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#include <iostream>
#include <sstream>
#include <vector>
#define ii pair<int, int>
using namespace std;

// DSK -> DSC
int n;
vector <ii> edge;
string s, num;

int main()
{
    cin >> n;
    cin.ignore();
    for(int i = 1; i <= n; i++)
    {
        getline(cin, s);
        stringstream ss(s);
        while(ss >> num)
            if(i < stoi(num))
                edge.push_back({i, stoi(num)});
    }
    return 0;
}

error: expected expression
                edge.push_back({i, stoi(num)});
                               ^

How can I fix the 'expected expression' error in my graph code when using VSCode to push a std::pair to a vector?

Mini Cake
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    Notes : Please do not use these `#define ii pair using namespace std;`. Make your variables as local as possible (e.g. move them into main). There is no need for your getline/stringstream you can do `cin >> num` directly. – Pepijn Kramer May 30 '23 at 04:18
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    There's no need for `cin.ignore()` either. And either read integers as integers from the input, or be ready to handle exceptions thrown for non-numeric input when calling `std::stoi`. – Some programmer dude May 30 '23 at 04:20
  • Use `edge.emplace_back(i,value);` like this : https://onlinegdb.com/Qs0x8Qq0b – Pepijn Kramer May 30 '23 at 04:24
  • looks like no problem for C++11(or more later?). May be, compiler version is old? – fana May 30 '23 at 04:27
  • You need to compile your code with C++11 or later enabled. You didn't do so. – user17732522 May 30 '23 at 04:28

2 Answers2

5

My psychic powers suggest you are on an older version of C++. Maybe C++11? Not sure which compiler you are using, but there are command line flags that will set the compiler environment to a newer standard than the default.

clang/g++: -std=c++17

MSVC: /std:c++17

Regardless, there's simpler ways to solve this than fighting the compiler. Just make it happy by giving it the type it expects instead of it trying to infer how to coerce your inline params.

For what's really just a couple of ints, this won't incur any performance penalty. And the compiler will likley optimize out the extra copy (if any) in a retail build anyway:

Instead of this:

edge.push_back({i, stoi(num)});

This:

std::pair<int,int> p = {i, stoi(num)};
edge.push_back(p);

Pro-tip unrelated to your quesiton. Always use curly braces for nested statements, even if it's just a single line. It will prevent bugs that easily occur later for not having a block explicitly declared. Instead of this:

        while(ss >> num)
            if(i < stoi(num))
                edge.push_back({i, stoi(num)});

This:

        while(ss >> num)
        {
            if(i < stoi(num))
            {
                edge.push_back({i, stoi(num)});
            }
        }

Putting it altogether:

    while(ss >> num)
    {
        if(i < stoi(num))
        {
            std::pair<int,int> p = {i, stoi(num)};
            edge.push_back(p);
        }
    }

The other answers and comments sugggesting to use make_pair or emplace_back are good suggestions too.

selbie
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1

I think you can use make_pair() function.

edge.push_back(make_pair(i, stoi(num)));
Sadiq Nayeem
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