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JavaScript : within a string, count the number of occurances of a character / character counting and string-position
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//www.lsauer.com 2012 | |
//Answer to: | |
//http://stackoverflow.com/questions/881085/count-the-number-of-occurances-of-a-character-in-a-string-in-javascript/10671743#10671743 | |
//There are at least four ways. The best option, which should also be the fastest -owing to the native RegEx engine -, is placed at the top. //jsperf.com is currently down, otherwise I would provide you with performance statistics. | |
#1. | |
("this is foo bar".match(/o/g)||[]).length | |
//>2 | |
#2. | |
"this is foo bar".split("o").length-1 | |
//>2 | |
//split is not recommended. Resource hungry. Allocates new instances of 'Array' for each match. Don't try that for a >100MB file via FileReader. | |
//You can actually easily observe the EXACT resource usage using **Chrome's profiler** option. | |
#3. | |
var stringsearch = "o" | |
,str = "this is foo bar"; | |
for(var count=-1,index=-2; index != -1; count++,index=str.indexOf(stringsearch,index+1) ); | |
//>count:2 | |
#4. | |
//searching for a single character | |
var stringsearch = "o" | |
,str = "this is foo bar"; | |
for(var i=count=0; i<str.length; count+=+(stringsearch===str[i++])); | |
//>count:2 | |
#5. | |
//element mapping and filtering; not recommended due to its overall resource pre-allocation vs. Pythonian 'generators' | |
//provides the position within the string | |
var str = "this is foo bar" | |
str.split('').map( function(e,i){ if(e === 'o') return i;} ) | |
.filter(Boolean) | |
//>[9, 10] | |
[9, 10].length | |
//>2 | |
#6 | |
//'deleting' the character out of the string and measuring the distance in length | |
var str = "this is foo bar"; | |
str.length - str.replace(/o/g,'').length | |
//>2 | |
#7 | |
//based on typed arrays; str2buffer is taken from 'is-lib'; See: https://gist.github.com/lsauer | |
//Converts an ASCII string to an typed-Array buffer | |
str2buffer = function(s){ var bu = new ArrayBuffer(s.length), aUint8 = new Uint8Array(bu ); | |
for(var i=0; i<bu.byteLength; aUint8[i]=s.charCodeAt(i),i++);return aUint8; | |
}; | |
var bstr = str2buffer ("this is foo bar") | |
,schar = 'o'.charCodeAt() | |
,cnt=0; | |
for(var i=0;i<bstr.byteLength;schar!==bstr[i++]||cnt++); | |
//>cnt | |
2 | |
#8 | |
//based on untyped Arrays. Is expected to be slower. Analogous to #7 | |
var ubstr = "this is foo bar".split('').map( function(e,i){ return e.charCodeAt();} ) | |
//>[116, 104, 105, 115, 32, 105, 115, 32, 102, 111, 111, 32, 98, 97, 114] | |
,schar = 'o'.charCodeAt() | |
,cnt=0; | |
for(var i=0;i<ubstr.length;schar!==ubstr[i++]||cnt++); | |
//>cnt | |
2 | |
#9 | |
//using reduce. Note: Element map functions are powerful but slow as they involve their own heap/stack allocation | |
//see: http://stackoverflow.com/questions/10293378/what-is-the-most-efficient-way-of-merging-1-2-and-7-8-into-1-7-2-8/17910641#17910641 | |
var str = "this is foo bar", | |
schar = 'o'; | |
str.split('').reduce( | |
function(p,c,i,a){ if(c === schar || p === schar){return isNaN(parseInt(p))? 1:+p+1;} return p;} | |
) | |
//Note: faster: c === schar || p === schar; slower: (c+p).indexOf(schar)>-1 | |
#10. dictionary character histogram | |
var str = "this is foo bar", | |
schar = 'o', | |
hist={}; | |
for(si in str){ | |
hist[str[si]] = hist[str[si]] ? 1+hist[str[si]]:1; | |
} | |
//>hist[schar] | |
2 | |
//Changelog > 11/2013: | |
// | |
// 24/11/2013 #3 bug fixed in initial index position; pointed out by Augustus@Stackoverflow |
Here's Another Way;
var text = "The quick brown fox jumps over the lazy dog";
text = text.toLowerCase();
var textLen = text.length;
var searchFor = "the";
var indexOfSearch = text.indexOf(searchFor);
var counter = 0;
for (var i = 0; i < textLen; i++) {
if (text.indexOf(text.charAt(i)) === indexOfSearch) {
counter++
}
}
console.log(counter);
which one is faster?
var text = "The quick brown fox jumps over the lazy dog";
document.write("Text: "+text+"
"+"There are "+text.match(/the/gi).length+" of word the");
Intuitively, it seems like this should be fastest:
const s = "this is foo bar";
const oCount = s.length - s.replaceAll('o', '').length;
If there are only two kinds of character in the string, then this is faster still:
const s = "001101001";
const oneCount = s.replaceAll('0', '').length;
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Can you explain the number 4 for loop.
count+=+(stringsearch===str[i++])