How to create auto iteration in for loop which would assign pointer instead of value to it

Question

I want to create auto iteration loop like this:

std::vector<sometype> vector1;
for(auto it : vector1)
{
   if(&it == &vector1.at(4))
      //do something...
   else
      continue;
}

but I found out that adress of "it" when it's equal to vector1.at(4) is not the same as adress of vector1.at(4). How can I do for loop which would assign to "it" not value but adress of value currently pointing to.

The answer below explains *how* to do this, but may I ask *why* you want to do this? e.g. in this particular case, you could just `// ... do something` with `vector.at(4)` directly. — cigien, Oct 09 '20 at 16:08

score 2 · Answer 1 · answered Oct 09 '20 at 16:04

2

You can use reference (add & to variable declarations) to do something like that.

std::vector<sometype> vector1;
for(auto& it : vector1) // add &
{
   if(&it == &vector1.at(4))
      //do something...
   else
      continue;
}

answered Oct 09 '20 at 16:04

MikeCAT

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score 0 · Answer 2 · answered Oct 09 '20 at 16:16

Looking at your code as pseudo-code for an algorithm, it basically attempts to solve

Check (via O(n) instead of O(1) cost) if the vector has at least 5 elements.
- If true, go to 2.
- Otherwise do nothing.
Do something with the 5th element.

If this is indeed the case, you don't actually need a for loop for this, you could just query the size of the vector prior to conditionally accessing (O(1) random access) the 5th element to //do something...:

#include <iostream>
#include <string>
#include <vector>

int main() {
    const std::vector<std::string> v{"a", "b", "c", "d", "e", "f"};
    if (v.size() > 4) { 
        auto s = v[4];
        // do something with the 5th element, as it exists.
        std::cout << e; // d
    }
}

Or, you could make use of the bounds-checking element accessor at() clong with exception handling in case of attempted out-of-bounds access.

score 0 · Answer 3 · answered Oct 09 '20 at 16:52

In the range-based for loop

std::vector<sometype> vector1;
for(auto it : vector1)
{
   if(&it == &vector1.at(4))
      //do something...
   else
      continue;
}

it is not an iterator. It is an object of the type sometype that stores the value of the corresponding element in the vector.

Instead of creating an object you should use a reference as for example

std::vector<sometype> vector1;
for( auto &value : vector1)
{
   if( &value == &vector1.at(4))
      //do something...
   else
      continue;
}

Pay attention that the else statement with continue does not make a great sense. You could write simply

std::vector<sometype> vector1;
for( auto &value : vector1)
{
   if( &value == &vector1.at(4))
      //do something...
}

Though it is unclear why you are using the range-based for loop when only one element of the vector will satisfy the condition.

How to create auto iteration in for loop which would assign pointer instead of value to it

3 Answers3