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content/CSE332S/CSE332S_L10.md

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+# CSE332S Lecture 10
+
+## Associative Containers
+
+| Container            | Sorted | Unique Key | Allow duplicates |
+| -------------------- | ------ | ---------- | ---------------- |
+| `set`                | Yes    | Yes        | No               |
+| `multiset`           | Yes    | Yes        | Yes              |
+| `unordered_set`      | No     | Yes        | No               |
+| `unordered_multiset` | No     | Yes        | Yes              |
+| `map`                | Yes    | Yes        | No               |
+| `multimap`           | Yes    | Yes        | Yes              |
+| `unordered_map`      | No     | Yes        | No               |
+| `unordered_multimap` | No     | Yes        | Yes              |
+
+Associative containers support efficient key lookup
+vs. sequence containers, which lookup by position
+Associative containers differ in 3 design dimensions
+
+- Ordered vs. unordered (tree vs. hash structured)
+- We’ll look at ordered containers today, unordered next time
+- Set vs. map (just the key or the key and a mapped type)
+- Unique vs. multiple instances of a key
+
+### Ordered Associative Containers
+
+Example: `set`, `multiset`, `map`, `multimap`
+
+Ordered associative containers are tree structured
+- Insert/delete maintain sorted order, e.g. `operator<`
+- Don’t use sequence algorithms like `sort` or `find` with them
+  - Already sorted, so sorting unnecessary (or harmful)
+  - `find` is more efficient (logarithmic time) as a container method
+Ordered associative containers are bidirectional
+- Can iterate through them in either direction, find sub-ranges
+- Can use as source or destination for algorithms like `copy`
+
+### Set vs. Map
+
+A set/multiset stores keys (the key is the entire value)
+
+- Used to collect single-level information (e.g., a set of words to ignore)
+- Avoid in-place modification of keys (especially in a set or multiset)  
+
+A map/multimap associates keys with mapped types
+
+- That style of data structure is sometimes called an associative array
+- Map subscripting operator takes key, returns reference to mapped type
+- E.g., `string s = employees[id]; // returns employee name`
+- If key does not exist, `[]` creates new entry with the key, value-initialized (0 if numeric, default initialized if class) instance of the mapped type
+
+### Unique vs. Multiple Instances of a Key
+
+In set and map containers, keys are unique
+
+- In set, keys are the entire value, so every element is unique
+- In map, multiple keys may map to same value, but can’t duplicate keys
+- Attempt to insert a duplicate key is ignored by the container (returns false)
+
+In multiset and multimap containers, duplicate keys ok
+
+- Since containers are ordered, duplicates are kept next to each other
+- Insertion will always succeed, at appropriate place in the order
+
+### Key Types, Comparators, Strict Weak Ordering
+
+Like `sort` algorithm, can modify container’s order ...
+... with any callable object that can be used correctly for sort
+
+Must establish a **strict weak ordering** over elements
+
+- Two keys cannot both be less than each other (inequality), so comparison operator must return `false` if they are equal
+- If `a  < b` and `b < c` then `a < c` (transitivity of inequality)
+- If `!(a < b)` and `! (b < a)` then `a == b` (equivalence)
+- If `a  == b` and `b == c` then `a == c` (transitivity of eqivalence)
+
+_Sounds like definition of order in math_
+
+Type of the callable object is used in container type
+
+- Cool example in LLM pp. 426 using `decltype` for a function
+  - Could do this by declaring your own pointer to function type
+  - But much easier to let compiler’s type inference figure it out for you
+
+### Pairs
+
+Maps use `pair` template to hold key, mapped type
+
+- A `pair` can be used hold any two types
+- Maps use the key type as the 1st element of the pair (`p.first`)
+- Maps use the mapped type as the 2nd element of the pair (`p.second`)
+
+Can compare `pair` variables using operators
+
+- Equivalence, less than, other relational operators
+
+Can declare `pair` variables several different ways
+
+- Easiest uses initialization list (curly braces around values) (e.g. `pair<string, int> p = {"hello", 1};`)
+- Can also default construct (value initialization) (e.g. `pair<string, int> p;`)
+- Can also construct with two values (e.g. `pair<string, int> p("hello", 1);`)
+- Can also use special `make_pair` function (e.g. `pair<string, int> p = make_pair("hello", 1);`)
+
+### Unordered Containers (UCs)
+
+Example: `unordered_set`, `unordered_multiset`, `unordered_map`, `unordered_multimap`
+
+UCs use `==` to compare elements instead of `<` to order them
+
+- Types in unordered containers must be equality comparable
+- When you write your own structs, overload `==` as well as `<`
+
+UCs store elements in indexed buckets instead of in a tree
+
+- Useful for types that don’t have an obvious ordering relation over their values
+
+UCs use hash functions to put and find elements in buckets
+
+- May improve performance in some cases (if performance profiling suggests so)
+- Declare UCs with pluggable hash functions via callable objects, decltype, etc.
