Passing values to an array within a loop

Question

I'm trying to make a program in C++ in which the number of mathematical signs are counted. I am using isdigit to figure this out, but when I pass the value of my string, it gives me a warning.

This is the code that I have. The line digito[i] = entrada[i] is where I think the problem lies, but I do not understand why.

cout << "Input the operation" << endl;
cin >> input;
string digit[] = { "" };
string sign[]={""};
int cn = 0, cs = 0;
for (int i = 0; i < input.size(); i++) {
    if (isdigit(input[i])) {
        cout << "There is a digit in position " << i << endl;
        cn += 1;
        digit[i] = input[i];
    }
    else {
        cout << "There is a sign in position " << i << endl;
        // sign[i] = input[i];
        cs += 1;
        sign[i] = input[i];
    }
}

It takes me to this code as the problem:

static _CONSTEXPR17 void assign(char& _Left, const char& _Right) noexcept
{ // assign an element
    _Left = _Right;
}

Hector, lo he convertido a inglés, pero es posible que desees comprobarlo, ya que usé Google Translate, que es notoriamente poco confiable. Alternativamente, hay un sitio SO en español si prefiere su idioma nativo: es.stackoverflow.com — paxdiablo
– paxdiablo, Commented Feb 5, 2019 at 1:32
I translated it to English! @paxdiablo (Spanish, my native language). — M.K
– M.K, Commented Feb 5, 2019 at 9:38

Bartek Banachewicz · Accepted Answer · 2019-02-05 09:34:02Z

Those two strings are problematic. You've unnecessarily declared them as arrays with one element each, and initialized each string to empty.

string digito[] = { "" };
string signo[]={""};

Yet afterwards, you're indexing them with non-zero indices:

digito[i] = entrada[i];

This line is problematic because of two reasons; going beyond the array bounds, and incompatible types.

digito[i] is the type of std::string (because digito is std::string[]), while entrada[i] is char (assuming entrada is std::string). std::string has an overload of its operator= that allows assigning to a single character, but that's not what you want here, I assume.

As for the second problem, std::string requires you to enlarge it before you random-access it at a given index. The best way to do this in this case would be during construction, dropping the erroneous array use:

std::cin >> entrada;
std::string digito(entrada.size(), ' ');
std::string signo(entrada.size(), ' ');

That being said, I'm not sure if this code does what you want it to. Given an input string of:

2+2/3

You'll get two such strings:

digito = "2 2 3"
signo  = " + / "

If your actual goal was to tokenize the input (divide into numbers and operators), a much better way would be to use either two std::vector<char>s, or, even better:

using Number = int;
enum class Operator { Plus, Minus, Div, Mul };
using Token = std::variant<Number, Operator>
using Sequence = std::vector<Token>;

A consistent, strongly-typed data model will make it much easier to write correct code that produces it and operates on the results afterwards.

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