CODEWITHSUNDEEP
c++string
Daniel Maydana
Daniel Maydana

Reputation: 43

Matching of strings with special characters

I need to generate a string that can match another both containing special characters. I wrote what I thought would be a simple method, but so far nothing has given me a successful match.

I know that specials characters in c++ are preceded with a "\". Per example a single quote would be written as "\'".

string json_string(const string& incoming_str)
{
    string str = "\\\"" + incoming_str + "\\\"";
    return str;
}

And this is the string I have to compare to:

bool comp = json_string("hello world") == "\"hello world\"";

I can see in the cout stream that in fact I'm generating the string as needed but the comparison still gives a false value.

What am I missing? Any help would be appreciated.

Upvotes: 2

Views: 1417

Answers (3)

Aconcagua
Aconcagua

Reputation: 25536

In C++ string literals are delimited by quotes.

Then the problem arises: How can I define a string literal that does itself contain quotes? In Python (for comparison), this can get easy (but there are other drawbacks with this approach not of interest here): 'a string with " (quote)'.

C++ doesn't have this alternative string representation1, instead, you are limited to using escape sequences (which are available in Python, too – just for completeness...): Within a string (or character) literal (but nowhere else!), the sequence \" will be replaced by a single quote in the resulting string.

So "\"hello world\"" defined as character array would be:

{ '"', 'h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd', '"', 0 };

Note that now the escape character is not necessary...

Within your json_string function, you append additional backslashes, though:

"\\\""
{ '\', '"', 0 }
//^^^

Note that I wrote '\' just for illustration! How would you define single quote? By escaping again! '\'' – but now you need to escape the escape character, too, so a single backslash actually needs to be written as '\\' here (wheras in comparison, you don't have to escape the single quote in a string literal: "i am 'singly quoted'" – just as you didn't have to escape the double quote in the character literal).

As JSON uses double quotes for strings, too, you'd most likely want to change your function:

return "\"" + incoming_str + "\"";

or even much simpler:

return '"' + incoming_str + '"';

Now

json_string("hello world") == "\"hello world\""

would yield true...

1 Side note (stolen from answer deleted in the meanwhile): Since C++11, there are raw string literals, too. Using these, you don't have to escape either.

Upvotes: 0

Elijah Dayan
Elijah Dayan

Reputation: 452

bool comp = json_string("hello world") == "\"hello world\"";

This will definitely yield false. You are creating string \"hello world\" by json_string("hello world") but comparing it to "hello world"

The problem is here:

 string str = "\\\"" + incoming_str + "\\\"";

In your first string literal of str, the first character backlash that you’re assuming to be treated like escape character is not actually being treated an escape character, rather just a backslash in your string literal. You do the same in your last string literal.

Do this:

string str = "\"" + incoming_str + "\"";

Upvotes: 0

IceFire
IceFire

Reputation: 4147

One way is to filter one string and compare this filtered string. For example:

#include <iostream>
#include <algorithm>

using namespace std;

std::string filterBy(std::string unfiltered, std::string specialChars)
{
    std::string filtered;

    std::copy_if(unfiltered.begin(), unfiltered.end(),
              std::back_inserter(filtered), [&specialChars](char c){return specialChars.find(c) == -1;});

    return filtered;
}

int main() {
    std::string specialChars = "\"";
    std::string string1 = "test";
    std::string string2 = "\"test\"";

    std::cout << (string1 == filterBy(string2, specialChars) ? "match" : "no match");

    return 0;
}

Output is match. This code also works if you add an arbitrary number of characters to specialChars.

If both strings contain special characters, you can also put string1 through the filterBy function. Then, something like:

"\"hello \" world \"" == "\"hello world "

will also match.

If the comparison is performance-critical, you might also have a comparison that uses two iterators, getting a comparison complexity of log(N+M), where N and M are the sizes of the two strings, respectively.

Upvotes: 1

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