c++ - 0 を返す pow() (C++)

Question

次のコードで、実際の計算ではなく、プログラムの実行時にpow()a が返される理由を誰かが説明できますか? 0私はプログラミングの初心者で、完全に困惑しています。

助けてくれてありがとう。

#include <iostream>
#include <math.h>
#include <windows.h>
using namespace std;
//Prototypes:

double phiExpo;

double phiNegExpo;    

double opt1f(double phi, double userInput){
      return userInput * phi;}    

double opt2f(double phi, double userInput){
      return userInput / phi;}  

double opt3f(){
      return phiExpo;}   

double opt4f(){
      return phiNegExpo;}   

double phiExpof(double phi, double userInput){
      pow(phi, userInput);}

double phiNegExpof(double phi, double userInput){
      pow(phi,-userInput);}

//Execute program:

int main()
{
    double userInput;
    int userChoice;
    double phi = 1.61803399;
    bool quit = false; 
    int userChoice2;
    cout << "I want to (press corresponding number, then enter):" << endl;
    cout << endl;
    startchoices: 
    cout << "1. Multiply by phi:" << endl;
    cout << "2. Divide by phi:" << endl;
    cout << "3. Exponentiate phi:" << endl;
    cout << "4. Negatively exponentiate phi:" << endl;
    cout << "5. Quit." << endl;    
    cout << endl;
    cin >> userChoice;
    cout << endl;  
    do {    
       switch (userChoice){

              case 1:
              cout << "Enter number for multiplication: ";
              cin >> userInput;
              cout << endl;
              cout << "Phi multiplied by " << userInput << ": ";   
              cout << opt1f(phi, userInput) << endl;
              cout << endl;
              Sleep(2000);
              cout << "1. Continue." << endl;
              cout << "2. Return to menu." << endl;
              cout << endl;
              cin >> userChoice2;
              cout << endl;
              if(userChoice2 > 1){
                  goto startchoices;}            
              break;

              case 2:
              cout << "Enter number for division: ";
              cin >> userInput;
              cout << endl;
              cout << "Phi divided by " << userInput << ": ";
              cout << opt2f(phi, userInput); 
              cout << endl;
              Sleep(2000);
              cout << "1. Continue." << endl;
              cout << "2. Return to menu." << endl;
              cout << endl;
              cin >> userChoice2;
              cout << endl;
              if(userChoice2 > 1){goto startchoices;}               
              break; 

              case 3:
              cout << "Enter number to exponentiate phi by: ";
              cin >> userInput;
              cout << endl;
              cout << "Phi to the power of " << userInput << ": ";
              cout << opt3f();
              cout << endl;
              Sleep(2000);
              cout<<endl;
              cout << "1. Continue." << endl;
              cout << "2. Return to menu." << endl;
              cout << endl;
              cin >> userChoice2;
              cout << endl;
              if(userChoice2 > 1){goto startchoices;}               
              break; 
        }
    }
}