Compact way to write if(..) statement with many equalities - c++

Question

Is there a better way to write code like this:
if (var == "first case" or var == "second case" or var == "third case" or ...)
In Python I can write:
if var in ("first case", "second case", "third case", ...)
which also gives me the opportunity to easily pass the list of good options:
good_values = "first case", "second case", "third case"
if var in good_values
This is just an example: the type of var may be different from a string, but I am only interested in alternative (or) comparisons (==). var may be non-const, while the list of options is known at compile time.
Pro bonus:
laziness of or
compile time loop unrolling
easy to extend to other operators than ==

Answer 1

if you want to expand it compile time you can use something like this
template<class T1, class T2>
bool isin(T1&& t1, T2&& t2) {
return t1 == t2;
}
template<class T1, class T2, class... Ts>
bool isin(T1&& t1 , T2&& t2, T2&&... ts) {
return t1 == t2 || isin(t1, ts...);
}
std::string my_var = ...; // somewhere in the code
...
bool b = isin(my_var, "fun", "gun", "hun");
I did not test it actually, and the idea comes from Alexandrescu's 'Variadic templates are funadic' talk. So for the details (and proper implementation) watch that.
Edit:
in c++17 they introduced a nice fold expression syntax
template<typename... Args>
bool all(Args... args) { return (... && args); }
bool b = all(true, true, true, false);
// within all(), the unary left fold expands as
// return ((true && true) && true) && false;
// b is false

Answer 2

The any_of algorithm could work reasonably well here:
#include <algorithm>
#include <initializer_list>
auto tokens = { "abc", "def", "ghi" };
bool b = std::any_of(tokens.begin(), tokens.end(),
[&var](const char * s) { return s == var; });
(You may wish to constrain the scope of tokens to the minimal required context.)
Or you create a wrapper template:
#include <algorithm>
#include <initializer_list>
#include <utility>
template <typename T, typename F>
bool any_of_c(const std::initializer_list<T> & il, F && f)
{
return std::any_of(il.begin(), il.end(), std::forward<F>(f));
}
Usage:
bool b = any_of_c({"abc", "def", "ghi"},
[&var](const char * s) { return s == var; });

Answer 3

Alrighty then, you want Radical Language Modification. Specifically, you want to create your own operator. Ready?
Syntax
I'm going to amend the syntax to use a C and C++-styled list:
if (x in {x0, ...}) ...
Additionally, we'll let our new in operator apply to any container for which begin() and end() are defined:
if (x in my_vector) ...
There is one caveat: it is not a true operator and so it must always be parenthesized as it's own expression:
bool ok = (x in my_array);
my_function( (x in some_sequence) );
The code
The first thing to be aware is that RLM often requires some macro and operator abuse. Fortunately, for a simple membership predicate, the abuse is actually not that bad.
#ifndef DUTHOMHAS_IN_OPERATOR_HPP
#define DUTHOMHAS_IN_OPERATOR_HPP
#include <algorithm>
#include <initializer_list>
#include <iterator>
#include <type_traits>
#include <vector>
//----------------------------------------------------------------------------
// The 'in' operator is magically defined to operate on any container you give it
#define in , in_container() =
//----------------------------------------------------------------------------
// The reverse-argument membership predicate is defined as the lowest-precedence
// operator available. And conveniently, it will not likely collide with anything.
template <typename T, typename Container>
typename std::enable_if <!std::is_same <Container, T> ::value, bool> ::type
operator , ( const T& x, const Container& xs )
{
using std::begin;
using std::end;
return std::find( begin(xs), end(xs), x ) != end(xs);
}
template <typename T, typename Container>
typename std::enable_if <std::is_same <Container, T> ::value, bool> ::type
operator , ( const T& x, const Container& y )
{
return x == y;
}
//----------------------------------------------------------------------------
// This thunk is used to accept any type of container without need for
// special syntax when used.
struct in_container
{
template <typename Container>
const Container& operator = ( const Container& container )
{
return container;
}
template <typename T>
std::vector <T> operator = ( std::initializer_list <T> xs )
{
return std::vector <T> ( xs );
}
};
#endif
Usage
Great! Now we can use it in all the ways you would expect an in operator to be useful. According to your particular interest, see example 3:
#include <iostream>
#include <set>
#include <string>
using namespace std;
void f( const string& s, const vector <string> & ss ) { cout << "nope\n\n"; }
void f( bool b ) { cout << "fooey!\n\n"; }
int main()
{
cout <<
"I understand three primes by digit or by name.\n"
"Type \"q\" to \"quit\".\n\n";
while (true)
{
string s;
cout << "s? ";
getline( cin, s );
// Example 1: arrays
const char* quits[] = { "quit", "q" };
if (s in quits)
break;
// Example 2: vectors
vector <string> digits { "2", "3", "5" };
if (s in digits)
{
cout << "a prime digit\n\n";
continue;
}
// Example 3: literals
if (s in {"two", "three", "five"})
{
cout << "a prime name!\n\n";
continue;
}
// Example 4: sets
set <const char*> favorites{ "7", "seven" };
if (s in favorites)
{
cout << "a favorite prime!\n\n";
continue;
}
// Example 5: sets, part deux
if (s in set <string> { "TWO", "THREE", "FIVE", "SEVEN" })
{
cout << "(ouch! don't shout!)\n\n";
continue;
}
// Example 6: operator weirdness
if (s[0] in string("014") + "689")
{
cout << "not prime\n\n";
continue;
}
// Example 7: argument lists unaffected
f( s, digits );
}
cout << "bye\n";
}
Potential improvements
There are always things that can be done to improve the code for your specific purposes. You can add a ni (not-in) operator (Add a new thunk container type). You can wrap the thunk containers in a namespace (a good idea). You can specialize on things like std::set to use the .count() member function instead of the O(n) search. Etc.
Your other concerns
const vs mutable : not an issue; both are usable with the operator
laziness of or : Technically, or is not lazy, it is short-circuited. The std::find() algorithm also short-circuits in the same way.
compile time loop unrolling : not really applicable here. Your original code did not use loops; while std::find() does, any loop unrolling that may occur is up to the compiler.
easy to extend to operators other than == : That actually is a separate issue; you are no longer looking at a simple membership predicate, but are now considering a functional fold-filter. It is entirely possible to create an algorithm that does that, but the Standard Library provides the any_of() function, which does exactly that. (It's just not as pretty as our RLM 'in' operator. That said, any C++ programmer will understand it easily. Such answers have already been proffered here.)
Hope this helps.

