It's not only the main function that you should try to improve upon. Rather, when you build your supporting functions in a nice way, a simplified main function follows naturally.

First, as general comments, your program suffers from a bad separation of logic. Your functions are doing too much: they do processing, print output to the user, and return some value. The same problem is echoed in the main: input parsing is tightly knit together with printing and processing.

• Don't include<bits/stdc++.h>, it's not portable. Also, seehere for more on why not do it.

• Your program crashes on multiple inputs due to out-of-indexing issues, so avoid C-arrays likeint numbers[33]. Also, avoid magic numbers like 33 which makes the reader go "Huh, what 33?".

• As explained in the other review(s), declare variables as late as possible because you write C++ and not C.

• Standard functions and data structures will simplify your code massively. We can logically think about the following: read the input, split it into sensible parts, and process each part. That's it!

So in full, we could write your program also as follows:

#include <iostream>#include <string>#include <vector>#include <algorithm>#include <bitset>#include <iterator>constexpr unsigned long MAX_BITSIZE = 32;int bits(unsigned long d){    if (d > 65535) {        return 32;    }    else if (d > 255) {        return 16;    }    else {        return 8;    }}// Return a bit string corresponding to the input.std::string to_binary(unsigned long decimal){    std::bitset<MAX_BITSIZE> bs(decimal);    return bs.to_string().substr(MAX_BITSIZE - bits(decimal));}// Split the input string into parts, // e.g., "abc123e6g" is split into ("abc", "123", "e", "6", "g").std::vector<std::string> get_parts(const std::string& str){    std::vector<std::string> parts;    for (auto first = str.cbegin(); first != str.cend(); )    {        auto change = std::adjacent_find(first, str.cend(),            [](char a, char b) { return isdigit(a) != isdigit(b); });        if (change != str.cend())        {            ++change;        }        parts.emplace_back(std::string(first, change));        first = change;    }    return parts;}int main(){    // Reading input could be done a function.    // We also omit all checks, and just assume it is valid.    std::string input;    std::cout << "------------ Input -----------\n";    std::cin >> input;    std::cout << "Input: " << input << "\n";    std::cout << "Length: " << input.length() << "\n";    const auto cont = get_parts(input);    std::vector<std::string> binary;    for (const auto& e : cont)    {        // Processing an integer        if (isdigit(e.front()))        {            const std::string b = to_binary(stoi(e));            std::cout << "------------ " << e << " ------------\n";            std::cout << "Bits..................... " << bits(stoi(e)) << "\n";            std::cout << "Binary value............. " << to_binary(stoi(e)) << "\n";            binary.push_back(b);        }        // Processing individual characters        else        {            for (const auto& ch : e)            {                const std::string b = to_binary(ch);                std::cout << "------------ " << ch << " ------------\n";                std::cout << "ASCII.................... " << int(ch) << "\n";                std::cout << "Bits..................... " << bits(ch) << "\n";                std::cout << "Binary value............. " << to_binary(ch) << "\n";                binary.push_back(b);            }        }    }    std::cout << "\n------- Binary value of " << input << " -------\n";    std::copy(binary.cbegin(), binary.cend(), std::ostream_iterator<std::string>(std::cout, " "));}

A few comments about the above program:

• Luckily,std::bitset has methods for printing a binary string that we can use to massively simplify your "to-binary" conversion function. Notice that this function doesno printing; it's not its job. Remember:one function, one responsibility. Whoever uses this function decides when, how, and what to print (if anything).

• The functionget_parts takes care of all that messy processing of your main function (which is buggy - but don't feel bad, it's hard to get it right when you go so low level). The magic is taken care ofstd::adjacent_find which is used by a suitable lambda function that checks if the two adjacent indices contain characters of "different type" (i.e., a digit and a non-digit). If we wanted to, we could also modify this slightly to further split "abc" to "a", "b", and "c".

• The main function is quite pretty now (and very readable compared to your original): just get the parts and process each. Printing could be further divided into more functions.

The first 3 lines inmain can be turned into a function:

std::string getInput(){    std::string input;    std::cout << "------------ Input -----------" << std::endl;    std::cin >> input;    return input;}int main()    std::string input = getInput();

There are several portions ofmain that could become functions.

Declare Variable when needed

In the functionconvertToBinary the integer variablesi andr are declared at the beginning of the function. They are only needed within thefor loop.

    for (int i = 0; i < l; i++) {        int r = decimal % 2;        decimal /= 2;        str += std::to_string(r);    }

This is also true in themain function fori and other variables.

The contents of thethen clause and theelse could both be moved into functions.

Does the innerif statement(if (i == length - 1)) need to be within thefor loop or could it be executed after thefor loop is done? You might want to think aboutloop invariants.

Reduce Complexity When You Can

In the functionbits, checking from the top down might make reading easier because the top limit isn't needed:

    if (d > 65535) {        l = 32;    } else if (d > 255) {        l = 16;    } else {        l = 8;    }

The use of the termASCII is misleading. What's used here is actually theexecution character coding, which may or may not be ASCII. C++ is intentionally portable enough to work on systems regardless of the encoding they use for characters.

• c++
• c++11
• number-systems