Algorithms and comparison contracts

The idea

Bring familiarity with vectors and loops. Standard algorithms let you state an operation without rewriting its traversal. A range described by begin and end includes the first element and excludes the end marker. sort rearranges the supplied range; count_if observes it and counts matches. The algorithm name tells you whether order or content can change, which helps callers reason about shared data.

A custom comparison must describe a strict ordering. For ordinary ascending numbers use less-than, not less-than-or-equal: an element must not be considered less than itself. When sorting records, make tie handling explicit if later output depends on it. A lambda can capture a threshold for a predicate, but captures do not automatically make external state safe to mutate during every possible algorithm.

#include <algorithm>
#include <iostream>
#include <vector>
int main(){std::vector<int> a{8,3,5}; std::sort(a.begin(),a.end());
 auto n=std::count_if(a.begin(),a.end(),[](int x){return x>=5;});
 std::cout<<a.front()<<" "<<a.back()<<" "<<n<<"\n";}

3 8 2

Sorting changes the vector to 3, 5, 8. The predicate counts 5 and 8 without removing any elements.