What is meant by this statement

[iamyiyaj]

bool res = false;
			cout << "Enter word to search" << endl;
string search; 
cin >> search;
			for (i = 0; i<line; i++)
			{
				size_t found = (data[i]).find(" " + search + " "); 
				if (found != string::npos)//what is this line
				{
					res = true;
					cout << data[i] << " " << found << endl; 
				}
			}

What i am trying to figure out is what does found!=string::npos mean and is there another way to write it?

[George P]

http://www.cplusplus.com/reference/string/string/npos/

If the string search (find) didn't go to the end of the string, something was found.

You can't use something else if you want to check if your search didn't find a match.

And with each search you do want to check for that condition.

[jonnin]

strstr lets you know if it is there or not using null==0 pointer as a boolean. If you need to know where it was, find is far superior. Sometimes, you just want to know if its in there.

[MikeStgt]

@Furry Guy

You can't use something else if you want to check if your search didn't find a match.

First: the test results true if the blank delimited search word was found.
2nd: The statement "you can't use something else.." made me thinking. How about (found > string::npos)? IMO even == will do. Hint: see the 3rd to 5th word of your a. m. statement.

[keskiverto]

MikeStgt wrote:

How about (found > string::npos)?

Assuming data[i] is a std::string:
http://www.cplusplus.com/reference/string/string/find/

Return Value
The position of the first character of the first match.
If no matches were found, the function returns string::npos.

And: http://www.cplusplus.com/reference/string/string/npos/

npos is a static member constant value with the greatest possible value for an element of type size_t.

Therefore, found > string::npos is always false.


@iamyiyaj: There is documentation for C++ Standard Library. The question is, does that help you to understand?

[jonnin]

yea, its a design fubar. They chose to return the array index where it is, which makes zero a no-go return value. If it had returned an iterator.... but no one asks me.

this works:
if (found < string::npos )
but it has zero merits over != which is more precise about what you want. The point is to get rid of the npos ugly, not to rewrite it and keep it in.

is risky and unportable to try to exploit npos being a negative number in a signed int result from find (int x = find (etc.. ) . It does work, but it may break on some systems where size_t is not the same bytesize as int. My work server broke on this, so I am sure that this is a real issue not just in theory.

You are basically stuck with ugly here.

[MikeStgt]

Therefore, found > string::npos is always false.

In http://www.cplusplus.com/reference/string/string/npos
I find "static const size_t npos = -1; [...] This constant is defined with a value of -1" -- so this [-1] is not {-1} when compared with integers >= 0? :(
I am still learning.

Edit: ok, sorry, got it. Unsigned -1 = FFFFFFFFFFFFFFFF. No harm if the reference mentioned that detail.

[George P]

First: the test results true if the blank delimited search word was found.

If what is being searched for is found, the function doesn't return string::npos. It returns the index of the first character of the found substring.

if the user entered word is "kumquat", what is passed into find in the OP's code is " kumquat ". A space is appended to the beginning and the end of the string before being used as the find parameter.

IMO even == will do.

If you want to do the INVERSE of a check for a substring being found, when when the substring is not found, then == string::npos works just fine. "Hey, did I come up a cropper looking for something?" If data[i] == "I like to eat a kumquat every day." a match is found.

The usual logic is structured to only check when something is FOUND. != string::npos

The exact meaning (of npos) depends on context, but it is generally used either as end of string indicator by the functions that expect a string index or as the error indicator by the functions that return a string index.

https://en.cppreference.com/w/cpp/string/basic_string/npos

[MikeStgt]

size_t found = (data[i]).find(" " + search + " ");
if (found == string::npos)                     // not found?
{}                                             // yes, nop
else                                           // found
{                                              // act...
	res = true;
	cout << data[i] << " " << found << endl; 
}

Just in case someone dislikes != (for aesthetical reasons or something else).