tmp.at(i);
}

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[d_stranz]

qSort() is obsolete and should not be used in new code. Besides, you can do the same thing with one line of code from the Standard Template Library (STL):

std::sort( d, d + num );

std::sort( d, d + num );

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[Cremers]

Thanks. But if i want to know the sorting order and need to sort a pair, what do i do with std::sort()?
I am using QPair for that now:

QList<QPair<double, int>> tmp;
for (int i = 0; i < num; i++)
  tmp.append(qMakePair(p->x, p->n));
qSort(tmp);
for (int i = 0; i < num; i++)
  {
  p->x = tmp.at(i).first;
  p->n = tmp.at(i).second;
  }

QList<QPair<double, int>> tmp;
for (int i = 0; i < num; i++)
  tmp.append(qMakePair(p->x, p->n));
qSort(tmp);
for (int i = 0; i < num; i++)
  {
  p->x = tmp.at(i).first;
  p->n = tmp.at(i).second;
  }

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Can I sort a structure like typedef struct { double x; int n; } mysortstruct; ?

[tuli]

Yes, you need to have a coparison operator for your struct or pass a lambda to the std::sort. See here: https://en.cppreference.com/w/cpp/algorithm/sort

[d_stranz]

Using lambdas is a more advanced technique than simply defining a comparison function. For example, using a comparison function:

// UNTESTED CODE
 
typedef struct
{
    double a;
    int b;
} MyStruct;
 
// Comparison function
bool myStructLess( const MyStruct & lhs, const MyStruct & rhs ) // "lhs" = "left-hand side"
{
    if ( lhs.a < rhs.a )
        return true;
    else if ( lhs.a > rhs.a )
        return false;
    else
        return lhs.b < rhs.b;
}
 
std::vector< MyStruct > myStructVector;
 
std::sort( myStructVector.begin(), myStructVector.end(), myStructLess() );

// UNTESTED CODE

typedef struct
{
    double a;
    int b;
} MyStruct;

// Comparison function
bool myStructLess( const MyStruct & lhs, const MyStruct & rhs ) // "lhs" = "left-hand side"
{
    if ( lhs.a < rhs.a )
        return true;
    else if ( lhs.a > rhs.a )
        return false;
    else
        return lhs.b < rhs.b;
}

std::vector< MyStruct > myStructVector;

std::sort( myStructVector.begin(), myStructVector.end(), myStructLess() );

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But if you understand lambdas or functors, then use whatever method works best for you.