Perhaps you are already using C++ as your main programming language to solve problems. This means that you have already used STL in a simple way, because arrays and strings are passed to your function as STL objects. You may have noticed, though, that many coders manage to write their code much more quickly and concisely than you. Or perhaps you are not a C++ programmer, but want to become one because of the great functionality of this language and its libraries.
Regardless of where you're coming from, this article can help. In it, we will review some of the powerful features of the Standard Template Library (STL) – a great tool that, sometimes, can save you a lot of time in an algorithm competition.
The simplest way to get familiar with STL is to begin from its containers.
Any time you need to operate with many elements you require some kind of container. In native C (not C++) there was only one type of container: the array.
The problem is not that arrays are limited (though, for example, it's impossible to determine the size of array at runtime). Instead, the main problem is that many problems require a container with greater functionality.
For example, we may need one or more of the following operations:
Of course, one can implement this functionality in an ordinal array. But the trivial implementation would be very inefficient. You can, for example, use a tree structure to solve it in a faster way, but think a bit: does the implementation of such a container depend on elements we are going to store? Do we have to re-implement the module to make it functional, for example, for points on a plane but not strings?
If not, we can develop the interface for such a container once, and then use everywhere for data of any type. That, in short, is the idea of STL containers.
When the program is using STL, it should #include the appropriate standard headers. For most containers the title of standard header matches the name of the container, and no extension is required. For example, if you are going to use stack, just add the following line at the beginning of your program:
#include <stack>
Container types (and algorithms, functors and all STL as well) are not defined in global namespace, but in special namespace called "std". To use them you need to write std:: before the name of the container, for example std::stack. You can, however, avoid that by adding the following line after your includes and before the code begin:
using namespace std;
Another important thing to remember is that the type of container is the template parameter. Template parameters are specified with the </> "brackets" in code. For example:
vector<int> N;
The simplest STL container is vector. Vector is just an array with extended functionality. By the way, vector is the only container that is backward-compatible to native C code – this means that vector actually IS the array, but with some additional features.
vector<int> v(10);
for (int i = 0; i < 10; i++) {
v[i] = (i + 1) * (i + 1);
}
for (int i = 9; i > 0; i–) {
v[i] -= v[i - 1];
}
Actually, when you type:
vector<int> v;
The empty vector is created. Be careful with constructions like this:
vector<int> v[10];
Here we declare v as an array of 10 vector<int>'s, which are initially empty. In most cases, this is not that we want. Use parentheses instead of brackets here. The most frequently used feature of vector is that it can report its size.
int elements_count = v.size();
Two remarks: first, size() is unsigned, which may sometimes cause problems. Accordingly, I usually define macros, something like sz(c) that returns size of c as ordinal signed int. Second, it's not a good practice to compare v.size() to zero if you want to know whether the container is empty. You're better off using empty() function:
bool is_nonempty_notgood = (v.size() >= 0); // Try to avoid this
bool is_nonempty_ok = !v.empty();
This is because not all the containers can report their size in , and you definitely should not require counting all elements in a double-linked list just to ensure that it contains at least one.
Another very popular function to use in vector is push_back. Push_back adds an element to the end of vector, increasing its size by one. Consider the following example:
vector<int> v;
for (int i = 1; i < 1000000; i *= 2) {
v.push_back(i);
}
int elements_count = v.size();
Don't worry about memory allocation – vector will not allocate just one element each time. Instead, vector allocates more memory than it actually needs when adding new elements with push_back. The only thing you should worry about is memory usage.
When you need to resize vector, use the resize() function:
vector<int> v(20);
for (int i = 0; i < 20; i++) {
v[i] = i + 1;
}
v.resize(25);
for (int i = 20; i < 25; i++) {
v[i] = i * 2;
}
The resize() function makes vector contain the required number of elements. If you require less elements than vector already contain, the last ones will be deleted. If you ask vector to grow, it will grow its size and fill the newly created elements with zeroes.
