I need...

I need an explanation of part of the following function that is highlighted in red (possible typo). At first I thought the parenthesis to the right of c was a typo(this is from the book Accelerated C++) but the full code compiles and runs successfully. I don't understand why that parenthesis is there.

Code:

bool not_url_char(char c)
{
	// characters, in addition to alphanumerics, that can appear in a \s-1URL\s0
	static const string url_ch = "~;/?:@=&$-_.+!*'(),";

	// see whether `c' can appear in a \s-1URL\s0 and return the negative
	return !(isalnum(c) ||
	         find(url_ch.begin(), url_ch.end(), c) != url_ch.end());
}

find takes three arguments; where to start, where to stop, and what to look for. The result is to not equal (!=) ch.end.

You need a better title.

Anyway... when you come across something like this break it into multiple lines.

Edit: Actually, if you don't understand it after breaking things up you should start over.

Soma

Code:

iter i1 = url_ch.begin();
iter i2 = url_ch.end();
iter i3 = find(v1, v2, c);
bool b1 = isalnum(c);
bool b2 = i3 != i2; // bool b2 = (i3 != i2);
bool b3 = b1 || b2;
bool b4 = !b3;
return b4;

You need a better title.

Sorry, I couldn't think of anything better. Actually, its the "c)" in the red that I don't understand. What is the end parenthesis for? It looks like a typo to me, but the code compiles and runs. The code in the book and in the provided code example each had that end parenthesis. If this is a concept in C++, I don't understand it. If it is a typo, I don't understand why it compiles and runs. Sorry if my original post wasn't clear.

Originally Posted by swappo

Sorry, I couldn't think of anything better. Actually, its the "c)" in the red that I don't understand. What is the end parenthesis for? It looks like a typo to me, but the code compiles and runs. The code in the book and in the provided code example each had that end parenthesis. If this is a concept in C++, I don't understand it. If it is a typo, I don't understand why it compiles and runs. Sorry if my original post wasn't clear.

What do you mean, what is the close parenthesis for? It closes the open parenthesis:

Code:

find(url_ch.begin(), url_ch.end(), c)
    ^--start                   end--^

std::find is defined as follows:
std::find(start_it, end_it, char_to_find)
If std::find cannot find char_to_find in the range start_it to end_it, it simply returns end_it.
And the parameter you pass to end_it is url_ch.end().
Makes sense?

You also need an editor that can highlight matching parentheses. Such as, well, nearly every programmer's editor out there, with the right settings. (I like jEdit.)

I think I understand it now. I think it was the return statement being on two lines that threw me off. It looked as if the != url_ch.end()) was part of find(). Sometimes I have trouble making sense of other peoples code.

Yes, the two-line return at first confused me It was the last closing parentheses that made me force myself to look at it twice.

Originally Posted by CodeMonkey

Yes, the two-line return at first confused me It was the last closing parentheses that made me force myself to look at it twice.

Which is exactly why, when I split a line of code, I put the related operator on the beginning of the next line instead of the end of the previous.

Code:

return !(isalnum(c)
    || find(url_ch.begin(), url_ch.end(), c) != url_ch.end());

Makes it pretty obvious that the second line is part of a larger expression.

Or even something like:

Code:

return 
	!(
		isalnum(c) 
		||
	        find(url_ch.begin(), url_ch.end(), c) != url_ch.end()
	);

One more thing. The canonical convention for boolean functions is the positive sense, eg:

Code:

bool url_char(char c);

In other words, negation would done by the user code, not the function itself.