C++ - Comparing Strings Function not working?_问答_开发者

C++ - Comparing Strings Function not working?

开发者 https://www.devze.com 2023-01-23 23:03 出处：网络

I am currently writing an HTML toke开发者_如何学Gon recognizer in c++. I am using stacks to check and see if there are beginning and matching tags.

Before I can match my tags I need to make sure that the tag is valid. But for some reason my checking function is not working. I step through each step in the algorithm and from everything i see this should work. Yet it is returning false every time. Here is my code:

const int NUM_TAGS = 12;
char *tagNames[NUM_TAGS] = 
{
    "<html>",
    "</html>",
    "<head>",
    "</head>",
    "<title>",
    "</title>",
    "<body>",
    "</body>",
    "<p>",
    "</p>",
    "</>",
    "<SPOT/>",
};

bool check_ValidTag(char check[])
{
    for (int i=0; i<=NUM_TAGS; i++)
    {       
        if (check==tagNames[i])
             return true;
    }

    return false;
};

I am calling the function like this:

 tok.isValid = check_ValidTag((char*)tok.str);

As Requested Here is what the struct "tok" looks like..

struct token 
{
  char name[MAX_NAME + 1];
  int type;
  bool isValid;
  char str[MAX_LENGTH + 1];
 };

Any Ideas? Thanks.

You're comparing pointers, not the strings' contents. Replace

 char *tagNames[NUM_TAGS] =

with

std::string tagNames[NUM_TAGS] =

The == operator on char * doesn't compare the contents of the strings. It compares the addresses of the strings. If you are working with C-style strings you should use strcmp to compare them.

However since this is tagged as C++ I would recommend that you use std::string instead of char *.

As other people pointed out, you are comparing a pointer to char array (not the actual string content). If you can change the implementation to use std::string, the "==" overloaded operator should do the trick for you.

However, if you need to stick with the char array. Do it c-style:

{
  if(strcmp(check, tagNames[i])==0)
}

Or if you like to stay c-plus-plussy, you could use the compare() function from char_traits<char>.

typedef std::char_traits<char> ct;

bool check_ValidTag(char check[])
{
    size_t check_len = ct::length(check);
    for (int i=0; i<=NUM_TAGS; i++)
    {       
        if( 0 == ct::compare(check, tagNames[i], check_len) )
            return true;
    }

    return false;
};

See the documentation at www.cplusplus.com.