Creating an array out of a string

Question

I'm new to C++ and I've encountered a problem... I can't seem to create an array of characters from a string using a for loop. For example, in JavaScript you would write something like this:

var arr = [];
function setString(s) {
    for(var i = s.length - 1; i >= 0; i--) {
       arr.push(s[i]);
    }
    return arr.join("");
}

setString("Hello World!"); //Returns !dlroW olleH

I know it's a bit complicated, I do have a little bit of background knowledge on how to do it but the syntax of it is still not too familiar to me.

Is there any way that I could do that in c++ using arrays? Could I join the array elements into one string as I do in JavaScript?

It would be greately appreciated if you could help. Thanks in advance. If anyone needs more information just tell me and I'll edit the post.

By the way, my code in c++ is really messy at the moment but I have an idea of what I'm doing... What I've tried is this:

function setString(s) {
    string arr[s.size() - 1];
    for(int i = s.size() - 1; i >= 0; i--) {
        arr[i] = s.at(i); //This is where I get stuck at... 
    //I don't know if I'm doing something wrong or not.
    }
}

It would be nice if someone told me what I'm doing wrong or what I need to put or take out of the code. It's a console application compiled in Code::Blocks

Are you doing this as a learning exercise? Because if not, it may be a case of a XY Problem - it is unlikely that it is useful to manually create an array of characters out of a string in this manner. What are you actually trying to do? — JBentley
– JBentley, Commented Oct 25, 2013 at 17:58
string arr[s.size() - 1]; ????!!!! Array of strings to store single characters, really? — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Oct 25, 2013 at 18:07

Zac Howland · Accepted Answer · 2013-10-25 18:38:36Z

3

std::string has the c_str() method that returns a C style string, which is just an array of characters.

Example:

std::string myString = "Hello, World!";
const char *characters = myString.c_str();

edited Oct 25, 2013 at 18:38

Zac Howland

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answered Oct 25, 2013 at 17:54

Carl Norum

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2 Comments

hyde Over a year ago

I think you misread the question (it is rather unclear). Desired output is the string reversed, c_str() is not really related to the problem in any way.

Zac Howland Over a year ago

c_str() returns a const char*, not a char*.

Mark Ransom · Accepted Answer · 2013-10-25 18:01:39Z

1

The closest thing to a direct translation of your function:

string setString(string s) {
    string arr;
    for(int i = s.length() - 1; i >= 0; i--) {
        arr.push_back(s[i]);
    }
    return arr;
}

answered Oct 25, 2013 at 18:01

Mark Ransom

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Comments

Zac Howland · Accepted Answer · 2013-10-25 18:42:27Z

A std::string is a dynamic array underneath a fairly thin wrapper. There is no need to copy character by character, as it will do it properly for you:

If the character array is null-terminated (that is, the last element is a '\0'):

const char* c = "Hello, world!"; // '\0' is implicit for string literals
std::string s = c; // this will copy the entire string - no need for your loop

If the character array is not null-terminated:

char c[4] = {'a', 'b', 'c', 'd'}; // creates a character array that will not work with cstdlib string functions (e.g. strlen)
std::string s(c, 4); // copies 4 characters from c into s - again, no need for your loop

If you cannot use std::string (e.g. if you are forced to use ANSI C):

const char* c = "Hello, World!";
// assume c2 is already properly allocated to strlen(c) + 1 and initialized to all zeros 
strcpy(c2, c);

In your javascript example, you are reversing the string, which can be done easily enough:

std::string s = "Hello, world!";
std::string s1(s.rbegin(), s.rend());

Additionally, you can cut your iterations in half (for both C++ and Javascript) if you fix your loop (pseudo-code below):

string s = "Hello, world!"
for i = 0 to s.Length / 2
    char t = s[i]
    s[i] = s[s.Length - 1 - t]
    s[s.Length - 1 - i] = t

Which will swap the ends of the string to reverse it. Instead of looping through N items, you loop through a maximum of N / 2 items.

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