replace and .size()

[EwCjE3v7]

So I`m having trouble understanding how the string::replace() function works. What I dont get is that in the functions nothing, in the while loop, when I call replace with firstLet and firstLet +oldVal.size(). I do not understand how that replaces "tho". Shouldnt in replace "tho " with a space?

void nothing(string s, string oldVal, string newVal) 
{
	string::size_type pos = 0;

	while (s.find(oldVal, pos) != string::npos) {
		string::size_type firstLet = s.find(oldVal, pos);

		s.replace(firstLet, firstLet + oldVal.size(), newVal);
		pos += oldVal.size();

	}
	cout << s << endl;
} 

int main()
{
	nothing("tho i didnt go thru", "tho", "though");
	return 0;
} 

"tho I hate"
 ^ ^^
 | ||
 | ||
 | |Shouldnt it point here after adding the size of oldVal to firstLet? Shouldnt it replace till here?
 | |
 | Why does it replace till here?
First letter

[naraku9333]

The second parameter to replace is the length of the replaced string.
So the second parameter your passing is wrong and should be s.replace(firstLet, oldVal.size(), newVal);

[keskiverto]

http://www.cplusplus.com/reference/string/string/find/ writes:

Note: The first character is denoted by a value of 0 (not 1)

Other "issues":

You find twice. They should give the same result, yes?

string::size_type pos = 0;
while ( string::npos != ( pos = s.find(oldVal, pos) ) )

How many characters do you want to replace? Say, you find a 3-char word on position 1000. Will you really remove characters 1000--2002?

The 'pos' points now to the first character of newVal substring that was just inserted into the 's'. From where will continue to search?
Try with this input: oldVal="tho", newVal="tho tho"