c++ string member function substr usage - c++

Question

Please tell me if I am understanding the the substr member function correctly?
result = result.substr(0, pos) + result.substr(pos + 1);
It takes the string from pos, 0 until (but not including), remove[i]
and then + result.substr(pos + 1); concatenates the rest of the string, except but not including the string / char in remove?
string removeLetters2(string text, string remove)
{
int pos;
string result = text;
for (int i = 0; i < remove.length(); i++)
{
while (true)
{
pos = result.find(remove[i]);
if (pos == string::npos)
{
break;
}
else
{
result = result.substr(0, pos) +
result.substr(pos + 1);
}
}
}
return result;
}

Answer 1

In short, you are asking if
result = result.substr(0, pos) +
result.substr(pos + 1);
removes the character at position pos, right?
Short Answer:
Yes.
Longer Answer:
The two-argument call takes the start index and the length (the one argument call goes to the end of string).
It helps to imagine the string like this:
F o o / B a r
0 1 2 3 4 5 6 <- indices
Now remove /:
F o o / B a r
0 1 2 3 4 5 6 <- indices
1 2 3 | <- 1st length
| 1 2 3 <- 2nd length
result = result.substr(0, 3) <- from index 0 with length 3
+ result.substr(4); <- from index 4 to end
As a programmer, always be aware of the difference between distance/index and length.
Better: If index is known:
Your code creates two new, temporary strings, which are then concatenated into a third temporary string, which is then copied to result.
It would be better to ask string to erase (wink wink) in place:
result.erase(pos,1);
// or by iterator
string::iterator it = ....;
result.erase(it,it+1);
This leaves more optimization freedom to the string implementer, who may choose to just move all characters after pos by one to the left. This could, in a specialized scenario, be implemented with a single assignment, a single loop, and within the loop with the x86 swap instruction.
Better: If characters to be deleted are known:
Or, but I am not sure if this gives better performance, but it may give better code, the algorithm remove_if:
#include <algorithm>
// this would remove all slashes, question marks and dots
....
std::string foobar = "ab/d?...";
std::remove_if (foobar.begin(), foobar.end(), [](char c) {
return c=='/' || c=='?' || '.';
});
remove_if accepts any function object.
If there is just one character, it gets easier:
// this would remove all slashes
std::remove (foobar.begin(), foobar.end(), '/');

Answer 2

Although the answer to your question is "yes", there is a better way to go about what you are trying to do. Use string::erase, like this:
result.erase(pos, 1);
This API is designed for removal of characters from the string; it achieves the same result much more efficiently.

Answer 3

Yes, this function removes all letters in remove from text.

Answer 4

since you seem to delete more than one type of character have a look at remove_if from <algorithm> with a special predicate too, although the response of dasblinkenlignt is the good one