C++ – Setting all values in a std::map

c++stl

How to set all the values in a std::map to the same value, without using a loop iterating over each value?

Best Solution

Using a loop is by far the simplest method. In fact, it’s a one-liner:^[C++17]

for (auto& [_, v] : mymap) v = value;

Unfortunately C++ algorithm support for associative containers isn’t great pre-C++20. As a consequence, we can’t directly use std::fill.

To use them anyway (pre-C++20), we need to write adapters — in the case of std::fill, an iterator adapter. Here’s a minimally viable (but not really conforming) implementation to illustrate how much effort this is. I do not advise using it as-is. Use a library (such as Boost.Iterator) for a more general, production-strength implementation.

template <typename M>
struct value_iter : std::iterator<std::bidirectional_iterator_tag, typename M::mapped_type> {
    using base_type = std::iterator<std::bidirectional_iterator_tag, typename M::mapped_type>;
    using underlying = typename M::iterator;
    using typename base_type::value_type;
    using typename base_type::reference;

    value_iter(underlying i) : i(i) {}

    value_iter& operator++() {
        ++i;
        return *this;
    }

    value_iter operator++(int) {
        auto copy = *this;
        i++;
        return copy;
    }

    reference operator*() { return i->second; }

    bool operator ==(value_iter other) const { return i == other.i; }
    bool operator !=(value_iter other) const { return i != other.i; }

private:
    underlying i;
};

template <typename M>
auto value_begin(M& map) { return value_iter<M>(map.begin()); }

template <typename M>
auto value_end(M& map) { return value_iter<M>(map.end()); }

With this, we can use std::fill:

std::fill(value_begin(mymap), value_end(mymap), value);

Related Solutions

C++ – How to retrieve all keys (or values) from a std::map and put them into a vector

While your solution should work, it can be difficult to read depending on the skill level of your fellow programmers. Additionally, it moves functionality away from the call site. Which can make maintenance a little more difficult.

I'm not sure if your goal is to get the keys into a vector or print them to cout so I'm doing both. You may try something like this:

std::map<int, int> m;
std::vector<int> key, value;
for(std::map<int,int>::iterator it = m.begin(); it != m.end(); ++it) {
  key.push_back(it->first);
  value.push_back(it->second);
  std::cout << "Key: " << it->first << std::endl();
  std::cout << "Value: " << it->second << std::endl();
}

Or even simpler, if you are using Boost:

map<int,int> m;
pair<int,int> me; // what a map<int, int> is made of
vector<int> v;
BOOST_FOREACH(me, m) {
  v.push_back(me.first);
  cout << me.first << "\n";
}

Personally, I like the BOOST_FOREACH version because there is less typing and it is very explicit about what it is doing.

C++ – How to convert an instance of std::string to lower case

Adapted from Not So Frequently Asked Questions:

#include <algorithm>
#include <cctype>
#include <string>

std::string data = "Abc";
std::transform(data.begin(), data.end(), data.begin(),
    [](unsigned char c){ return std::tolower(c); });

You're really not going to get away without iterating through each character. There's no way to know whether the character is lowercase or uppercase otherwise.

If you really hate tolower(), here's a specialized ASCII-only alternative that I don't recommend you use:

char asciitolower(char in) {
    if (in <= 'Z' && in >= 'A')
        return in - ('Z' - 'z');
    return in;
}

std::transform(data.begin(), data.end(), data.begin(), asciitolower);

Be aware that tolower() can only do a per-single-byte-character substitution, which is ill-fitting for many scripts, especially if using a multi-byte-encoding like UTF-8.

Best Solution

Related Solutions

C++ – How to retrieve all keys (or values) from a std::map and put them into a vector

C++ – How to convert an instance of std::string to lower case

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