1. CS212: Object Oriented Analysis and Design
STL Algorithms

2. Introduction
Read the content of the file
Add the values together
Divide by number of elements
Problem statement: Suppose that we want to write a program that reads in a list of integers from a file (data.txt), then prints out the average of those values
ifstream input("data.txt");
multiset<int> values;
double total = 0.0;
for (multiset<int>::iterator itr = values.begin();
itr != values.end(); ++itr)
total += *itr;
int currValue;
while (input >> currValue)
values.insert(currValue);
cout << "Average is: " << total / values.size() << endl;

3. Discussion Intent In practice
“read the contents of the file into the multiset“ “sum the elements together”
“create an integer, and then while it's possible to read another element out of the file, do so and insert it into the multiset.” “initialize the total to zero, then iterate over the elements of the multiset, increasing the total by the value of the current element at each step.”
“Mechanical model”

4. Accumulate Averaging program Using “accumulate” algorithm
double total = 0.0;
for (multiset::iterator itr = values.begin();
itr != values.end(); ++itr)
total += *itr;
cout << "Average is: " << total / values.size()
<< endl;
Using “accumulate” algorithm
cout <<
accumulate(values.begin(), values.end(), 0.0) / values.size()
<< endl;

5. More on “accumulate” Defined in the <numeric> header
Three parameters (arguments) are passed to it
Iterator 1: Beginning of the range of values
Iterator 2: End of the range of values
Initial value to be used in the summation
Demonstration (AccumSum.cpp)
template <class InputIterator, class T>
T accumulate (InputIterator first, InputIterator last, T init);

6. Do we really need them? There are reasons behind using STL algorithms
Simplicity
Leverage existing codes
A great time-saver
Correctness
Reduces the chance of slip up and making mistakes
Speed
Optimized to work as fast as possible
Clarity
Ease of understanding of existing modules

7. Reordering algorithms
Reorder but preserve the elements in a container
sort(RandomAccessIterator first, RandomAccessIterator last );
random_shuffle(RandomAccessIterator first, RandomAccessIterator last));
rotate(ForwardIterator first, ForwardIterator middle, ForwardIterator last);
Demonstration (ReorderAlgo.cpp)

8. Searching Algorithms Checking membership in a container
InputIterator find (InputIterator first, InputIterator last, const T& val);
Returns an iterator to the first element in the range that compares equal to val
If no such element is found, the function returns last.
bool binary_search (ForwardIterator first, ForwardIterator last, const T& val);
The elements in the range shall already be sorted
Demonstration (SearchAlgo.cpp) 

9. Removal Algorithms For removing elements from containers
Removal algorithms do not actually remove elements from containers
Algorithms accept iterators, not containers
Do not know how to erase elements from containers
Removal functions work by shuffling down the contents of the container

10. Removal algorithm: example1 2 3 4 3
ForwardIterator remove (ForwardIterator first, ForwardIterator last, const T& val);
The removal is done by replacing the elements that compare equal to val by the next element that does not F L 1 2 3 4 4 == == == == == == == 3 3 3 3 3 3 3
Demonstration (RemoveAlgo.cpp)