Note that if you use push_back() after resize(), it will add elements AFTER the newly allocated size, but not INTO it. In the example above the size of the resulting vector is 25, while if we use push_back() in a second loop, it would be 30.
vector<int> v(20);
for (int i = 0; i < 20; i++) {
v[i] = i + 1;
}
v.resize(25);
for (int i = 20; i < 25; i++) {
v.push_back(i * 2); // Writes to elements with indices [25...30), not [20...25) !
}
To clear a vector use clear() member function. This function makes vector to contain 0 elements. It does not make elements zeroes – watch out – it completely erases the container.
There are many ways to initialize vector. You may create vector from another vector:
vector<int> v1;
// ...
vector<int> v2 = v1;
vector<int> v3(v1);
The initialization of v2 and v3 in the example above are exactly the same.
If you want to create a vector of specific size, use the following constructor:
vector<int> data(1000);
In the example above, data will contain 1000 zeroes after creation. If you want vector to be initialized with something else, write it in such manner:
vector<string> names(20, "Unknown");
Remember that you can create vectors of any type.
Multidimensional arrays are very important. The simplest way to create the two-dimensional array via vector is to create a vector of vectors.
vector<vector<int>> matrix;
It should be clear to you now how to create the two-dimensional vector of given size:
int N, M;
// ...
vector<vector<int>> matrix(N, vector<int>(M, -1));
Here we create a matrix of size and fill it with -1.
You should remember one more very important thing: when vector is passed as a parameter to some function, a copy of vector is actually created. It may take a lot of time and memory to create new vectors when they are not really needed. Actually, it's hard to find a task where the copying of vector is REALLY needed when passing it as a parameter. So, you should never write:
void some_function(vector<int> v) { // Never do it unless you're sure what you do!
// ...
}
Instead, use the following construction:
void some_function(const vector<int>& v) { // OK
// ...
}
If you are going to change the contents of vector in the function, just omit the const modifier.
void modify_vector(vector<int>& v) { // Correct
v[0]++;
}
Pairs are widely used in STL. Simple problems usually require some simple data structure that fits well with pair. STL pair is just a pair of elements. The simplest form would be the following:
template<typename T1, typename T2>
struct pair {
T1 first;
T2 second;
};
In general pair<int, int> is a pair of integer values. At a more complex level, pair<string, pair<int, int>> is a pair of string and two integers. In the second case, the usage may be like this:
pair<string, pair<int, int>> P;
string s = P.first; // extract string
int x = P.second.first; // extract first int
int y = P.second.second; // extract second int
The great advantage of pairs is that they have built-in operations to compare themselves. Pairs are compared first-to-second element. If the first elements are not equal, the result will be based on the comparison of the first elements only; the second elements will be compared only if the first ones are equal. The array (or vector) of pairs can easily be sorted by STL internal functions.
For example, if you want to sort the array of integer points so that they form a polygon, it's a good idea to put them to the vector<pair<double, pair<int, int>>>, where each element of vector is { angle, { x, y } }. One call to the STL sorting function will give you the desired order of points.
Pairs are also widely used in associative containers, which we will speak about later in this article.
What are iterators? In STL iterators are the most general way to access data in containers. Consider the simple problem: reverse the array A of N integers. Let's begin from a C-like solution:
void reverse_array_simple(int* A, int N) {
int first = 0, last = N - 1; // First and last indices of elements to be swapped
while (first < last) { // Loop while there is something to swap
swap(A[first], A[last]); // swap(a,b) is the standard STL function
first++; // Move first index forward
last–; // Move last index back
}
}
This code should be clear to you. It's pretty easy to rewrite it in terms of pointers:
void reverse_array(int* A, int N) {
int *first = A, *last = A + N - 1;
while (first < last) {
swap(*first, *last);
first++;
last–;
}
}
Look at this code, at its main loop. It uses only four distinct operations on pointers first and last:
first < last);*first, *last);Now imagine that you are facing the second problem: reverse the contents of a double-linked list, or a part of it. The first code, which uses indexing, will definitely not work. At least, it will not work in time, because it's impossible to get element by index in a double-linked list in , only in , so the whole algorithm will work in . Errr...
