Javascript -> Recursively find a substring within a string

Question

I'm trying to write a recursive program to count the number of times a substring ("cat") appears in a string ("catdogcowcat"). Not sure what I'm doing wrong, but I keep getting the following error:

TypeError: Cannot read property 'length' of undefined

Here's my code:

function strCount (str, sub) {
    var subLen = sub.length;
    var strLen = str.length;
    if (strLen < subLen) {
        return 0;
    } else if (str.slice(0, subLen) === sub) {
        return 1 + strCount(str.substring(subLen));
    } else return strCount(str.substring(1));
}

I think it's breaking when I try to get the length of the substring on this line, but that's just my guess based on my infantile understanding of devtools debugging:

return 1 + strCount(str.substring(subLen));

Thanks!

p.s.w.g · Accepted Answer · 2014-01-28 05:29:00Z

4

Your strCount function takes 2 arguments, so make sure you pass sub when you call it recursively:

function strCount (str, sub) {
    var subLen = sub.length;
    var strLen = str.length;
    if (strLen < subLen) {
        return 0;
    } else if (str.slice(0, subLen) === sub) {
        return 1 + strCount(str.substring(subLen), sub);
    } else return strCount(str.substring(1), sub);
}

answered Jan 28, 2014 at 5:29

p.s.w.g

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1 Comment

i_made_that Over a year ago

Oops, shouldn't have missed that. Thanks!

Patrick Evans · Accepted Answer · 2014-01-28 05:35:03Z

0

You could use regular expression, with match instead of having to do a recursive function

function strCount(needle,haystack){
   var r = new RegExp(needle,"g");
   var r2 = haystack.match(r);
   return (r2?r2.length:0);
}

console.log( strCount("cat","catdowcowcat") );

answered Jan 28, 2014 at 5:35

Patrick Evans

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Comments

Myth · Accepted Answer · 2014-01-28 05:37:22Z

0

you need to pass value for sub ..

 function strCount(str, sub) {                
            var subLen = sub.length;
            var strLen = str.length;
            if (strLen < subLen) {
                return 0;
            } else if (str.slice(0, subLen) === sub) {
                return 1 + strCount(str.substring(subLen),sub);
            } else
                return strCount(str.substring(1),sub);
        }

you can call the method

 $(document).ready(function(){
               alert(strCount("catdogcowcat","cat"));
            });

It will return the result as 2. Fiddle http://jsfiddle.net/deepaksuresh3003/RDmby/

answered Jan 28, 2014 at 5:37

Myth

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Comments

Abhishek Gahlout · Accepted Answer · 2014-01-28 05:42:53Z

0

you are calling your own method strCount(str.substring(subLen)) without providing second parameter and hence the function is taking 'sub' parameter as undefined.

Update your code as follows and it will start working:

 function strCount(str, sub) {
        var subLen = sub.length;
        var strLen = str.length;
        if (strLen < subLen) {
            return 0;
        } else if (str.slice(0, subLen) === sub) {
            return 1 + strCount(str.substring(subLen), sub);
        } else return strCount(str.substring(1),sub);
    }

answered Jan 28, 2014 at 5:42

Abhishek Gahlout

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