C++ String dissapears outside if statement [closed] - c++

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I'm quite a newbie regarding C++ and I'm struggling with one (maybe simple) thing. I want to write a code that checks the first word of the string line and copy the second word of the string line to the string x_start, x_end or num_steps if the first word of the string is equal to "x_start", "x_end" or "num_steps".
The problem I've to solve is the following. In the if statement the value of the second word (label) is copied to the string x_start, but when I go further to below in my debugger, the content of the string x_start disappears after the if statement end bracket. The first sentence of the output (cout) is now equal to: x_start =, where it should be: x_start = 0.
Does anyone know why this happens and how it can be solved? Thanks in advance!
#include <iostream>
#include <fstream>
#include <cassert>
#include <sstream>
#include <string>
int main(int argc, char** argv) {
std::string x_end;
std::string num_steps;
std::string x_start;
std::string line = "x_start 0";
std::istringstream linestream(line);
while (!linestream.eof()) {
std::string label;
std::string value;
linestream >> label >> value;
if(label.compare("x_start") == 0){
std::string x_start = value;
}
else if(label.compare("x_end") == 0){
std::string x_end = value;
}
else if(label.compare("num_steps") == 0){
std::string num_steps = value;
}
else{
std::cout<<"The format of the text in the file 'params.in' ";
std::cout<<"is not correct"<<std::endl";
break;
}
std::cout<<"x_start = "<<x_start<<std::endl;
std::cout<<"x_end = "<<x_end<<std::endl;
std::cout<<"num_steps = "<<num_steps<<std::endl;
}
return 0;
}

Answer 1

Instead of this
std::string x_start = value;
Write this
x_start = value;
Your code introduces new variable x_start that "hides" the existing variable with the same name. This new variable is set, and immediately afterwards destroyed. So, just don't introduce new variable, and all will be fine.
Repeat this also for x_end and num_steps.

Answer 2

When you do this
if(label.compare("x_start") == 0){
std::string x_start = value;
} //^^^^^^^^^^^
you are telling the compiler the following: "I need a new string variable called x_start. This variable will replace x_start from the outside until the end of the if statement".
This is not what you want, though: you want to assign the existing variable! Removing the declaration will fix this problem:
if(label.compare("x_start") == 0){
x_start = value;
}
Now you are assigning an existing variable - the one that would remain there after the if statement. Of course you need to do the same with other declarations that shadow variables outside of their statements.
Note: the technical term for which variable is visible at what part of your program is scope. Look it up to see what it means and how to work with it.

Answer 3

if(label.compare("x_start") == 0){
std::string x_start = value;
}
Should be
if(label.compare("x_start") == 0){
x_start = value;
}