JavaScript - Convert a four-character array to an integer

Question

How can i convert a four-character array to an integer?

Where's the most significant digit? At [0] or at [3]? Also: what base? — Linus Kleen
– Linus Kleen, Commented Jan 11, 2011 at 12:22
Just to be sure: you're not secretly hoping for JavaScript characters to be 8-bit, are you? — Victor Nicollet
– Victor Nicollet, Commented Jan 11, 2011 at 12:23
Using which base? The answers you're getting assume binary hex, decimal ,etc. — winwaed
– winwaed, Commented Jan 11, 2011 at 16:23

Victor Nicollet · Accepted Answer · 2011-01-11 12:37:31Z

3

You're trying to turn those characters into ASCII character codes and using the codes as byte values. This can be done using charCodeAt. For instance:

var str = "x7={";
var result = ( str.charCodeAt(0) << 24 )
           + ( str.charCodeAt(1) << 16 )
           + ( str.charCodeAt(2) << 8 )
           + ( str.charCodeAt(3) );

This returns 2016886139 as expected.

However, bear in mind that unlike C++, JavaScript will not necessarily use a one-byte, 256-character set. For instance, '€'.charCodeAt(0) returns 8364, well beyond the maximum of 256 that your equivalent C++ program would allow. As such, any character outside the 0-255 range will cause the above code to behave erraticaly.

Using Unicode, you can represent the above as "砷㵻" instead.

answered Jan 11, 2011 at 12:37

Victor Nicollet

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2 Comments

jAndy Over a year ago

numbers are stored as 64bit Integers in Javascript. Using bitwise operators, they are converted into 32bit.

Victor Nicollet Over a year ago

jAndy: I agree, and the above is identical whether run on 64-bit or 32-bit. My point is that having bytes with values larger than 255 would cause them to spill over in the byte above.

jAndy · Accepted Answer · 2011-01-11 12:22:11Z

2

var arr = [5,2,4,0],
    foo = +arr.join('');

console.log(foo, typeof foo);

answered Jan 11, 2011 at 12:22

jAndy

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1 Comment

cdmckay Over a year ago

Doesn't the poster specifically say four characters, not numbers?

unwind · Accepted Answer · 2011-01-11 13:09:45Z

1

I guess that depends on how you want to map the character values to the integer's bits.

One straight-forward solution would be:

var myArray = ['1', '2', '3', '4']
var myInt = (myArray[0].charCodeAt(0) << 24) | (myArray[1].charCodeAt(0) << 16) | (myArray[2].charCodeAt(0) << 8) | myArray[3].charCodeAt(0);

This produces the integer 0x01020304. This uses integers in the input array, for characters the result might be different depending on the characters used.

Update: use charCodeAt() to convert characters to code points.

edited Jan 11, 2011 at 13:09

answered Jan 11, 2011 at 12:24

unwind

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2 Comments

Victor Nicollet Over a year ago

The characters would be converted to integers by <<, so any character outside [0-9] would be treated as zero.

Sotiris Over a year ago

I am trying to do something like this , but in your array don't you use numbers? //That's a C++ code int toInt(const char* bytes) { return (int)(((unsigned char)bytes[3] << 24) | ((unsigned char)bytes[2] << 16) | ((unsigned char)bytes[1] << 8) | (unsigned char)bytes[0]); }

Gabriele Petrioli · Accepted Answer · 2011-01-11 12:31:53Z

0

var chArr = ['1','2','3','4'];
var num = parseInt( chArr.join(''), 10);

or

var num = parseInt( chArr.reverse().join(''), 10);

if depending on the order you array is filled..

answered Jan 11, 2011 at 12:31

Gabriele Petrioli

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