String.at() not comparing to character ' ' - c++

Question

I am writing a C++ function that takes in a string of the form "let_varname_=_20" and then it will isolate "varname" to make a new string with my name variable name. I am trying to tell the .at() function to stop iterating through a while loop when it hits a space (the underscores are spaces, I just wanted it to be abundantly clear there was a space). However, it isn't comparing them properly, as it goes to the end of the string completely without stopping (passing by 2 spaces).
void store(std::string exp) {
int finalcount = 4;
char placeholder = ' ';
while (exp.at(finalcount)!=placeholder) {
finalcount++;
}
for (int i = 0; i < exp.size(); i++) {
std::cout << exp.at(i) << std::endl;
}
std::string varname = exp.substr(4, finalcount+1);
std::cout << finalcount + 1 << std::endl;
std::cout << varname << std::endl;
}
I started at index 4 because I know that indexes 0-3 of teh string will be 'l' 'e' 't' and ' '. The print statements were just me checking to see what it was reading versus what I input (and it was reading everything fine, just not comparing properly). I also tried have my while loop condition say while the char was >65 && <90 to work with ASCII codes but that also didn't work.
Thanks in advance for the help.

Answer 1

You could use istringstream and treat the string as a stream:
const std::string test_data = "let varname = 20";
std::istringstream test_stream(test_data);
std::string let_text;
std::string var_name;
char equals_sign;
unsigned int value;
test_stream >> let_text >> var_name >> equals_sign >> value;
This may be a lot easier than your code.
Edit 1: Searching the string
You could also use the std::string methods, find_first_of and find_first_not_of.
std::string::size_type position = test_data.find_first_of(' ');
position = test_data.find_first_not_of(' ', position);
std::string::size_type end_position = test_data.find_first_of(' ');
let_text = test_data.substr(position, end_position - position);

Answer 2

The problem is that you aren't using substr() properly, as I mentioned in a comment. Also, as Pete Becker mentioned in a comment, you should also be checking for = and stopping when you reach the end of the string, so that you don't overrun your string if there aren't any more spaces in it. Additionally, you don't want to add 1 to finalcount when determining substring length, because then your substring will include the space or = that made the check fail.
Try this:
void store(std::string exp) {
const int start = 4; // <-- Enter starting position here.
const char placeholder_space = ' '; // Check for space.
const char placeholder_equal = '='; // Check for equals sign, as pointed out by Pete Becker.
int finalcount = start; // <-- Use starting position.
bool found = false;
while (finalcount < exp.size() && !found) {
if (!((exp.at(finalcount) == placeholder_space) ||
(exp.at(finalcount) == placeholder_equal))) {
finalcount++;
} else {
found = true;
}
}
if (!found) {
std::cout << "Input is invalid.\n"; // No ' ' or '=' found, put error message here.
return;
}
for (int i = 0; i < exp.size(); i++) {
std::cout << exp.at(i) << std::endl;
}
std::string varname = exp.substr(4, finalcount - start); // <-- Use starting position.
std::cout << finalcount - start << std::endl; // Length of varname.
std::cout << varname << std::endl;
}