# 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.
