how to traversing through vector using iterator and accessing element value?

Question

I started to use iterators for traversing through a vector. Usually, I would use this to go through a vector:

for( int i=0; i< vector.size(); i++){
    cout<<vector[i];
}

I learned if I want to use iterators I would have to do something like this:

vector<int>::iterator col;
for(col = vector.begin(); col != vector.end(); col++){
    cout << *col;
}

But, if I want to access the value at which the iterator is pointing, how would I do that?

I tried to do this:

 int temp = *col;

but this gives an error

Assigning to 'int' from incompatible type 'std::__1::vector<int, std::__1::allocator<int> >'

I am trying this because I have a 2d vector and I'm trying to find the sum of individual columns.

The shown code in this question does not meet stackoverflow.com's requirements for a minimal reproducible example, and because of that it is unlikely that anyone here can conclusively determine the problem, but only guess, at the most. This question must be edited to show a minimal example, no more than one or two pages of code (the "minimal" part), that everyone else can cut/paste, compile, run, and reproduce the described problem (the "reproducible" part) exactly as shown (this includes any ancillary information, like the input to the program). See How to Ask for more information. — Sam Varshavchik
– Sam Varshavchik, Commented Apr 13, 2020 at 23:38

Remy Lebeau · Accepted Answer · 2020-04-14 00:11:19Z

0

The code you showed is correct ... for a 1D vector. But, you say you are actually using a 2D vector, in which case the outer vector's elements are themselves vectors, and so dereferencing an iterator for the outer vector gives you a reference to an inner vector. That is exactly what the error message is complaining about - you are trying to assign an inner vector to an int, which will obviously not work. You would have to iterate that inner vector separately, same as you would have to do when using indexes instead of iterators, eg:

std::vector<std::vector<int> > rows_vec;
...
/*
for(size_t row_idx = 0; row_idx < rows_vec.size(); ++row_idx){
    std::vector<int> &row_vec = rows_vec[row_idx];
    int sum_cols = 0;
    for(size_t col_iter = 0; col_idx < row_vec.size(); ++col_idx){
        sum_cols += row_vec[col_idx];
    }
    // use sum_cols as needed...
}
*/
std::vector<std::vector<int> >::iterator row_iter;
for(row_iter = rows_vec.begin(); row_iter != rows_vec.end(); ++row_iter){
    std::vector<int> &row_vec = *row_iter; 
    std::vector<int>::iterator col_iter;
    int sum_cols = 0;
    for(col_iter = row_vec.begin(); col_iter != row_vec.end(); ++col_iter){
        sum_cols += *col_iter;
    }
    // use sum_cols as needed...
}

If you are using C++11 or later, this can be greatly simplified using range-based for loops:

std::vector<std::vector<int>> rows_vec;
...
for(auto &row_vec : rows_vec){
    int sum_cols = 0;
    for(int col_val : row_vec){
        sum_cols += col_val;
    }
    // use sum_cols as needed...
}

Which could be simplified more by using the standard std::accumulate() algorithm, eg:

std::vector<std::vector<int>> rows_vec;
...
for(auto &row_vec : rows_vec){
    int sum_cols = std::accumulate(row_vec.begin(), row_vec.end(), 0);
    // use sum_cols as needed...
}

edited Apr 14, 2020 at 0:11

answered Apr 13, 2020 at 23:52

Remy Lebeau

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4 Comments

Vibhor Sagar Over a year ago

Thank you, this helped a lot, but if I want to find the sum of the rows and columns individually in a 2d vector, how would I do that?

Remy Lebeau Over a year ago

Obviously, you would have to declare an integer variable, and then have the inner loop increment that variable. And then repeat that for each iteration of the outer loop. See the edit I just made to my answer.

Vibhor Sagar Over a year ago

sorry, I am a bit confused. If you can, can you explain through some code so I can understand better? Thank you.

Vibhor Sagar Over a year ago

Thank you for your help, I am able to get the sum of columns but how can I get the sum of Rows? I tried to run a range based loop just for the row iterator but it just gave me the sum of cols as it seems

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