Logic operators in if statments - c++

Question

So i am writing a calculator as an exercise in c++, were you first select operators +, -, * or / by inputting a, s, m or d. The calculator works fine, except for a filter i set up to respond with an error if the user inputs something other than a, s, m or d. The filter is an if statement:
if(Opperator=='a'||'s'||'m'||'d')
{
//some code
}
else
{
//"Operatorfault"
cout <<"opperatorfeil";
}
Even though the "operator" has other char values than those the if statement is suppose to execute, the code within the if statement is still executed. The whole code is below. The outputs and variables are in Norwegian, but i have tried to translate in the comments.
#include <iostream>
using namespace std;
//The calculator function
int Kalkulator(int IN1, int IN2, char Opperator)
{
int Svar;
//Detects witch operator the user choose, and preforms the assigned operation
if(Opperator=='a')
{
Svar=IN1+IN2;
}
if(Opperator=='s')
{
Svar=IN1-IN2;
}
if(Opperator=='m')
{
Svar=IN1*IN2;
}
if(Opperator=='d')
{
Svar=IN1/IN2;
}
//Returns the answer
return Svar;
}
int main()
{
//Input a, s, m or d for addition, subtraction, multiplication or division, respectively
cout <<"Skriv \"a\" for addisjon, \"s\" for subtraksjon, \"m\" for multipliksjon, og \"d\" for divisjon";
cout <<endl;
cout <<endl;
char Opperator;
cin >>Opperator;
cout <<endl;
//Checks if the input is valid
if(Opperator=='a'||'s'||'m'||'d')
{
cout <<"Skriv inn det første tallet du vil gjøre opperasjonen på, trykk derreter enter, og skriv inn det andre";
cout <<endl;
cout <<endl;
int IN1;
int IN2;
cin >>IN1;
cin >>IN2;
cout <<endl;
//"The answer is"
cout << "Svaret er: ";
//Calls the calculator function, and inputs the values it has gathered, then prints the answer
cout <<Kalkulator(IN1, IN2, Opperator);
}
else
{
//"Operatorfault"
cout <<"opperatorfeil";
}
return 0;
}

Answer 1

The error is in your if statement. In C++, when you are comparing the same variable to multiple values, for example if opperator is a, m, s, d, you need to restate the variable for each comparison.
if( Opperator=='a' || Opperator=='s' || Opperator=='m' || Opperator=='d' )
Instead of your current if statement.
Also, as a tip, a switch statement is much better in these cases, you can simply state
int ans;
switch (customerPackage) {
case 'a':
ans = int1 + int2;
case 's':
ans = int1 - int2;
case 'd':
ans = int1 * int2;
case 'm':
ans = int1 / int2;
default:
string ans;
ans = "invalid input";
}
cout <<ans;

Answer 2

This is wrong:
if(Opperator=='a'||'s'||'m'||'d')
you can NOT test for equality (or inequality) like this. The code executes as the equivalent of
if (Operator == (result of boolean ORs))
You have to test for equality individually:
if ((op == 'a') || (op == 's') || etc...)

Answer 3

Why not just break it out like so,
if (Opperator == 'a' ||
Opperator == 's' ||
Opperator == 'm' ||
Opperator == 'd')
{
// Good values
}
else
{
// Bad values
}

Answer 4

if(Opperator=='a'||'s'||'m'||'d')
It checks whether Opperator is equal to 'a', if not then 's' always returns true and so all logic becomes true.
You can use if statement:
if (Opperator == 'a' || Opperator == 's' || Opperator == 'm' || Opperator == 'd') {
// my calc works well
} else {
// again my calc works well
}
OR you can use switch:
switch(Opperator) {
case 'a':
case 's':
case 'm':
case 'd': // do normal operation here
break;
default: // error handle
break;
}

Answer 5

You must test each char for equality:
if(Opperator=='a'||Opperator=='s'||Opperator=='m'||Opperator=='d') {
...execute
} else {
...execute something else
}
If you want to be fancy you can use algorithm::anyOf().
#include <algorithm>
const std::vector<char> validValues{'a','s','m','d'};
if(any_of(validValues.begin(),
validValues.end(),
[&](const char &x) { return x == Opperator; }) {
// good values
} else {
// bad values
}