Elements of a String

[universal exports]

I'm trying to declare specific elements of a string and failing horribly.

#include <iostream>
#include <string>

using namespace std;

int main ()
{
    string s1;
    string s2;
   
    s1 = "some words";
    s2[0] = s1[2];
    s2[1] = s1[3];

    cout << s2;
}

That's basically what I want to do and it compiles just fine but if you try to run it it freaks out. "Debug Assertion Failed!". So why doesn't this work and how can I work around it? Thanks.

[GodPyro]

The problem lies in the fact that you haven't allocated any memory to s2. Try adding:

s2 = "ww";

[kfmfe04]

GodPyro is right.

You need to be aware of a subtlety in std::string - read here:

http://www.cplusplus.com/reference/string/string/operator[]/

This means, if you do this:

#include <iostream>
#include <string>

using namespace std;

int main ()
{
    string s1;
    string s2;
   
    s1 = "some words";
    s2.at(0) = s1[2];
    s2.at(1) = s1[3];

    cout << s2;
}

you will crap out during runtime, as expected.

terminate called after throwing an instance of 'std::out_of_range'
  what():  basic_string::at
Abort trap

So you can use [] when you can ensure that you are indexed within s2.size()-1.
Otherwise, use at().

[universal exports]

Yeah I thought that might be it but still no luck...

[GodPyro]

Still no luck with what? That is definitely the problem.

[universal exports]

Sorry, you're right. I got it working using [] as long as the string was initialized at the value in the brackets, but I'm having trouble understanding how to use .at() for values that haven't been initialized

string s1;
string s2;

s2 = "ww";
s1= "some words";

size_t i = 2;

s2[0] = s1[2];
s2[1] = s1[3];

s2.at(i) = s1[4];

cout << s2;

That doesn't work and I can't tell what I should be doing differently from the reference page for string::at.

[GodPyro]

at(i) will insert data at the position, i, in the string but you still have to allocate memory to it. The at() command won't do that for you automatically.

[kfmfe04]

The reason your code doesn't work is because Line 4 only initializes s2[0] and s2[1].
s2[2] is '\0' and "out-of-bounds" for .at().

I'm having trouble understanding how to use .at() for values that haven't been initialized

You shouldn't use .at() or [] for values that haven't been initialized!!!

Initialize the string first. At least do something like this:

string s2(10,' '); // fill s2 with 10 blanks - initialize out as many as you plan to read

[universal exports]

haha ok I understand now. Thanks for your help guys.

[GodPyro]

You're welcome :)