But look: the second code can work for ANY pointer-like object. The only restriction is that that object can perform the operations described above: take value (unary *), comparison (<), and increment/decrement (++/–). Objects with these properties that are associated with containers are called iterators. Any STL container may be traversed by an iterator. Although not often needed for vector, it's very important for other container types.
So, what do we have? An object with syntax very much like a pointer. The following operations are defined for iterators:
int x = *it;it1++, it2–;!= and by <;it += 20 (move 20 elements forward);int n = it2 - it1.But instead of pointers, iterators provide much greater functionality. Not only can they operate on any container, they may also perform, for example, range checking and profiling of container usage.
And the main advantage of iterators, of course, is that they greatly increase the reuse of code: your own algorithms, based on iterators, will work on a wide range of containers, and your own containers, which provide iterators, may be passed to a wide range of standard functions.
Not all types of iterators provide all the potential functionality. In fact, there are so-called "normal iterators" and "random access iterators". Simply put, normal iterators may be compared with == and !=, and they may also be incremented and decremented. They may not be subtracted, and we can not add a value to the normal iterator. Basically, it's impossible to implement the described operations in for all container types. In spite of this, the function that reverses array should look like this:
template<typename T>
void reverse_array(T* first, T* last) {
if (first != last) {
while (true) {
swap(*first, *last);
first++;
if (first == last) {
break;
}
last–;
if (first == last) {
break;
}
}
}
}
The main difference between this code and the previous one is that we don't use the < comparison on iterators, just the == one. Again, don't panic if you are surprised by the function prototype: template is just a way to declare a function, which works on any appropriate parameter types. This function should work perfectly on pointers and with all normal iterators.
Let's return to the STL. STL algorithms always use two iterators, called "begin" and "end." The end iterator is pointing not to the last object, however, but to the first invalid object, or the object directly following the last one. It's often very convenient.
Each STL container has member functions begin() and end() that return the begin and end iterators for that container.
Based on these principles, c.begin() == c.end() if and only if c is empty, and c.end() - c.begin() will always be equal to c.size(). (The last sentence is valid in cases when iterators can be subtracted, i.e. begin() and end() return random access iterators, which is not true for all kinds of containers. See the prior example of the double-linked list.)
The STL-compliant reverse function should be written as follows:
template<typename T>
void reverse_array_stl_compliant(T* begin, T* end) {
// We should at first decrement 'end'
// But only for non-empty range
if (begin != end) {
end–;
if (begin != end) {
while (true) {
swap(*begin, *end);
begin++;
if (begin == end) {
break;
}
end–;
if (begin == end) {
break;
}
}
}
}
}
Note that this function does the same thing as the standard function std::reverse(T begin, T end) that can be found in algorithms module (#include <algorithm>).
In addition, any object with enough functionality can be passed as an iterator to STL algorithms and functions. That is where the power of templates comes in! See the following examples:
vector<int> v;
// ...
vector<int> v2(v);
vector<int> v3(v.begin(), v.end()); // v3 equals to v2
int data[] = { 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31 };
vector<int> primes(data, data + (sizeof(data) / sizeof(data[0])));
The last line performs a construction of vector from an ordinal C array. The term data without index is treated as a pointer to the beginning of the array. The term data + N points to N-th element, so, when N if the size of array, data + N points to first element not in array, so data + length of data can be treated as end iterator for array data. The expression sizeof(data) / sizeof(data[0]) returns the size of the array data, but only in a few cases, so don't use it anywhere except in such constructions. (C programmers will agree with me!)
Furthermore, we can even use the following constructions:
vector<int> v;
// ...
vector<int> v2(v.begin(), v.begin() + (v.size() / 2));
It creates the vector v2 that is equal to the first half of vector v.
Here is an example of reverse() function:
int data[10] = {
1, 3, 5, 7, 9, 11, 13, 15, 17, 19
};
reverse(data + 2, data + 6);
// the range { 5, 7, 9, 11 } is now { 11, 9, 7, 5 };
Each container also has the rbegin()/rend() functions, which return reverse iterators. Reverse iterators are used to traverse the container in backward order. Thus:
vector<int> v;
vector<int> v2(v.rbegin() + (v.size() / 2), v.rend());
will create v2 with first half of v, ordered back-to-front.
