How to make a 2d vector from input string? - c++

Question

I got this problem in one of the coding questions in my university exam where I knew the answer but couldn't follow as I didn't know how to parse the string and convert it into a 2d vector.
So I have got a 2d vector in the form of a string
"[[1,4], [5,7], [4,1], [8,9]]"
I want to convert this string into a vector<vector<int>>
EDIT
Maybe I wasn't clear the last time and there's one thing I missed. The string is
"[[1,4], [5,7], [4,1], [8,9]]"
I want this string to be in the form of a 2d vector. So, let's say I have a vector vec defined as vector<vector<int>> vec. Then vec[0]={1,4}, vec[1]={5,7}, vec[2]={4,1}, vec[3]={8,9}. Below is what I did and it shows incorrect output
#include<iostream>
#include<sstream>
#include<vector>
using namespace std;
int main()
{
string str="[[1,4], [5,7], [4,1], [8,9]]";
stringstream ss(str);
vector<vector<int>> vec;
string temp;
while(ss>>temp){
cout<<temp<<endl;
vector<int> t;
int x,y;
stringstream ss2(temp);
while(ss2>>x>>y)
{
t.push_back(x);
t.push_back(y);
}
vec.push_back(t);
}
}

Answer 1

The generic answer is to write a recursive descent parser (look it up). It's a fancy way of saying that you write a function for each of the non-terminals (vector2, vector, int) then you usually look at just the first byte to figure out to do. In this case your grammar might be:
vector2 = "[" vector (, " " vector) "]"
vector = "[" int (, number) "]"
number = 0 | 1 [0-9]
Then you implement number, vector and vector_vector similar to how #StPiere showed you previously.
I usually use c, and I found this generic function signature to be a useful start:
char *parse_something(const char *s, something *v)
where s is the string you are parsing and the return value is what the next thing is you want to parse.

Answer 2

Here is quick implementation Godbolt:
Note: This answer does not handle cases where integers have more than one digit. It is not generic or easily extensible, but as this is a homework question, you can understand and improve the code.
#include <string>
#include <vector>
#include <istream>
#include <sstream>
#include <cctype>
#include <iostream>
#include <iterator>
int main()
{
std::string input{"[[1,4], [5,7], [4,1], [8,9]]"};
std::stringstream ss{input};
auto it = std::istream_iterator<char>{ss};
std::vector<std::vector<int>> output;
std::vector<int> inner;
// Loop through the string, one character at a time
while (it != std::istream_iterator<char>{})
{
++it;
// If we encounter a digit, push it into the inner vector
if (std::isdigit(*it))
{
// Dirty hack to convert char to integer (assumes ASCII)
inner.push_back(*it - 48);
}
// If you encounter a ']' char, then push inner vector into the outer vector
// and then clear the inner vector
// For the final ']', the inner vector will be size 0, ignore this case
if (*it == ']')
{
if (inner.size() != 0)
{
output.push_back(inner);
}
inner.clear();
}
}
}

Answer 3

#include <iostream>
#include <vector>
using namespace std;
template <typename Range>
void print_range(const Range& r)
{
for (const auto& e : r)
cout << e << " ";
cout << endl;
}
int main()
{
vector<int> v;
vector<vector<int>> vv;
char c;
int cnt = 0;
while (cin >> c)
{
if (c == ',' || c == ' ' || c == '\n')
{
continue;
}
if (c == '[')
{
++cnt;
v.clear();
continue;
}
if (c == ']')
{
--cnt;
if (cnt == 0)
break;
vv.push_back(v);
v.clear();
continue;
}
cin.unget();
int x;
if (cin >> x)
v.push_back(x);
}
for (const auto& vec : vv)
print_range(vec);
}