Newb Array Question

**hollywoodjrc** · 11-25-2011 04:04 PM

Hi Everyone,

I'm just learning JavaScript through a Lynda.com tutorial, and I'm trying to push myself beyond some of the examples in each chapter.

Right now I'm on arrays. One of the examples they went through was adding all of the values in the array:

Code:

var myArray = [500, 500, 500, 500, 500];

var total = 0;

for ( var i = 0; i < myArray.length; i++ ) {
	total = total + myArray[i];
}

document.write("The total is: " + total);

Okay so I understand this. My challenge, which I can't seem to solve by myself, is this: What if I had an array with 6 values, and I wanted to add the first two numbers, then subtract the third? And loop through this each time.

For example:

Code:

var myArray = [35, 250, 75, 60, 90, 100];

My desired output would be:

210 (result from 35 + 250 - 75)
260 (result from 210 + 60 + 90 - 100)

What are some ways I could accomplish this?

Thank you!

John

**xelawho** · 11-25-2011 04:46 PM

hmmm... I suspect that this is not what you are looking for,but it may be of interest anyway. You can access array values directly by their position in the array:

Code:

<html>
<title></title>
</head>
<body>

<script language="javascript">
var myArray = [35, 250, 75, 60, 90, 100];
sum1=myArray[0]+myArray[1]-myArray[2];
alert("sum 1= "+sum1)
sum2=sum1+myArray[3]+myArray[4]-myArray[5]
alert("sum 2= "+sum2)
</script>
</body>
</html>

**007Julien** · 11-25-2011 06:26 PM

You can too change the sign after each addition

Code:

var myArray = [],i,sign=1;
for (i=0;i<2000;i++) myArray[i] =1/(2*i+1);
var alternatedTotal = 0,oldAlternatedTotal;
for ( var i = 0; i < myArray.length; i++ ) {
	oldAlternatedTotal=alternatedTotal;
	alternatedTotal += sign*myArray[i];
	sign =- sign;
}
document.write("<p>The alternatedTotal is : " + alternatedTotal+" and<br>"+(4*alternatedTotal)+"< Pi <"+(4*oldAlternatedTotal)+"</p>");

And you notice that Pi/4 = 1-1/3+1/5-1/7+1/9....

**JMRKER** · 11-25-2011 07:46 PM

Another alternative solution...

Code:

<html>
<title></title>
</head>
<body>
<script language="javascript">
var myArray = [35, 250, 75, 60, 90, 100];
var sum = 0;
var total = 0;
for (var i=0; i<myArray.length; i=i+3) {
  sum = myArray[i]+myArray[i+1]-myArray[i+2];
  total += sum;
  alert(sum+' : '+total);
}
alert('Total: '+total);
</script>
</body>
</html>

**hollywoodjrc** · 11-26-2011 12:38 AM

Thank you, everyone, for your time! This is extremely helpful.

Kindly,
John

**JunkMale** · 11-26-2011 04:09 PM

Another method might include...

Code:

var myArray = [35, 250, 75, 60, 90, 100];
// add first two and then subtract the third & repeat...
modVal = myArray.length / 2;
for(c=0,total=0; c<myArray.length; c++)
	 total += ( (c+1)&#37;modVal == 0 )? -(myArray[c]) : myArray[c];

alert(total);// outputs 260

**Ayşe** · 11-26-2011 05:07 PM

Code:

   
<script type="text/javascript">

var B = ["+","+","-","+","+","-"];
var A = [35, 250, 75, 60, 90, 100];
var str = "";
for(var i=0; i< A.length; i++) {
str += B[i] + A[i];
}
alert( "str = " + str ); // +35+250-75+60+90-100

alert( "eval( str ) = " + eval( str ) ); // 260

</script>

http://www.w3schools.com/jsref/jsref_eval.asp

**JunkMale** · 11-26-2011 10:08 PM

Originally Posted by Ayşe

Code:

   
<script type="text/javascript">

var B = ["+","+","-","+","+","-"];
var A = [35, 250, 75, 60, 90, 100];
var str = "";
for(var i=0; i< A.length; i++) {
str += B[i] + A[i];
}
alert( "str = " + str ); // +35+250-75+60+90-100

alert( "eval( str ) = " + eval( str ) ); // 260

</script>

http://www.w3schools.com/jsref/jsref_eval.asp

I am afraid that THIS is another example of an abuse of eval and why it gets such a bad name. eval should only be used as a last resort.

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