+- Or specialize the `std::hash()` template for your type, used by default
+
+### Summary
+
+Use associative containers for key based lookup
+
+- Ordering of elements is maintained over the keys
+- Think ranges and ordering rather than position indexes
+- A sorted vector may be a better alternative (depends on which operations you will use most often, and their costs)
+
+Ordered associative containers use strict weak order
+
+- Operator `<` or any callable object that acts like `<` over `int` can be used
+
+Maps allow two-level (dictionary-like) lookup
+
+- Vs. sets which are used for “there or not there” lookup
+- Map uses a `pair` to associate key with mapped type
+
+Can enforce uniqueness or allow duplicates
+
+- Duplicates are still stored in order, creating “equal ranges”
+
+## IO Libraries
+
+### `std::copy()`
+
+`std::copy()`
+
+http://www.cplusplus.com/reference/algorithm/copy/
+
+Takes 3 parameters:
+
+- `copy(InputIterator first, InputIterator last, OutputIterator result);`
+  - `[first, last)` specifies the range of elements to copy.
+  - `result` specifies where we are copying to.
+
+Example:
+
+```cpp
+vector<int> v = {1, 2, 3, 4, 5};
+// copy v to cout
+std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, " "));
+```
+
+Some useful destination iterator types:
+
+1. `ostream_iterator` - iterator over an output stream (like `cout`)
+2. `insert_iterator` - inserts elements directly into an STL container (will be practiced in studio)
+
+```cpp
+#include <iostream>
+#include <string>
+#include <fstream>
+#include <iterator>
+#include <algorithm>
+
+using namespace std;
+
+int main(int argc, char *argv[]) {
+    if (argc != 3) {
+        cerr << "Usage: " << argv[0] << " <input file> <output file>" << endl;
+        return 1;
+    }
+
+    string input_file = argv[1];
+    string output_file = argv[2];
+
+    ifstream input_file(input_file.c_str());
+    ofstream output_file(output_file.c_str());
+
+    // don't skip whitespace
+    input_file >> noskipws;
+
+    istream_iterator<char> i (input_file);
+    ostream_iterator<char> o (output_file);
+
+    // copy the input file to the output file: copy(InputIterator first, InputIterator last, OutputIterator result);
+    copy(i, istream_iterator<char>(), o);
+
+    cout << "Copied input file" << input_file << " to " << output_file << endl;
+
+    return 0;
+}
+```
+
+### IO reviews
+
+How to move data into and out of a program:
+
+- Using `argc` and `argv` to pass command line args
+- Using `cout` to print data out to the terminal
+- Using `cin` to obtain data from the user at run-time
+- Using an `ifstream` to read data in from a file
+- Using an `ofstream` to write data out to a file
+
+How to move data between strings, basic types
+
+- Using an `istringstream` to extract formatted int values
+- Using an `ostringstream` to assemble a string
+
+### Streams
+
+Simply a buffer of data (array of bytes).
+
+Insertion operator (`<<`) specifies how to move data from a variable into an output stream
+Extraction operator (`>>`) specifies how to pull data off of an input stream and store it into a variable
+
+Both operators defined for built-in types:
+
+- Numeric types
+- Pointers
+- Pointers to char (char *)
+
+Cannot copy or assign stream objects
+
+- Copy construction or assignment syntax using them results in a compile-time error
+
+Extraction operator consumes data from input stream
+
+- "Destructive read" that reads a different element each time
+- Use a variable if you want to read same value repeatedly
+
+Need to test streams’ condition states
+
+- E.g., calling the `is_open` method on a file stream
+- E.g., use the stream object in a while or if test
+- Insertion and extraction operators return a reference to a stream object, so can test them too
+
+File stream destructor calls close automatically
+
+### Flushing and stream manipulators
+
+An output stream may hold onto data for a while, internally
+
+- E.g., writing chunks of text rather than a character at a time is efficient
+- When it writes data out (e.g., to a file, the terminal window, etc.) is entirely up to the stream, **unless you tell it to flush out its buffers**
+- If a program crashes, any un-flushed stream data is lost
+- So, flushing streams reasonably often is an excellent debugging trick
+
+Can tie an input stream directly to an output stream
+
+- Output stream is then flushed by call to input stream extraction operator
+- E.g., `my_istream.tie(&my_ostream);`
+- `cout` is already tied to `cin` (useful for prompting the user, getting input)
+
+Also can flush streams directly using stream manipulators
+
+- E.g., `cout << flush;` or `cout << endl;` or `cout << unitbuf;`
+
+Other stream manipulators are useful for formatting streams
+
+- Field layout: `setwidth`, `setprecision`, etc.
+- Display notation: `oct`, `hex`, `dec`, `boolalpha`, `nobooleanalpha`, `scientific`, etc.