Test for multiple conditions in same if test?

问题

I tried googling and failed.

Using C, I have an IF statement. I want to test a variable against two non-consecutive values.

Is it possible to do

if (state == 1 || 3)

Meaning if state is 1 or if state is 3.

Or does it have to be

if (state == 1 || state == 3)

I'm thinking the first actually means if state is 1, or 3, which means the test will always be true (if true or true).

Is there a way to write this without having to rewrite the variable name multiple times?

No, I don't want to use a case / switch statement. I'm trying to type less.

回答1:

you have to use the long variant

if (state == 1 || 3)

evaluates always to true because it is interpreted as

if ((state == 1) || (3))

EDIT:

Because for C++ was asked in the comments an M.M mentioned operator overloading, a C++ solution

#include <cstdlib>
struct State {
    struct Proxy {
        int v;
        bool res;
        Proxy(int v, bool res) : v(v), res(res) {}

        Proxy operator || (int a) const {
            return Proxy(v, res || (a == v));
        }

        operator bool() const { return res; }
    };
    int v;
    State(int v) : v(v) {}

    Proxy operator == (int a) {
        return Proxy(v, a == v);
    }
};


int main(int argc, char *argv[])
{
    State   state(atoi(argv[1]));

    if (state == 1 || 3 || 5)
        return 1;
    else
        return 0;
}

回答2:

@cde, 'C' programming doesn't have a construct to use the expression like in the first form, so you have to use the second form, as many have already mentioned above.

You might try the following approach, if the values are known ahead of comparison, either by including them at the time of declaration or populate with the values at a later time (which might of fixed or variable set, in which case the array has to be allocated dynamically and size must have already been available by then).

Note: inline, static, const aren't really necessary for the solution to work. Also, it has the advantage that the function returns as soon as the condition evaluates to true skips the rest of the comparisons, unless the state doesn't match with any of the given states.

#include <iostream>

using namespace std;


inline bool isInKnownStates(int state, const int ds[], const int sz)
{
    int i = 0;

    for (; (i < sz) && (state != ds[i]); i++);

    return (i < sz);
}

int main(int argc, const char * argv[]) {

    static const int discrete_states[] = {1, 3};
    static const int sz = sizeof(discrete_states)/sizeof(discrete_states[0]);
    int state0 = 0;
    int state1 = 1;
    int state3 = 3;

    cout << state0 << " " << (isInKnownStates(state0, discrete_states, sz) ? "True" : "False") << endl << flush;
    cout << state1 << " " << (isInKnownStates(state1, discrete_states, sz) ? "True" : "False") << endl << flush;
    cout << state3 << " " << (isInKnownStates(state3, discrete_states, sz) ? "True" : "False") << endl << flush;

    return 0;
}

回答3:

Is there a way to write this without having to rewrite the variable name multiple times?

Well, yes. #include <string.h> and you can use

if(memchr(&(char []){1, 3}, state, 2))

if(memchr(&(char []){1, 3, 5, 7, 9}, state, 5))

Note that state mustn't contain values beyond the scope of char(signedness can be specified as need), or information might be lost. You cannot use an array of type T when sizeof (T) != 1. The reduce of readability will also be taken into consideration.

来源：https://stackoverflow.com/questions/35950990/test-for-multiple-conditions-in-same-if-test

标签

if-statement

logical-operators