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Copy pathmap.cpp
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349 lines (290 loc) · 8.79 KB
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/*
# map
http://www.cplusplus.com/reference/map/map/
Also comes in an unordered version `unordered_map`.
Ordered.
Also comes in an multiple value input version `multimap`.
Does not require a hash function. Usually implemented as a self balancing tree such as a rb tree.
# hashmap
There seems to be no explicit hashmap container, only a generic map interface,
However unordered_map is likely to be hashmap based.
A nonstandard `hash_map` already provided with gcc and msvc++.
It is placed in the `std::` namespace, but it is *not* ISO.
*/
#include "common.hpp"
template <class K, class V>
std::string map_to_str(std::map<K,V> map) {
std::stringstream result;
for (auto& pair : map) {
result << pair.first << ":" << pair.second << ", ";
}
return result.str();
}
// How to use the range pattern from a method.
// The key is to static initialize the lambda map.
class RangeSwitch {
public:
int method(int x) {
static const std::map<int,std::function<void(int&)>> m{
{0, [&](int &x){
x = -1;
}},
{2, [&](int &x){
x = 0;
}},
{5, [&](int &x){
x = 2;
}},
{7, [&](int &x){
x = 5;
}}
};
static const auto end = m.end();
auto it = m.upper_bound(x);
if (it == end) {
x = 7;
} else {
it->second(x);
}
return x;
}
};
int main() {
/*
Create initialized map.
*/
{
std::map<int,std::string> m{
{0, "zero"},
{1, "one"},
};
assert(m[0] == "zero");
assert(m[1] == "one");
}
/*
emplace
Put a value pair into the map without creating the pair explicitly.
Only inserts if not already present.
Needs gcc 4.8: http://stackoverflow.com/questions/15812276/stdset-has-no-member-emplace
*/
{
std::map<int,std::string> m;
assert((m.emplace(0, "zero").second));
assert((m.emplace(1, "one").second));
assert(!(m.emplace(1, "one2").second));
assert(m[0] == "zero");
assert(m[1] == "one");
}
/*
# operator[]
Get value from a given key.
Create if not present, so avoid this if possible and prefer the more restrictive methods:
- use at() or find () for fetching and updating
- emplace() for putting new values
*/
{
std::map<int,std::string> m{
{0, "zero"},
{1, "one"},
};
assert(m[0] == "zero");
assert(m[1] == "one");
// Returns a reference that can override the value.
m[1] = "one2";
assert(m[1] == "one2");
// WARNING: if the key does not exist, it is inserted with a value with default constructor.
// This can be avoided by using `find` or `at` instead of `[]`.
// Inserts `(2,"")` because `""` is the value for the default String constructor.
// http://stackoverflow.com/questions/10124679/what-happens-if-i-read-a-maps-value-where-the-key-does-not-exist
{
assert(m[2] == "");
assert(m.size() == 3);
// This behaviour is however very convenient for nested containers.
{
std::map<int,std::map<int,int>> m;
// Create the empty map at m[0], and immediately add a (0,0) pair to it.
m[0][0] = 0;
// map at m[0] already exists, now just add a new (1, 1) pair to it.
m[0][1] = 1;
m[1][0] = 2;
assert(m[0][0] == 0);
assert(m[0][1] == 1);
assert(m[1][0] == 2);
}
}
}
/*
# find
# check if in map
Similar to `std::set` find with respect to the keys:
returns an iterator pointing to the pair which has given key, not the value.
If not found, returns `map::end()`
This is preferable to `[]` since it does not insert non-existent elements.
*/
{
std::map<int,std::string> m{
{0, "zero"},
{1, "one"},
};
auto found = m.find(0);
assert(found != m.end());
assert(found->second == "zero");
assert(m.find(2) == m.end());
assert(m.size() == 2);
/*
Get default provided value if key not present
TODO: any less verbose way than finding and check != end? Like:
m.get(key, default)
I know I can define a helper, but come on...
*/
{
std::map<int,int> m{};
int default_ = 42;
int result;
auto f = m.find(1);
if (f == m.end()) {
result = default_;
} else {
result = f->second;
}
}
}
/*
# at()
A convenient version of find() that returns the item directly.
Throws if not present, so better when the key is supposed to be there.
C++11.
*/
{
std::map<int,std::string> m{
{0, "zero"},
{1, "one"},
};
// Returns a reference, so we can modify it.
m.at(1) = "one2";
assert(m.at(1) == "one2");
}
/*
# insert
Insert pair into map.
The return value is similar to that of a set insertion with respect to the key.
Just use emplace instead, less verbose as it was added after perfect forwarding.
http://stackoverflow.com/questions/17172080/insert-vs-emplace-vs-operator-in-c-map
*/
{
std::map<int,std::string> m;
std::pair<std::map<int,std::string>::iterator,bool> ret;
ret = m.insert(std::make_pair(0, "zero"));
assert(ret.first == m.find(0));
assert(ret.second == true);
ret = m.insert(std::make_pair(1, "one"));
assert(ret.first == m.find(1));
assert(ret.second == true);
//key already present
ret = m.insert(std::make_pair(1, "one2"));
assert(m[1] == "one");
assert(ret.first == m.find(1));
assert(ret.second == false);
}
/*
# iterate
Map is ordered:
http://stackoverflow.com/questions/7648756/is-the-order-of-iterating-through-stdmap-known-and-guaranteed-by-the-standard
It is iterated in key `<` order.
So this basically requires implementations to use balanced
trees intead of hashmap.
Iteration returns key value pairs.
*/
{
std::map<int,std::string> m{
{1, "one"},
{0, "zero"},
};
int i = 0;
int is[] = {0, 1};
for (auto& im : m) {
assert(im.first == is[i]);
//cout << im->second << endl;
++i;
}
assert(i == 2);
assert(map_to_str(m) == "0:zero, 1:one, ");
}
/*
# erase
Remove element from map.
*/
{
// key version. Returns number of elements removed (0 or 1).
{
std::map<int,std::string> m{
{0, "zero"},
{1, "one"},
};
int ret;
ret = m.erase(1);
assert(ret = 1);
assert((m == std::map<int,std::string>{{0, "zero"}}));
ret = m.erase(1);
assert(ret == 0);
}
// iterator version. Returns iterator to next element.
// Does not invalidate other iterators.
// http://stackoverflow.com/questions/6438086/iterator-invalidation-rules
{
std::map<int,std::string> m{
{0, "zero"},
{1, "one"},
};
auto itNext = m.find(1);
auto it = m.find(0);
assert(m.erase(it) == itNext);
assert((m == std::map<int,std::string>{{1, "one"}}));
}
}
// # Range switch case.
// http://stackoverflow.com/questions/9432226/how-do-i-select-a-range-of-values-in-a-switch-statement/42331563#42331563
{
const std::map<int, int> result{
{-1, -1},
{ 0, 0},
{ 1, 0},
{ 2, 2},
{ 3, 2},
{ 4, 2},
{ 5, 5},
{ 6, 5},
{ 7, 7},
};
int x;
const std::map<int,std::function<void()>> m{
{0, [&](){
x = -1;
}},
{2, [&](){
x = 0;
}},
{5, [&](){
x = 2;
}},
{7, [&](){
x = 5;
}}
};
const auto end = m.end();
for (auto i = -1; i < 8; ++i) {
auto it = m.upper_bound(i);
if (it == end) {
x = 7;
} else {
it->second();
}
assert(x == result.at(i));
}
RangeSwitch rangeSwitch;
for (auto i = -1; i < 8; ++i) {
x = rangeSwitch.method(i);
assert(x == result.at(i));
}
}
}