Comparing a char to chars in a list in C++ - c++

Question

Is there a way to compare a char to each element in a list of chars?
char ch;
if(ch == 'a' || ch == 'b' || ch == 'c')
Is there some way to just do
if(ch is one of {a, b, c})

Answer 1

Why would you write lambdas or use a throwaway string object when you can just:
if (strchr("abc", ch))

Answer 2

Use : std::any_of
With C++11 :
std::string str="abc";
if(std::any_of(str.cbegin(), str.cend(),
[ch](const char& x){return x==ch; } ))
{
}
Or use a functor:
struct comp
{
comp(char x) :ch(x){}
bool operator()(const char& x) const
{
return x == ch;
}
char ch;
};
And then,
if(std::any_of(str.cbegin(), str.cend(),comp(ch) ))
{
}
Edit : std::any_of might not be efficient enough, just for sake of C++'s <algorithm> one can try this out too .

Answer 3

You could use std::find. Assuming chars is your character array and you need to find ch.
if(std::find(std::begin(chars), std::end(chars), ch) != std::end(chars))

Answer 4

One way to do it is searching a string, like this:
string abc("abc");
if (abc.find(ch) != string::npos) {
...
}

Answer 5

(This answer really only applies if you don't want to use C++ std lib constructs.)
In your specific case, you should be able to do:
if(ch >= 'a' && ch <= 'c')
I also have employed fall-through switch for this case:
switch(ch)
{
case 'a':
case 'b':
case 'c':
case 'e':
...
break;
}
Some people don't like fall-through switch/case statements, but I think its less error prone than a massive piece of boolean logic and will perform better than using a data structure for this purpose. The compiler is really good with dealing with switch statements.

Answer 6

If you can use variadic template arguments, which were introduced in C++11, then you can do something like this:
template <typename Key, typename Value>
inline bool in(const Key& key, const Value& value) {
return key == value;
}
template <typename Key, typename Value0, typename ...ValueN>
inline bool in(const Key& key, const Value0& value, ValueN &&...args) {
return (key == value ? true : in(key, std::forward<ValueN>(args)...));
}
I use it for strings like this:
if (in(some_string, "base", "os", "io", "coroutine", "debug")) ...
But other types that support comparison (char is one of them) should also work.
Hope it helps. Good Luck!

Answer 7

Just as another option, create a set with the characters, and check if it's contained there;
std::set<char> mySet = {'a','b','c'}; // C++11 initializer list
if(mySet.find('d') != mySet.end()) {
...
}

Answer 8

I'm sort of surprised that nobody suggested find_first_of.
char c('e');
// we can check if c is undesirable
const std::string unwanted("abc");
bool undesirable = (unwanted.find_first_of(c) != std::string::npos);
// OR we can check if c is desirable
const std::string wanted("def");
bool desirable = (wanted.find_first_of(c) != std::string::npos); //..or check if it's desirable.
I use this (maybe I shouldn't? folks?) for ignoring unwanted characters from a string iterator...
/** in and out are string iterators.
* skip over any undesirable characters by matching
* against desirable and looking for npos.
**/
const std::string ok("!+-./0123456789:^ABFIORmn");
while (ok.find_first_of(*in) == string::npos && in < out) {
in++;
}
A nice benefit of this is that by putting more frequent characters at the front of the string, there's a little bit of saving in time.