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);
}

[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qSort() is obsolete and should not be used in new code.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 );

[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;
}


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() );


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