why vector does not updates in loop?

Question

I want to update vector 'v' so that I can iterate from count 0-100.

I know this is not allowed, but what if I want to do this only? Is there any way?

int main() {
    // your code goes here
    vector<int> v;
    v.push_back(1);
    int count = 0;

    for(int elem: v){
        if(count<100)
        v.push_back(count);
        count++;
    }

    for(int elem: v)
    cout << elem << endl;

    return 0;
}

The output is:

1    
0

Answer 1

As you can see from the definition of the range-based for loop, the end_expr does not update between iterations. Therefore you only have one iteration. push_back invalidates v.end() (which is what end_expr is as described in the linked page), so what you have is actually undefined behaviour.

The arguably simplest way to fill vector with 0..100 would be:

vector<int> v(101);
std::iota(v.begin(), v.end(), 0);

Answer 2

You should use this code instead

int count = 0;
while (v.size() < 100) {
   v.push_back(count++)
}

Modifying vector while iterate through it is not allowed

Answer 3

Best efective way for this operation

vector<int> v;
v.resize(100);
for(unsigned int i = 0; i < v.size(); i++)
{
    v[i] = i;
}

same as above.

Answer 4

using your code :

for(int elem: v){
    if(count<100)
        v.push_back(count);
    count++;
}

is like using this :

int i = v.size();
for(int j = 0; j < i; j++){
     v.push_back(j);
}

I don't really know why... v.size() might be keep in memory for optimization and data protection

Edit after OP comment :

Try this

int i = v.size();
for(int j = 0; j < i; j++){
    if(j<100)
        i = v.size();
    v.push_back(count);
}

Answer 5

A range-based for loop produces code similar to this:

{
auto && __range = range_expression ; 
    for (auto __begin = begin_expr,__end = end_expr; __begin != __end; ++__begin) { 
        range_declaration = *__begin; 
        loop_statement 
  } 
} 

As you can see the range will not be updated as you're iterating over your container.

Additionally you're most likely ending up with undefined behaviour because as you're pushing back values to your vector these iterators will be invalidated in case of a resize.

See @user2079303 for a better way to fill your vector.