How to implement a function that looks for a substring in a string?

Question

C++ / C#

I'm taking an online course in C++ and again I'm in a stupor. The task reads like this:
Another common task with strings is to find a substring (some word or combination of characters) within another string. Implement a function that searches for a given substring in a string and returns the position of its first occurrence (remember that in C++ the convention is to count from 0) if the substring is found, or -1 if no such substring exists.
Sample Input:
Hello world!
ello
Sample Output:
1
Memory Limit: 256 MB
Time Limit: 5 seconds
Solve using pointers and arrays.
I still don’t understand C++ well, I didn’t manage to google anything sensible.

Answer 1

Google strcpy implementation. At least he can give some ideas. It is very difficult to explain the task "on the fingers" to a person with low knowledge of c / c ++. In short: you
take the first letter of the substring, go along the line, as soon as you find the letter, you start comparing element by element.

Answer 2

In general, it can be implemented with a couple of nested forloops: in the outer one, we go along the line, as soon as we find the 1st character, we make an inner loop in which we check that all the others match.

Answer 3

google didn't work out anything

I should have started with a googling course.

Answer 4

google Knuth-Morris-Pratt algorithm

Answer 5

I sketched a function (possibly with errors), but I think the algorithm is clear:

int FindPattern(const char* arg_Str, const char* arg_Ptrn)
{
    int StrSz = strlen(arg_Str);
    int PSz = strlen(arg_Ptrn);
    if(StrSz < PSz || StrSz == 0 || PSz == 0)
        return -1;
    for(int StrIt = 0; StrIt <= StrSz - PSz; StrIt++)
    {
        if(arg_Str[StrIt] == arg_Ptrn[0])
        {
            bool IsBad = false;
            for(int PtrnIt = 1; PtrnIt < PSz; PtrnIt++)
                if(arg_Ptrn[PtrnIt] != arg_Str[StrIt + PtrnIt])
                {
                    IsBad = true;
                    break;
                }
            if(!IsBad) return StrIt;
        }
    }

    return -1;
}