To create an iterator object, we must specify its type. The type of iterator can be constructed by a type of container by appending ::iterator, ::const_iterator, ::reverse_iterator or ::const_reverse_iterator to it. Thus, vector can be traversed in the following way:
vector<int> v;
// ...
// Traverse all container, from begin() to end()
for (vector<int>::iterator it = v.begin(); it != v.end(); it++) {
*it++; // Increment the value iterator is pointing to
}
However, most times you can simply use the auto keyword which will automatically determine the correct type:
for (auto it = v.begin(); it != v.end(); it++) {
*it++;
}
I recommend you use != instead of <, and empty() instead of size() != 0 – for some container types, it's just very inefficient to determine which of the iterators precedes another.
Moreover, C++ has a nice syntax for iterating through all elements of a container: range-based for loops. This syntax works for any container that has begin() and end() functions.
for (auto& x : v) {
x++;
}
for (const auto& x : v) { // If you don't need to modify the elements
// ...
}
Now you know of STL algorithm reverse(). Many STL algorithms are declared in the same way: they get a pair of iterators – the beginning and end of a range – and return an iterator.
The find() algorithm looks for appropriate elements in an interval. If the element is found, the iterator pointing to the first occurrence of the element is returned. Otherwise, the return value equals the end of interval. See the code:
vector<int> v;
for (int i = 1; i < 100; i++) {
v.push_back(i * i);
}
if (find(v.begin(), v.end(), 49) != v.end()) {
// ...
}
To get the index of element found, one should subtract the beginning iterator from the result of find():
int i = find(v.begin(), v.end(), 49) - v.begin();
if (i < v.size()) {
// ...
}
Remember to #include <algorithm> in your source when using STL algorithms.
The min_element and max_element algorithms return an iterator to the respective element. To get the value of min/max element, like in find(), use *min_element(...) or *max_element(...), to get index in array subtract the begin iterator of a container or range:
int data[5] = { 1, 5, 2, 4, 3 };
vector<int> X(data, data + 5);
int v1 = *max_element(X.begin(), X.end()); // Returns value of max element in vector
int i1 = min_element(X.begin(), X.end()) - X.begin(); // Returns index of min element in vector
int v2 = *max_element(data, data + 5); // Returns value of max element in array
int i2 = min_element(data, data + 5) - data; // Returns index of min element in array
Now you may see that the useful macros would be:
#define all(c) c.begin(), c.end()
Don't put the whole right-hand side of these macros into parentheses – that would be wrong!
Another good algorithm is sort(). It's very easy to use. Consider the following examples:
vector<int> X;
// ...
sort(X.begin(), X.end()); // Sort array in ascending order
sort(all(X)); // Sort array in ascending order, use our #define
sort(X.rbegin(), X.rend()); // Sort array in descending order using with reverse iterators
One important last thing about iterators is that iterators can be invalidated. This can happen when the container is modified, for example, by adding or removing elements:
vector<int> v(10);
auto it = v.begin();
v.push_back(42); // it is no longer valid!!!
*it++; // ops, the program crashed :(
Keep in mind that for loops use iterators under the hood, so if you modify the container in the loop, iterators may be invalidated:
vector<int> v(10);
v[0] = 3;
for (auto x : v) {
if (x == 3) {
v.push_back(42); // this will cause the program to crash :(
}
}
One can insert an element to vector by using the insert() function:
vector<int> v;
// ...
v.insert(1, 42); // Insert value 42 after the first
All elements from second (index 1) to the last will be shifted right one element to leave a place for a new element. If you are planning to add many elements, it's not good to do many shifts – you're better off calling insert() one time. So, insert() has an interval form:
vector<int> v;
vector<int> v2;
// ...
// Shift all elements from second to last to the appropriate number of elements.
// Then copy the contents of v2 into v.
v.insert(1, all(v2));
Vector also has a member function erase, which has two forms. Guess what they are:
erase(iterator);
erase(begin iterator, end iterator);
At first case, single element of vector is deleted. At second case, the interval, specified by two iterators, is erased from vector.
