Simple calculator using Arduino UNO

Arduino coding:
#include<Keypad.h>

const byte ROWS = 4; //four rows

const byte COLS = 4; //four columns

char keys[ROWS][COLS] = {

  {'7','8','9','/'},

  {'4','5','6','*'},

  {'1','2','3','-'},

  {'c','0','=','+'}

};

byte rowPins[ROWS] = {2,3,4,5}; //connect to the row pinouts of the keypad

byte colPins[COLS] = {6,7,8,9}; //connect to the column pinouts of the keypad

int i=0;

double d=0,result=0;

char arr[10], prev_key=0;

Keypad keypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS );

void setup(){

   Serial.begin(9600);

}

void loop(){

  char key = keypad.getKey();

  

  if (key){

     Serial.print(key);

     if(key == '+' || key == '-' || key == '*' || key == '/')

     {

        if(prev_key==0)

          result=atol(arr);

        i=0;  

     }

     else

     {

       arr[i++]=key;

       arr[i]=NULL;

     }

     

     if(key == '+' || key == '-' || key == '*' || key == '/')

     { 

        calculate(prev_key);

        prev_key=key;  

     }

      

     if(key == '=')

     {

       calculate(prev_key);

       d=(long int)result;

       if(d==result)

       {

         Serial.println(result,0); 

       }

       else

       {

         Serial.println(result);        

       }

       arr[0]=NULL;

     }

     if(key == 'c')

     {

      Serial.println(" ");

     }

  }

}

  int calculate(char ch)

  {

   switch(ch){

    case '+':

     result=result+atol(arr);

     break;

    case '-':

     result=result-atol(arr);

     break; 

    case '*':

     result=result*atol(arr);

     break; 

    case '/':

     result=result/atol(arr);

     d=1;

     break; 

     case 'c':

      result=0;

      arr[0]=NULL;

      i=0;

      prev_key=0;

     break;

     default:

      int j=0;

   }

  }

Embedded systems basics:
Embedded systems step by step video guide
-using PROTEUS ISIS professional and Arduino IDE software

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https://youtu.be/cnPb4pObkXc

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