Why does inbuilt sort function for c++ does not function when we use equals '=' sign in comparator function?

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Because the ≤ relation is not a strict weak ordering.

Strict weak ordering is is defined by:

A strict weak ordering is a binary relation < on a set S that is a strict partial order (a transitive relation that is irreflexive, or equivalently,[6] that is asymmetric) in which the relation "neither a < b nor b < a" is transitive.

Note the requirement of irreflexivity.

A binary relation is called irreflexive, or anti-reflexive, if it doesn't relate any element to itself.

It is easy to show that ≤ does not have this property. Consider for example: 0 ≤ 0. Clearly, this number (and also all other integers) is related with itself, so ≤ is not irreflexive (it is reflexive) and therefore is not a strict partial order and therefore it is not a strict weak ordering.

The comparison function of std::sort is required to be a strict weak ording. Using ≤ violates this requirement, and behaviour of using it is undefined.

Comparator function of qsort() in C, Standard C library provides qsort() that can be used for sorting an array. As the name suggests, the function uses QuickSort algorithm to sort the given array. Using the Built-in Sort and Search Functions in C++ Being able to implement your own versions of quicksort (as well as other sorts that might be suitable for the occasion) and binary search algorithms is important (you can check my own implementations here ).


Built in functions for sorting arrays in C, Are there any built in functions in C programming language for sorting arrays? Or do I have to write my own functions? share. Standard C library provides qsort() that can be used for sorting an array. As the name suggests, the function uses QuickSort algorithm to sort the given array. As the name suggests, the function uses QuickSort algorithm to sort the given array.


Here is a layman's explanation of why using <= isn't going to work.

I am the sort function that requires a true or false answer to determine if an item comes before the other. I give you two items that happen to be equal, call them item A and item B. I ask you "which comes before the other?" You say "A", because a <= b (you return true).

Then I take the same two equal items, but give them to you first B, then A. You now tell me that "B" comes before A because again, a <= b. Huh? That's ambiguous. You first told me one thing, that A comes before B, and then contradicted yourself by telling me that "B comes before A", and all I did was hand you the same two items, just in a different order.

That's why using <= isn't going to work -- it goes completely wild if the two items are equal.

BTW, the Visual C++ debug runtime does this very same test I described, to determine if your sort criteria follows a strict-weak-order. If VC++ detects this ambiguity, an assertion is raised and you get that scary dialog that you have invalid comparison criteria.

C library function - qsort(), The C library function void qsort(void *base, size_t nitems, size_t size, int (*​compar)(const void *, const void*)) sorts an array. Declaration. Following is the  First take a look at each of these functions and see if we can figure out what each parameter does. Example 1 ~ std::sort(myvector.begin(), myvector.end()) Example 2 ~ std::sort(myvector.begin(), myvector.end(), myCompFunction) About The Function So let’s go dig into these and figure out what each does and why it does it.


The way the comparator determines if two elements are equal is if neither one is less then the other. In other words, if we have a and b, then if a < b and b < a are both false the values are considered equal.

For simplicity, let's assume we have ints. If the comparator uses <= and a = b = 4, then a <= b and b <= a are both true.

You can read more details on the requirements on cppreference.

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Sort requires strict weak ordering to work:

Almost all stl containers rely on strict weak ordering. A strict weak ordering defines the relative position of elements in terms of precedence of one item over other. For eg. if you have a room full of person and you have to form a queue based on their height, a person with "lesser" height will "precede" the person with greater height. For a function to be satisfying strict weak ordering following conditions need to be met:

a < b => !(b < a) 
!(a < b) && !(b < a) => a≡b

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