Answer 4

First, I recommend using a for loop, which is both the easiest and
most readable solution:
for (i = 0; i < n; i++) {
if (var == eq[i]) {
// if true
break;
}
}
However, some other methods also available, e.g., std::all_of, std::any_of, std::none_of (in #include <algorithm>).
Let us look at the simple example program which contains all the above keywords
#include <vector>
#include <numeric>
#include <algorithm>
#include <iterator>
#include <iostream>
#include <functional>
int main()
{
std::vector<int> v(10, 2);
std::partial_sum(v.cbegin(), v.cend(), v.begin());
std::cout << "Among the numbers: ";
std::copy(v.cbegin(), v.cend(), std::ostream_iterator<int>(std::cout, " "));
std::cout << '\\n';
if (std::all_of(v.cbegin(), v.cend(), [](int i){ return i % 2 == 0; }))
{
std::cout << "All numbers are even\\n";
}
if (std::none_of(v.cbegin(), v.cend(), std::bind(std::modulus<int>(),
std::placeholders::_1, 2)))
{
std::cout << "None of them are odd\\n";
}
struct DivisibleBy
{
const int d;
DivisibleBy(int n) : d(n) {}
bool operator()(int n) const { return n % d == 0; }
};
if (std::any_of(v.cbegin(), v.cend(), DivisibleBy(7)))
{
std::cout << "At least one number is divisible by 7\\n";
}
}

Answer 5

You may use std::set to test if var belongs to it. (Compile with c++11 enabled)
#include <iostream>
#include <set>
int main()
{
std::string el = "abc";
if (std::set<std::string>({"abc", "def", "ghi"}).count(el))
std::cout << "abc belongs to {\"abc\", \"def\", \"ghi\"}" << std::endl;
return 0;
}
The advantage is that std::set<std::string>::count works in O(log(n)) time (where is n is number of strings to test) comparing to non compact if witch is O(n) in general. The disadvantage is that construction of the set takes O(n*log(n)). So, construct it once, like:
static std::set<std::string> the_set = {"abc", "def", "ghi"};
But, IMO it would be better to leave the condition as is, unless it contains more than 10 strings to check. The performance advantages of using std::set for such a test appears only for big n. Also, simple non compact if is easier to read for average c++ developer.

Answer 6

The closest thing would be something like:
template <class K, class U, class = decltype(std::declval<K>() == std::declval<U>())>
bool in(K&& key, std::initializer_list<U> vals)
{
return std::find(vals.begin(), vals.end(), key) != vals.end();
}
We need to take an argument of type initializer_list<U> so that we can pass in a braced-init-list like {a,b,c}. This copies the elements, but presumably we're going doing this because we're providing literals so probably not a big deal.
We can use that like so:
std::string var = "hi";
bool b = in(var, {"abc", "def", "ghi", "hi"});
std::cout << b << std::endl; // true

Answer 7

If you have access to C++14 (not sure if this works with C++11) you could write something like this:
template <typename T, typename L = std::initializer_list<T>>
constexpr bool is_one_of(const T& value, const L& list)
{
return std::any_of(std::begin(list), std::end(list), [&value](const T& element) { return element == value; });
};
A call would look like this:
std::string test_case = ...;
if (is_one_of<std::string>(test_case, { "first case", "second case", "third case" })) {...}
or like this
std::string test_case = ...;
std::vector<std::string> allowedCases{ "first case", "second case", "third case" };
if (is_one_of<std::string>(test_case, allowedCases)) {...}
If you don't like to "wrap" the allowed cases into a list type you can also write a little helper function like this:
template <typename T, typename...L>
constexpr bool is_one_of(const T& value, const T& first, const L&... next) //First is used to be distinct
{
return is_one_of(value, std::initializer_list<T>{first, next...});
};
This will allow you to call it like this:
std::string test_case = ...;
if (is_one_of<std::string>(test_case, "first case", "second case", "third case" )) {...}
Complete example on Coliru

Answer 8

Worth noting that in most Java and C++ code I've seen, listing 3 or so conditionals out is the accepted practice. It's certainly more readable than "clever" solutions. If this happens so often it's a major drag, that's a design smell anyway and a templated or polymorphic approach would probably help avoid this.
So my answer is the "null" operation. Just keep doing the more verbose thing, it's most accepted.

Answer 9

You could use a switch case. Instead of having a list of separate cases you could have :
include
using namespace std;
int main ()
{
char grade = 'B';
switch(grade)
{
case 'A' :
case 'B' :
case 'C' :
cout << "Well done" << endl;
break;
case 'D' :
cout << "You passed" << endl;
break;
case 'F' :
cout << "Better try again" << endl;
break;
default :
cout << "Invalid grade" << endl;
}
cout << "Your grade is " << grade << endl;
return 0;
}
So you can group your results together: A, B and C will output "well done".
I took this example from Tutorials Point:
http://www.tutorialspoint.com/cplusplus/cpp_switch_statement.htm