C++ get string between two delimiters and replace it - c++

Question

I want to replace a substring in a string with something that depends on the substring between to delimiters. Little example:
I got the string
The result is __--__3__--__.
and a function
int square(int x): { return x*x };
Now I want to output just the string with the result without delimiters, so:
The result is 9.
I already tried several algorithms but none of them worked yet.
Best regard
My best attempt to far:
const std::string emptyString = "";
std::string ExtractString(std::string source, std::string start, std::string end)
{
std::size_t startIndex = source.find(start);
// If the starting delimiter is not found on the string
// stop the process, you're done!
//
if (startIndex == std::string::npos)
{
return emptyString;
}
// Adding the length of the delimiter to our starting index
// this will move us to the beginning of our sub-string.
//
startIndex += start.length();
// Looking for the end delimiter
//
std::string::size_type endIndex = source.find(end, startIndex);
// Returning the substring between the start index and
// the end index. If the endindex is invalid then the
// returned value is empty string.
return source.substr(startIndex, endIndex - startIndex);
}
int square(int x): { return x*x };
int main() {
std::string str = "The result is __--__3__--__.";
std::string foundNum = ExtractString(str, "__--__", "__--__");
int foundNumInt = atoi(foundNum.c_str());
int result = square(foundNumInt);
std::string toReplace = "__--__";
toReplace.append(foundNumInt);
toReplace.append("__--__");
str.replace(str.begin(), str.end(), toReplace, result);
}
The Question is: How to take the first string given ( The result is __--__<number>__--__.>, get the number from it, preform a function on that number, and then end with a string that looks like this The result is <number squared>.

Answer 1

Here is a way to take the first string, find the number. I then just squared the number, but you could plug that into your own function of you wanted to.
std::string s = "The result is __--__3__--__.";
std::regex r( "[0-9]+");
std::smatch m;
//
std::sregex_iterator iter(s.begin(), s.end(), r);
std::sregex_iterator end;
std::string value;
//
int index = 0;
while (iter != end)
{
for (unsigned i = 0; i < iter->size(); ++i)
{
value = (*iter)[i];
}
++iter;
index++;
}
int num = stoi(value);
int answer = num*num;
s = s.substr(0, s.find('_'));
s = s + " " + std::to_string(answer);
std::cout << s << std::endl;

Answer 2

Have you tried std::string::find?
const std::string example_data = "The result is __--__3__--__.";
static const char text_to_find[] = "__--__";
const std::string::size_type start_position = example_data.find(text_to_find);
if (start_position != std::string::npos)
{
const std::string::size_type replacement_start_position = start_position + sizeof(text_to_find) - 1;
if (replacement_start_position < example_data.length())
{
// Perform replacement
}
}
The "sizeof(text_to_find) - 1" returns the length of the text, without counting the terminating nul character.
To skip past the number, you could do something like:
const std::string after_number_position = example_data.find(replacement_start_position, "_");
The substring between replacement_start_position and after_number_position will contain your number. You can use a variety of functions to convert the substring to a number.
See also std::ostringstream for converting numbers to text.
Edit 1:
Corrected declaration of replacement_start_position.

Answer 3

You must need these functions(for c++17, much faster):
auto replace_all
(std::string str, std::string_view from, std::string_view to) noexcept -> decltype(str) {
unsigned start_pos{ 0 };
while ((start_pos = str.find(from, start_pos)) != std::string::npos) {
str.replace(start_pos, from.length(), to);
start_pos += to.length();
}
return str;
}
auto remove_all
(std::string str, std::string_view from) noexcept -> decltype(str) {
return replace_all(str, from, "");
}
and for later versions:
std::string replace_all
(std::string str, std::string from, std::string to) noexcept {
unsigned start_pos{ 0 };
while ((start_pos = str.find(from, start_pos)) != std::string::npos) {
str.replace(start_pos, from.length(), to);
start_pos += to.length();
}
return str;
}
std::string remove_all
(std::string str, std::string from) noexcept {
return replace_all(str, from, "");
}
I tested:
int main() {
std::string str = "__+__hello__+__";
std::cout << remove_all(str, "__+__");
std::cin.get();
return 0;
}
and my output was:
hello