Predicate Functions

INFO

An expression that returns a value that can be used as a condition

• Unary: Takes 1 parameter
• Binary: Takes 2 parameters
• Tertiary: Takes 3 parameters

NOTE

STL Algorithms only uses the Unary and Binary predicate functions

Example

std::sort has 2 templates

• Default → template <class RandomAccessIterator> void sort(RandomAccessIterator first, RandomAccessIterator last)
• Custom → template <class RandomAccessIterator, class Compare> void sort(RandomAccessIterator first, RandomAccessIterator last, Compare comp); Notice how the Custom template of the std::sort has an extra comp argument? This is the Binary Function that accepts 2 elements in the range as arguments, and returns a value convertible to bool. Also known as Predicate Functions

With the Default sort (ascending order)

vector<int> v = {3, 1, 2, 4, 7, 5, 6};
sort(v.begin(), v.end()); // 1, 2, 3, 4, 5, 6, 7

But what if we want to sort the vector v in descending order?

bool descending(int a, int b) {return a > b;} // The Predicate Function
sort(v.begin(), v.end(), descending); // 7, 6, 5, 4, 3, 2, 1

• For each 2 elements (a, b) in vector v, the sort function will call the predicate function → compares the elements and return true or false
• Depending on the returned Boolean value, sort() will rearrange elements in vector
  • if (a > b) {return true;} → a comes before b