what is the best way to return a string in C++ where size of string already known?

Question

When I am using str[] I get compilation error of mismatch type.When I use str+push_back instead of str[] everything works fine.

   class Solution {
public:
   string bits(int n){
       string str[32];//size already known thats why used str[32]
        for(long long int i=31;i>=0;i--)
        {
            if(n & 1<<i){
                str[i]=('1');
            }else{
                str[i]=('0');
            }
            
        }
       return str;
    };
    int findMaximumXOR(vector<int>& nums) {
        queue<int>q;
        vector<string>s(nums.size());
        for(int i=0;i<nums.size();i++ ){
           s[i]=bits(nums[i]);
            cout<<s[i]<<"\n";
        }
        return 0;
    }
};

string str[32]; is an array of 32 strings named str. It's not a string with 32 characters. std::string handles memory management for you. Just declare a std::string str; and call it a day. Alternative constructors here if you want to create it with a certain size. — JohnFilleau
– JohnFilleau, Commented Sep 30, 2020 at 1:06
In this case str[i] refers to string #i in that array of strings. You end up trying to assign a char to a std::string. — tadman
– tadman, Commented Sep 30, 2020 at 1:17

Remy Lebeau · Accepted Answer · 2020-09-30 01:25:32Z

4

string str[32]; is declaring an array of 32 std::string objects. But the bits() method is declared as returning a single std::string, which is why return str; does not work.

You don't want an array of 32 strings. You want a single string of 32 characters in length, eg:

std::string bits(int n) {
    std::string str(32, '0');
    for(long long int i = 31; i >= 0; i--) {
        if (n & (1 << i)) {
            str[i] = '1';
        }
    }
    return str;
}

That being said, As @Slava mentioned in comments, if you want a better way, use std::bitset instead:

std::string bits(int n) {
    return std::bitset<32>(n).to_string();
}

edited Sep 30, 2020 at 1:25

answered Sep 30, 2020 at 1:11

Remy Lebeau

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8 Comments

tadman Over a year ago

A string filled with NUL?

JohnFilleau Over a year ago

@tadman if you want to allocate up front in one line you have to fill it with something. OP has a comment about setting the array to 32 because they know how many characters they need. Looks like a leethaxxorchef.ru website thing so performance matters.

tadman Over a year ago

If performance matters, fill it with '0' so you don't need to do anything for those. On average 50% less work!

tadman Over a year ago

Seems like wasted effort. You know it's going to be '1' or '0', there's no ambiguity there, so pick one and pre-fill it. Then just skip those during the iteration.

Slava Over a year ago

@tadman actually if str is filled with '0' it allows: for(int i = 31; n; i--, n >>= 1 ) str[i] = (n & 1) + '0'; :)

|

tadman · Accepted Answer · 2020-09-30 01:12:20Z

3

If you want a string representing a 32-bit binary expression you need a single string. In C++ [] in a declaration generally refers to an array of something, as in int x[N] is an array of N int values.

Reworked:

  string bits(int n) {
    std::string str(32, '0'); // Reserve with 32 x 0

    for(long long int i=31;i>=0;i--) {
      if(n & 1<<i) {
          str[i]=('1');
      }
    }

    return str;
  };

There's a lot of quirks in this code, too, which I'll remove here:

  // Avoid using namespace std; and embrace the std:: prefix
  std::string bits(int n) {
    string str(32, '0');

    // You do not need a long long int to iterate to 31
    // Also try and express your loops as going from 0..N-1 unless
    // doing so overly complicates the code.
    for (int i=0; i<32; ++i) {
      // Although not necessary, the brackets around (1 << i)
      // make it abundantly clear this is not to be read as
      // something like (n & 1) << i
      if (n & (1 << i)) {
        // The braces around the '1' serve no purpose
        str[i]='1';
      }
    }

    return str;
  };

edited Sep 30, 2020 at 1:12

answered Sep 30, 2020 at 1:09

tadman

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4 Comments

Slava Over a year ago

Or simply one line: return std::bitset<32>( n ).to_string();

tadman Over a year ago

@Slava Yeah, if doing so is permitted. This looks like an assignment.

Slava Over a year ago

OP is asking for the best way, nothing about restrictions

tadman Over a year ago

@Slava The Solution name of the class suggests to me it's an assignment, but your point is valid.

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