STL Lab

STL

The STL is the Standard Template Library. It wasn’t originally part of C++; it was a library written at HP. It’s now an official part of C++, but the name remains.

We will discuss these containers (there are more):

vector: A linear collection, like an array.
string: Pretty much a vector<char>, with specialized methods
list: A doubly-linked list
set / multiset: A sorted collection (a tree)
map / multimap: A sorted collection of pairs, a mapping

Documentation

https://cplusplus.com is great:

Incomplete type

There is no type called simply vector. You can’t do this:

vector v;

c.cc:1: error: class template argument deduction failed:

It’s a vector of … what? You have to decide. Give it a type:

vector<int> a;
vector<string> b;
vector<const char *> c;
cout << "Hooray!\n";

Hooray!

`.size()`

All STL containers have a current size, initially zero.

#include <vector>
#include <string>
#include <set>
#include <iostream>
using namespace std;
int main() {
    vector<int> vi;
    string str;
    set<double> sd;
    cout << "vector size: " << vi.size() << '\n'
         << "string size: " << str.size() << '\n'
         << "set size: " << sd.size() << "\n\n";
    vi.push_back(10);
    str.push_back('J'); str += "ck"; str.insert(1, "a");
    sd.insert(7.6); sd.insert(1.2); sd.insert(7.6);
    cout << "vector size: " << vi.size() << '\n'
         << "string size: " << str.size() << '\n'
         << "set size: " << sd.size() << '\n';
}

vector size: 0
string size: 0
set size: 0

vector size: 1
string size: 4
set size: 2

Iterating over a vector:

vector<int> v = {2024, 1492, 1957};
v.push_back(42);
for (size_t i=0; i<v.size(); i++)
    cout << v[i] << '\n';

Why size_t? Why not just int? Because vector::size() returns an unsigned type, and the compiler will complain if we compare signed and unsigned integers. size_t is an appropriate unsigned type.

[subscript ] only works for vector and string. Indexing would be expensive for set and list. For map, the subscript represents the key, and returns the value.

Easier iteration

If you don’t need the index:

vector<int> v = {2024, 1492, 1957};
for (auto val : v)
    cout << val << '\n';

2024
1492
1957

set

set<char> s;
string large = "bigness";
for (char c : large)
    s.insert(c);
for (auto val : s)
    cout << val;

begins

How did “bigness” become “begins”⁇

multiset

multiset<char> ms;
ms.insert('b');
ms.insert('a');
ms.insert('n');
ms.insert('a');
ms.insert('n');
ms.insert('a');
for (auto val : ms)
    cout << val;

aaabnn

map

map<int, string> m;
m[253] = "Jack Applin";
m[320] = "Wim Bohm";
m[152] = "Dave Matthews";
for (auto val : m)
    cout << val.second << " is teaching CS" << val.first << endl;
cout << "CS253 is taught by " << m[253] << endl;
cout << "CS222 is taught by " << m[222] << endl;

Dave Matthews is teaching CS152
Jack Applin is teaching CS253
Wim Bohm is teaching CS320
CS253 is taught by Jack Applin
CS222 is taught by

Why did the for-loop produce the lines in that order?

Linked List

list<int> l;
for (int i=0; i<10; i++)
    l.push_back(rand() % 100);
for (auto val : l)
    cout << val << ' ';

83 86 77 15 93 35 86 92 49 21

Now …

Create a program results.cc that:

Reads integers, until the user enters zero, into a vector<int>.
Write the integers from the vector<int>.
Read all the characters from the file /etc/resolv.conf into one big string.
Copy the characters from the string to a multiset<char>.
Copy the characters from the multiset<char> to a set<char>.
Display the .size() and characters from the string, set<char>, and multiset<char>.
Add a // comment block to your program, titled “QUESTION 7”, that explains why the sizes from the previous question aren’t all the same. I want an explanation—merely stating “this one is 10, but this one is 20” is not good enough.

How to submit your work:

In Canvas, check in the file results.cc to the assignment “Lab03”.

How to receive negative points:

Turn in someone else’s work.

CS253: Software Development with C++

Fall 2020

STL