The insert/erase technique is common, but not identical for all STL containers.
There is a special container to manipulate with strings. The string container has a few differences from vector<char>. Most of the differences come down to string manipulation functions and memory management policy.
String has a substring function without iterators, just indices:
string s = "hello";
string s1 = s.substr(0, 3); // "hel"
string s2 = s.substr(1, 3); // "ell"
string s3 = s.substr(0, s.length() - 1); // "hell"
string s4 = s.substr(1); // "ello"
Beware of s.length() - 1 on empty string because s.length() is unsigned and (unsigned)0 - 1 is definitely not what you are expecting!
It's always hard to decide which kind of container to describe first – set or map. My opinion is that, if the reader has a basic knowledge of algorithms, beginning from set should be easier to understand.
Consider we need a container with the following features:
This is quite a frequently used task. STL provides the special container for it: set. Sets can add, remove and check the presence of particular element in , where N is the count of objects in the set. While adding elements to set, the duplicates are discarded. A count of the elements in the set, N, is returned in . We will speak of the algorithmic implementation of set and map later – for now, let's investigate its interface:
set<int> s;
for (int i = 1; i <= 100; i++) {
s.insert(i); // Insert 100 elements, [1...100]
}
s.insert(42); // does nothing, 42 already exists in set
for (int i = 2; i <= 100; i += 2) {
s.erase(i); // Erase even values
}
int n = (int)s.size(); // n will be 50
The push_back() member may not be used with set. It make sense: since the order of elements in set does not matter, push_back() is not applicable here.
Since set is not a linear container, it's impossible to take the element in set by index quickly, but you can still iterate a set using either for loops or iterators.
// Calculate the sum of elements in set
set<int> S;
// ...
int r = 0;
for (auto x : S) {
r += x;
}
To determine whether some element is present in set use find() member function. Don't be confused, though: there are several find() in STL. There is a global algorithm find(), which takes two iterators, element, and works for . It is possible to use it for searching for element in set, but why use an algorithm while there exists an one? While searching in set and map (and also in multiset/multimap, hash_map/hash_set, etc.) do not use global find – instead, use member function set::find(). As 'ordinal' find, set::find will return an iterator, either to the element found, or to end(). So, the element presence check looks like this:
set<int> s;
// ...
if (s.find(42) != s.end()) {
// 42 presents in set
} else {
// 42 not presents in set
}
The same check can be done with the count() function. count() returns the number of elements in the set equal to the given one, which is either 0 or 1. This function is not particularly useful for set, other than as a syntax shortcut, but it exists for compatibility with multiset. Since C++20, there is also a contains() function, which is the same as count() but returns a boolean value.
if (s.count(42)) {
// ...
}
if (s.contains(42)) {
// ...
}
To erase an element from set use the erase() function.
set<int> s;
// ...
s.insert(54);
s.erase(29);
The erase() function also has the interval form:
set<int> s;
// ...
auto it1 = s.find(10);
auto it2 = s.find(100);
// Will work if it1 and it2 are valid iterators, i.e. values 10 and 100 present in set.
s.erase(it1, it2); // Note that 10 will be deleted, but 100 will remain in the container
Set has an interval constructor:
int data[5] = { 5, 1, 4, 2, 3 };
set<int> S(data, data + 5);
It gives us a simple way to get rid of duplicates in vector, and sort it:
vector<int> v;
// ...
set<int> s(all(v));
vector<int> v2(all(s));
Here v2 will contain the same elements as v but sorted in ascending order and with duplicates removed.
Any comparable elements can be stored in set. This will be described later.
There are two explanation of map. The simple explanation is the following:
map<string, int> M;
M["Algo"] = 1;
M["Badge"] = 2;
M["Diamond"] = 10;
int x = M["Algo"] + M["Badge"];
if (M.contains("Diamond")) {
M.erase(M.find("Diamond")); // or even M.erase("Diamond")
}
Very simple, isn't it?
