Extracting a random word in a string

I would like to extract a random in a given line so I called a random number to use it as an index. But it keeps giving me an error.

terminated called after throwing an instance of 'std::out_of_range'.

I know it occurs because the index value is exceeded size of the string. But I can not find any mistake in my code.

do {
    int size = line.length();
    srand(time(NULL));
    index = rand() % size;
} while (isspace(line.at(index)));

This is a random number generator. I extract a line from a text file.

I used do { } while (isspace(line.at(index))); because I do not want it to be space, in order to generate a random number again if it is space.

What I want is this.

0123456789

Example... Index : 4

F---i----L F : First, L : Last

if (index == 0) {
    while (!isspace(line.at(last + 1))) {
        last = last + 1;
    }
}

else if (index == (size - 1))) {

    while (!isspace(line.at(first - 1))) {
        first = first - 1;
    }
}

else {
    while (!isspace(line.at(first - 1))) {
        first = first - 1;
    }

    while (!isspace(line.at(last + 1))) {
        last = last + 1;
    }
}

for (int i = first; i <= last; i++) {
    targetword += line.at(i);
}

This is how I calculate first and last index and print out the chosen word and it should not make any error...

please help me thank you