通过char写入字符串数组char

Question

i'm working on a project and i came to a deadlock. i'm having trouble figuring out how to write a string char by char in an array?

i mean:

string my_str[10];

i want to access my_str[0]'s first char.

i tried this:

    string tokens[10];
    (&tokens[0])[0] = 'a';
    (&tokens[0])[1] = 's';
    (&tokens[0])[2] = 'd';

it works like tokens[0]='a', tokens[1]='s' etc.

then i tried:

string tokens[10];
(&tokens[0])[0][0] = 'a';
(&tokens[0])[0][1] = 's';
(&tokens[0])[0][2] = 'd';

when i compile it errors (string substring out of range) but when i click abort it works. what is wrong with this code?

Answer 1

If you want to add a character at a time to a string, you can use either push_back or the += operator. Eg:

tokens[0] += 'a';
tokens[0] += 's';
tokens[0].push_back('d');

Answer 2

The string you created using

string tokens[10];

are empty. That is, you have 10 empty string s. If you want to access a character in there you'll need to resize them, eg:

string tokens[10];
for (string& str: tokens) {
     str.resize(3);
}
tokens[0][0] = 'a';
tokens[0][1] = 's';
tokens[0][2] = 'd';

When you use, eg,

(&tokens[0])[2] = 'd';

you first get a pointer to the first string in the array, dereference that index 2 , and assign a string which is converted from the character 'd' , ie, string('d') to it. That is, this statement is equivalent to

token[2] = 'd';

Answer 3

You'll want to use string::at for this. Like so:

char x;
string s[10];
s[0] = "asdf";
x = s[0].at(0);
printf("%c", x);

prints a.

Answer 4

You can use the public member function in the String class name : c_str(). It returns in fact Returns a pointer to an array that contains a null-terminated sequence of characters (ie, a C-string) representing the current value of the string object: to be specific, it's a char array that contains the exact equivalent of the string object value. Let's say we have a :

string array_of_strings[10];

To access the first char of array_of_strings[0] and print it is simply to write:

std::cout << array_of_strings[0].c_str()[0] << endl;