void find_substring(char * str_for_search_in, char * substring)
{
    bool flag = false;
    for (int i = 0; i < strlen(str_for_search_in) - strlen(substring); i  )
    {
        if (str_for_search_in.substr(i, strlen(substring)) == substring)
        {
            cout << i << " ";
            flag = true;
            break;
        }
    }
    if (flag == false)
        cout << "NONE";
}

I am converting my program from C strings to C strings and stuck on this issue in line 6

What's the issue here?

It was length.substring everywhere previously, i changed them to strlen() so they fit char *

I've tried changing data types somewhere but that didn't work

CodePudding user response：

For starters the function in C should be declared like

int find_substring( const char * str_for_search_in, const char * substring);

It does not change the passed strings so its parameters should be declared with the qualifier const. And the function should not output any message. It is the caller of the function will decide whether to output a message. The function just should return a value (0 or 1) that the caller of the function can check.

This for loop

for (int i = 0; i < strlen(str_for_search_in) - strlen(substring); i  )

at least has a wrong condition, You should write it like for example

for (int i = 0; i <= strlen(str_for_search_in) - strlen(substring); i  )

But this work only in case when strlen(str_for_search_in) is indeed greater than or equal to strlen(substring).

Also there are redundant calls of strlen.

Character arrays do not have member functions. So this expression

str_for_search_in.substr(i, strlen(substring))

is invalid.

And to compare strings in your for loop you need to use standard C string function strncmp declared in the header <string.h>.

Pay attention to that within your function you could use standard C string function strstr for example the following way

int find_substring( const char * str_for_search_in, const char * substring)
{
    return strstr( str_for_search_in, substring ) != NULL;
}

CodePudding user response：

You were using std::string which has substr method, but now switched to raw-strings (char*), that are plain data types. You can make use of pointer-arithmetic for a comparison.

In the same way, there is no operator== for raw-strings (in this case you will be comparing pointers), in general prefer strcmp, though in this case it may not be possible because your substrings may not end in a null character.

As a side note, take into consideration that strlen has O(n) complexity, so calling it every time in your for loop is inefficient, pre-compute it.

I recommend you to take a look at string operations in C, rather than C .