comparing characters in c++ - c++

Question

If I run my program, it doesn't compare the value entered after "Bitte Funktion waehlen" to sin, cos or tan. Instead, it always jumps directly to the else statement.
#include <iostream>
#include <string>
using namespace::std;
int funktions_wahl(char funktion, int genauigkeit)
{
char sin;
char cos;
char tan;
int x;
if (funktion==sin) {
cout << "sin von " << x << " =" << 2*x*genauigkeit; //Wie sin als fkt hier benutzen? 2x vorübergehend.
}
else if (funktion==cos) {
cout << "cos von " << x << " =" << 4*x*genauigkeit; //Gleiches Spiel wie oben.
}
else if (funktion==tan) {
cout << "tan von " << x << " =" << 8*x*genauigkeit; //Gleiches Spiel wie oben;
}
else {
cout << "Keine gültige Variable eingegeben\r";
}
return 0;
}
int main() {
char f;
int g=0;
cout << "Taschenrechner 1.0\r" << "Bitte Funktion eingeben: ";
cin >> f;
cout << "Genauigkeit wählen: ";
cin >> g;
funktions_wahl(f, g);
}
I don't understand why. Does someone has an advice for me?

Answer 1

char sin;
char cos;
char tan;
int x;
if (funktion==sin) {
cout << "sin von " << x << " =" << 2*x*genauigkeit;
You forgot to initialize sin, cos, tan and x and are therefore reading uninitialized memory.
You probably intended something like char sin = 's'; and so on.

Answer 2

The problem is that you didn't initialize the variables sin, cos and tan (which should be constants actually), which leads to undefined behavior:
For a debug build, the compiler may indeed execute step-by-step your program, and thus compare uninitialized, random numbers;
For an optimized build, the compiler is likely to optimize away these comparisons between uninitialized local variables and only implement the error case (cout << "Keine gültige Variable eingegeben\r";).
To correct this, you should instead define and initialize these constants as follows:
const char sin = 's';
const char cos = 'c';
const char tan = 't';
UPDATE
To answer your updated question (how to consume "sin" rather than 's'), you simply need to change the types of your variables from char to string, and change the literals from char literals (between simple quotes) to string literals (between double quotes), as exemplified below:
string f;
...
const string sin = "sin";
const string cos = "cos";
const string tan = "tan";

Answer 3

Thanks for your answers.
I changed it as you suggested. But still isn´t it possible to enter e.G sin instead of s ?
#include <iostream>
#include <string>
using namespace::std;
int funktions_wahl(char funktion, int genauigkeit, int x)
{
char sin='s';
char cos='c';
char tan='t';
if (funktion==sin) {
cout << "sin von " << x << " =" << 2*x*genauigkeit << "\r"; //Wie sin als fkt hier benutzen? 2x vorübergehend.
}
else if (funktion==cos) {
cout << "cos von " << x << " =" << 4*x*genauigkeit << "\r"; //Gleiches Spiel wie oben.
}
else if (funktion==tan) {
cout << "tan von " << x << " =" << 8*x*genauigkeit << "\r"; //Gleiches Spiel wie oben;
}
else {
cout << "Keine gültige Variable eingegeben\r";
}
return 0;
}
int main() {
char f;
int g=0;
int x=0;
cout << "Taschenrechner 1.0\r" << "Bitte Funktion eingeben: ";
cin >> f;
cout << "Genauigkeit wählen: ";
cin >> g;
cout << "x wählen: ";
cin >> x;
funktions_wahl(f, g, x);