Actually map is very much like set, except it contains not just values but pairs <key, value>. Map ensures that at most one pair with specific key exists. In fact, if you take an iterator, the underlying type is just a pair:
map<string, int> M;
M["Diamond"] = 250;
auto it = M.find("Diamond");
cout << it->first << " " << it->second << endl; // Will print "Diamond 250"
Notice the it->first syntax. Since it is an iterator, we need to take an object from it before operating. So, the correct syntax would be (*it).first. However, it's easier to write something-> than (*something).. The full explanation will be quite long – just remember that, for iterators, both syntaxes are allowed.
Traversing a map is made easy by modern C++ syntax:
map<string, int> M;
// ...
int r = 0;
for (const auto& [key, value] : M) {
r += value;
}
As for other containers, avoid modifying the map while iterating through it to prevent unexpected behavior.
Another quite pleasant thing is that map has operator[] defined, there is one important difference between map::find() and map::operator[] however. While map::find() will never change the contents of map, operator[] will create an element if it does not exist. In some cases this could be very convenient, but it's definitely a bad idea to use operator[] many times in a loop, when you do not want to add new elements. That's why operator[] may not be used if map is passed as a const reference parameter to some function:
void f(const map<string, int>& M) {
if (M["the meaning"] == 42) { // Error! Cannot use [] on const map objects!
}
if (M.contains("the meaning") && M.find("the meaning")-> second == 42) { // Correct
cout << "Don' t Panic!" << endl;
}
}
Internally map and set are almost always stored as red-black trees. We do not need to worry about the internal structure, the thing to remember is that the elements of map and set are always sorted in ascending order while traversing these containers.
It's also possible to change this order, this is particularly useful when you need to sort elements in descending order:
set<int, greater<int>> S; // Set in descending order
Another important thing is that operators ++ and – are defined on iterators in map and set. Thus, if the value 42 presents in set, and it's not the first and the last one, than the following code will work:
set<int> S;
// ...
auto it = S.find(42);
auto it1 = it, it2 = it;
it1–;
it2++;
int a = *it1, b = *it2;
Here a will contain the first neighbor of 42 to the left and b the first one to the right.
It's time to speak about algorithms a bit more deeply. Most algorithms are declared in the #include <algorithm> standard header. At first, STL provides three very simple algorithms: min(a, b), max(a, b), swap(a, b). Here min(a, b) and max(a, b) returns the minimum and maximum of two elements, while swap(a, b) swaps two elements.
Algorithm sort() is also widely used. The call to sort(begin, end) sorts an interval in ascending order. Notice that sort() requires random access iterators, so it will not work on all containers. However, you probably won't ever call sort() on set, which is already ordered.
You've already heard of algorithm find(). The call to find(begin, end, element) returns the iterator where 'element' first occurs, or end if the element is not found. Instead of find(...), count(begin, end, element) returns the number of occurrences of an element in a container or a part of a container. Remember that set and map have the member functions find() and count(), which works in , while std::find() and std::count() take .
Other useful algorithms are next_permutation() and prev_permutation(). Let's speak about next_permutation. The call to next_permutation(begin, end) makes the interval hold the next permutation of the same elements, or returns false if the current permutation is the last one. Accordingly, next_permutation makes many tasks quite easy. If you want to check all permutations, just write:
vector<int> v;
for (int i = 0; i < 10; i++) {
v[i] = rand();
}
sort(all(v));
do {
solve(v);
} while (next_permutation(all(v)));
Don't forget to ensure that the elements in a container are sorted before your first call to next_permutation(...). Their initial state should form the very first permutation; otherwise, some permutations will not be checked.
To go on with STL, I would like to summarize the list of templates to be used. This will simplify the reading of code samples and, I hope, improve your skills. The short list of templates and macros follows:
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int, int> ii;
#define sz(a) int((a).size())
#define pb push_back
#defile all(c) (c).begin(), (c).end()
The container vector<int> is here because it's really very popular. Actually, I found it convenient to have short aliases to many containers (especially for vector<string>, vector<ii>, vector<pair<double, ii>>). But this list only includes the macros that are required to understand the following text.
Another note to keep in mind: when a token from the left-hand side of #define appears in the right-hand side, it should be placed in braces to avoid many nontrivial problems.
This article is an adaptation of a TopCoder article, updated with modern C++ features.