Evaluate a string with a switch in C++ [duplicate]
Solution 1:
As said before, switch can be used only with integer values. So, you just need to convert your "case" values to integer. You can achieve it by using constexpr from c++11, thus some calls of constexpr functions can be calculated in compile time.
something like that...
switch (str2int(s))
{
case str2int("Value1"):
break;
case str2int("Value2"):
break;
}
where str2int is like (implementation from here):
constexpr unsigned int str2int(const char* str, int h = 0)
{
return !str[h] ? 5381 : (str2int(str, h+1) * 33) ^ str[h];
}
Another example, the next function can be calculated in compile time:
constexpr int factorial(int n)
{
return n <= 1 ? 1 : (n * factorial(n-1));
}
int f5{factorial(5)};
// Compiler will run factorial(5)
// and f5 will be initialized by this value.
// so programm instead of wasting time for running function,
// just will put the precalculated constant to f5
Solution 2:
You can map the strings to enum values, then switch on the enum:
enum Options {
Option_Invalid,
Option1,
Option2,
//others...
};
Options resolveOption(string input);
// ...later...
switch( resolveOption(input) )
{
case Option1: {
//...
break;
}
case Option2: {
//...
break;
}
// handles Option_Invalid and any other missing/unmapped cases
default: {
//...
break;
}
}
Resolving the enum can be implemented as a series of if
checks:
Options resolveOption(std::string input) {
if( input == "option1" ) return Option1;
if( input == "option2" ) return Option2;
//...
return Option_Invalid;
}
Or a map lookup:
Options resolveOption(std::string input) {
static const std::map<std::string, Option> optionStrings {
{ "option1", Option1 },
{ "option2", Option2 },
//...
};
auto itr = optionStrings.find(input);
if( itr != optionStrings.end() ) {
return itr->second;
}
return Option_Invalid;
}
Solution 3:
A switch
statement can only be used for integral values, not for values of user-defined type. (And even if it could, your input operation doesn't work, either. The >>
operation extracts single tokens, separated by whitespace, so it can never retrieve a value "Option 1"
.)
You might want this:
#include <string>
#include <iostream>
std::string input;
if (!std::getline(std::cin, input)) { /* error, abort! */ }
if (input == "Option 1")
{
// ...
}
else if (input == "Option 2")
{
// ...
}
// etc.