Arduino coin counter from vending machine does not work

Projects General Guidance
1

Hi guys,
Me and my friend want to build a little arduino based vending machine. The main problems are the lcd display and the keypad. Normally you should press a button and a product name with the corresponding price should appear. Then coins have to be inserted and the coin counter detects them and subtract the coins value from the products price (this should be seen on the lcd). When the price hits zero the product is given to the customer. We used arrays in order to make the code easier to understand. The idea behind the code is that the productIndex should help the program, so that it knows which product is being selected at all time. The button is only activated by the corresponding coin. CurrentlyThis text will be hidden the code is only for one button, in order to test it.

Code: 
#include <Keypad.h>
#include <LiquidCrystal.h> 

const byte ROWS= 3;
const byte COLS= 3;

char hexaKeys [ROWS][COLS] = {
  {'1','2','3'},
  {'4','5','6'},
  {'7','8','9'}
};

byte rowPins[ROWS]= {10,8,7};
byte colPins[COLS]= {6,5,4};

const int rs = 0,en = 1, d4 = 2, d5 = 3, d6 = 12, d7 = 13;
LiquidCrystal lcd( rs, en, d4, d5, d6, d7);

char customKey;
Keypad customKeypad= Keypad(makeKeymap(hexaKeys),rowPins,colPins,ROWS,COLS);
const int buttonPin1 = 22;
const int buttonPin2 = 23;
boolean lastbuttonState1;
boolean lastbuttonState2;
int produktIndex;
int verbleibenderPreis;
String produkte[]= {"Kinderriegel", "Kindercountry", "Mentos", "KitKat"};
double preise []= { 1.0, 0.80, 0.80, 0.70};


void setup() {
  lcd.begin (16,2);
  pinMode(buttonPin1, INPUT);
  pinMode(buttonPin2, INPUT);
  char customKey = customKeypad.getKey();
}

  
 void loop() {
 
 
  if (customKey == '1'){
    produktIndex = 0;
  
  }else if (customKey == '2'){
    produktIndex = 0;
  }else if (customKey == '3'){
    produktIndex = 0;
  }else if (customKey == '4'){
    produktIndex = 0;
  }
  if (produktIndex >= 0){
    lcd.setCursor (0,0);
    lcd.print ("Produkt:");
    lcd.setCursor(0,10);
    lcd.print( produkte[produktIndex]);
    lcd.setCursor (1,0);
    lcd.print ("Preis:");
    lcd.setCursor(1,8);
    lcd.print(preise[produktIndex]);
   
  }
  if (digitalRead (buttonPin1) ==HIGH && lastbuttonState1 == false){
    lastbuttonState1 = true;
    if (produktIndex>0){
      verbleibenderPreis = preise[produktIndex];
      produktIndex = -1;
      
    }
    verbleibenderPreis -= 0.10;
  }else if( digitalRead (buttonPin1)== LOW){
    lastbuttonState1 = false; 
  }
  }`Preformatted text`
2

As most new members You mix up things.
Number one is managing the hardware needed, like the keyboard, and the display.

Make a test code using example code for the keyboard.
Then do the same for the display.
By then You have gained knowledge about the hardware.

The next will be working out what the hardware will do, the code.
There are topics giving examples regarding buttons. "Buttons" looks like being a very important thing for new members.

3

You get one key in setup() and put it in a local variable and then you look for the key in a global variable that you did not set?!?

Did you want to wait in setup() until a key is pressed and place that key in the global variable for use in loop()? That would be more like:

void setup() {
  do
  {
    customKey = customKeypad.getKey();
  } while (customKey == 0);
}