Trouble with finding a c-string substring in C++ - c++

Question

long time lurker, first time poster. I have been working on this problem for the last six hours hours.
Problem:
Implement the following functions. Each function deals with null terminated C-Style strings. You can assume that any char array passed into the functions will contain null terminated data. Place all of the functions in a single file and then create a main() function that tests the functions thoroughly.
Note: You may not use any c-string functions other than strlen().
I am having trouble with the fourth function.
The desired behavior is: This function returns the index in string s where the substring can first be found. For example if s is "Skyscraper" and substring is "ysc" the function would return 2. It should return -1 if the substring does not appear in the string.
prototype:
int findSubstring(char *str, char substring[])
Here's my two starts for function definitions, I'm not really sure if either is going in the right direction, I'm having a lot of trouble keeping the loop iterations in my head, any help would be TREMENDOUSLY appreciated.
int findSubstring(char *str, char substring[]){
int subS = -1, index1 = 0, index2 = 0;
int length1 = (strlen(str) - 1);
int length2 = (strlen(substring) - 1);
if(length1 > length2){
for(int i = 0; i <= length2; i++){
for(int j = 0; j <= length1; j++){
if(*(substring + i) == *(str + j) && *(substring +i) != '\0' ){
i++;
if(index1 == 0){
index1 = i;
}
}
if( *(substring + i) == '\0'){
subS = i + 2;
}
}
}
}
if (length1 < length2){
cout << "Invalid, substring exceeds size of string!" << endl;
}
return subS;
}
int findSubstring(char *str, char substring[]){
int index = -1;
int lengthStr = (strlen(str) - 1);
int lengthSub = (strlen(substring) - 1);
if (lengthStr < lengthSub){
cout << "Invalid input, substring exceeds size of string!" << endl;
}
if( lengthSub == 0){
cout << "";
}
if (lengthStr > lengthSub){
for(int i = 0; i <= lengthSub; i++){
for(int j = 0; j <= lengthStr; j++){
}
return index;
}

Answer 1

//You can replace my str.size() and subString.size() by the size of each c-string.
int stringPointerOperation( string str, string subString )
{
int pos=0;
bool notFound;
for(int i = 0; i < str.size() ; i++)
{
notFound= false;
if(str[i] == subString[0])
{
pos=i;
for(int k = 0 ; k < subString.size() && k < str.size() ; k++,i++)
{
if(subString[k] != str[i] )
{
notFound=true;
break;
}
}
}
}
if(notFound)
return -1;
else
return pos;
}

Answer 2

You are using the wrong strategy for finding a sub-string in a string. The outer for loop needs to iterate over the main string and the inner for loop needs to iterate over the sub-string.
Say you are looking for "de" in "abcdef". The strategy that I find easier to understand and implement is:
Can I find "de" starting from 0 of "abcdef". No, I can't.
Can I find "de" starting from 1 of "abcdef". No, I can't.
Can I find "de" starting from 2 of "abcdef". No, I can't.
Can I find "de" starting from 3 of "abcdef". Yes, I can. Return 3.
Here's a version that works for me.
int findSubstring(char *str, char substring[]){
int i;
int j;
int length1 = strlen(str);
int length2 = strlen(substring);
if(length1 < length2){
std::cout << "Invalid, substring exceeds size of string!" << std::endl;
return -1;
}
for(i = 0; i < length1; i++){
for(j = 0; j < length2; j++){
// The index to use access the element of str
// needs to be offset by i.
if( str[i+j] != substring[j] )
{
break;
}
}
if ( j == length2 )
{
return i;
}
}
return -1;
}