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              <link href="https://fonts.googleapis.com/css?family=Lobster" rel="stylesheet" type="text/css">

<body>  
  <div class="container-fluid text-center">
       
    <div class="row"> <!-- for title -->
      <h1> Calculator </h1>
      <h3> Math is Fun </h3>
    </div> <!-- end title -->
    
    <div class="row"> <!-- for main -->
      <!-- Start calculator -->
      <div id="calculator">
        	<div class="top">
		          <span class="clear">C</span>
		          <div class="screen"></div>
	        </div> <!-- end class top-->
	
        	<div class="keys">
		        <!-- operators and other keys -->
		          <span>7</span>
		          <span>8</span>
          <span>9</span>
          <span class="operator">+</span>
          <span>4</span>
          <span>5</span>
          <span>6</span>
          <span class="operator">-</span>
          <span>1</span>
          <span>2</span>
          <span>3</span>
          <span class="operator">÷</span>
          <span>0</span>
          <span>.</span>
          <span class="eval">=</span>
          <span class="operator">x</span>
        </div>
      </div>

      <!-- End calculator -->
    </div> <!-- end main -->
    
    <div class="row text-center"> <!-- for footer -->
      <h5><a href="http://www.eleftheriabatsou.com" target="_blank">Eleftheria </a> | <a href="https://codepen.io/EleftheriaBatsou" target="_blank">Projects</a> | <a href="https://www.youtube.com/channel/UCC-WwYv3DEW7Nkm_IP6VeQQ" target="_blank"> Coding videos </a></h5>
    </div> <!-- end footer -->
  
  </div> <!-- end class container-fluid --> 
</body>
   
            
          
!
            
              body {
  background-color: #B2127D; /* purple */
  color: #14C6CC; /* blue */
}

a:link    {color:#14C6CC;  text-decoration:none}
a:visited {color:#14C6CC; text-decoration:none}
a:hover   {color:#b8ff05; text-decoration:underline} /* Yellow */

h1, h3 {
    font-family: Lobster, Monospace;
}

/* Style for the calculator*/

/* Using box shadows to create 3D effects */
#calculator {
	width: 325px;
	height: auto;
	margin: 5% auto;
	padding: 20px 20px 10px;
	
	background: #14C6CC;
	background: linear-gradient(#14C6CC, #FFDF27); /* blue and yellow*/
	border-radius: 50px;	
}

/* Top portion */
.top span.clear {
	float: left;
}

/* Inset shadow on the screen to create indent */
.top .screen {
	height: 40px;
	width: 212px;
	
	float: right;
	
	padding: 0 10px;
	
	background: #1BADB2; /* darker blue*/
	border-radius: 50px;
	box-shadow: inset 0px 4px  #1BADB2;
	
	/* Typography */
	font-size: 20px;
	line-height: 40px;
	color: white;
	text-align: right;
	letter-spacing: 1px;
}

/* Clear floats */
.keys, .top {overflow: hidden;}

/* Applying same to the keys */
.keys span, .top span.clear {
	float: left;
	position: relative;
	top: 0;
	
	cursor: pointer;
	
	width: 66px;
	height: 36px;
	
	background: white;
	border-radius: 20px;
	
	margin: 0 7px 11px 0;
	
	color: #B2127D;
	line-height: 36px;
	text-align: center;
	
	/* prevent selection of text inside keys */
	user-select: none;
	
	/* Smoothing out hover and active states using css3 transitions */
	transition: all 0.2s ease;
}

/* Remove right margins from operator keys */
/* style different type of keys (operators/evaluate/clear) differently */

.keys span.operator {
	background: #FFF0F5; /* yellow */
	margin-right: 0;
}

.keys span.eval {
	background: #14C6CC;
	color: #B2127D;
}

.top span.clear {
	background: #FFDF27;
	color: B2127D;
}

/* Some hover effects */
.keys span:hover {
	background: #14C6CC;
	color: white;
}

.keys span.eval:hover {
	background: #B2127D;
	color: #ffffff;
}

.top span.clear:hover {
	background: #B2127D;
	color: white;
}

/* End calculator's style*/

            
          
!
            
              // Get all the keys from document
var keys = document.querySelectorAll('#calculator span');
var operators = ['+', '-', 'x', '÷'];
var decimalAdded = false;

// Add onclick event to all the keys and perform operations
for(var i = 0; i < keys.length; i++) {
	keys[i].onclick = function(e) {
		// Get the input and button values
		var input = document.querySelector('.screen');
		var inputVal = input.innerHTML;
		var btnVal = this.innerHTML;
		
		// Now, just append the key values (btnValue) to the input string and finally use javascript's eval function to get the result
		// If clear key is pressed, erase everything
		if(btnVal == 'C') {
			input.innerHTML = '';
			decimalAdded = false;
		}
		
		// If eval key is pressed, calculate and display the result
		else if(btnVal == '=') {
			var equation = inputVal;
			var lastChar = equation[equation.length - 1];
			
			// Replace all instances of x and ÷ with * and / respectively. This can be done easily using regex and the 'g' tag which will replace all instances of the matched character/substring
			equation = equation.replace(/x/g, '*').replace(/÷/g, '/');
			
			// Final thing left to do is checking the last character of the equation. If it's an operator or a decimal, remove it
			if(operators.indexOf(lastChar) > -1 || lastChar == '.')
				equation = equation.replace(/.$/, '');
			
			if(equation)
				input.innerHTML = eval(equation);
				
			decimalAdded = false;
		}
		
		// Basic functionality of the calculator is complete. But there are some problems like 
		// 1. No two operators should be added consecutively.
		// 2. The equation shouldn't start from an operator except minus
		// 3. not more than 1 decimal should be there in a number		
		// We'll fix these issues using some simple checks
		
		// indexOf works only in IE9+
		else if(operators.indexOf(btnVal) > -1) {
			// Operator is clicked
			// Get the last character from the equation
			var lastChar = inputVal[inputVal.length - 1];
			
			// Only add operator if input is not empty and there is no operator at the last
			if(inputVal != '' && operators.indexOf(lastChar) == -1) 
				input.innerHTML += btnVal;
			
			// Allow minus if the string is empty
			else if(inputVal == '' && btnVal == '-') 
				input.innerHTML += btnVal;
			
			// Replace the last operator (if exists) with the newly pressed operator
			if(operators.indexOf(lastChar) > -1 && inputVal.length > 1) {
				input.innerHTML = inputVal.replace(/.$/, btnVal);
			}
			
			decimalAdded = false;
		}
		
		// Now only the decimal problem is left. We can solve it easily using a flag 'decimalAdded' which we'll set once the decimal is added and prevent more decimals to be added once it's set. It will be reset when an operator, eval or clear key is pressed.
		else if(btnVal == '.') {
			if(!decimalAdded) {
				input.innerHTML += btnVal;
				decimalAdded = true;
			}
		}
		
		// if any other key is pressed, just append it
		else {
			input.innerHTML += btnVal;
		}
		
		// prevent page jumps
		e.preventDefault();
	} 
}
            
          
!
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