Double for loop adding values

Question

I have 2 vectors that only have 4 values in each of them. I want to loop through them and process them together, as the values in both vectors go together in those positions in the vectors.

for (std::vector<boost::filesystem::path>::iterator i = volumeVec.begin(); i != volumeVec.end(); ++i) {         
        for (std::vector<boost::filesystem::path>::iterator j = sliceHeaderVec.begin(); j != sliceHeaderVec.end(); ++j) {
            ParseHeader(j->string(), i->string(), sourceDir);  
        }             
    }

However, when I run ParseHeader it process many more values then what I was expecting. I think I need to fix the arrangement of my for loops but don't know what to do

your loops go through all combinations of i and j while your text sounds as if you want i going from 0 to 3 and j==i — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Apr 6, 2017 at 15:12
btw to get the indices right, a much simpler example would be sufficient — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Apr 6, 2017 at 15:13

Jarod42 · Accepted Answer · 2017-04-06 15:30:08Z

2

You probably want something like:

assert(volumeVec.size() == sliceHeaderVec.size());
for (std::size_t i = 0; i != volumeVec.size(); ++i) {
    ParseHeader(volumeVec[i].string(), sliceHeaderVec[i].string(), sourceDir);
}

Or with range-v3:

for (const auto& p : ranges::view::zip(volumeVec, sliceHeaderVec)) {
    ParseHeader(std::get<0>(p).string(), std::get<1>(p).string(), sourceDir);
}

edited Apr 6, 2017 at 15:30

answered Apr 6, 2017 at 15:21

Jarod42

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Dusteh · Accepted Answer · 2017-04-06 15:24:01Z

2

If I understand you correctly you have a situation like this:

v1 = { v11, v12, v13, v14}

v2 = { v21, v22, v23, v24}

And you want a loop which process elements from both v1 and v2 in pairs {v11, v21}, {v12, v22}, {v13, v23}, {v14, v24}.

In such case you can simply use a indexed loop:

for (size_t i = 0; i < v1.size(); ++i)
{
    process(v1[i], v2[i]);
}

Before that you might want to check that v1.size() == v2.size() to avoid problems.

answered Apr 6, 2017 at 15:24

Dusteh

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Alexander Stante · Accepted Answer · 2017-04-06 16:00:18Z

0

If you have two collection which you want to run an operation (aka transformation) on, my suggestion would be to use std::transform from <algorithm>. In my opinion code written with that is well readable:

Suppose you have vector<int> v1 and vector<int> v2 and want to add them, you can do:

std::transform(v1.begin(), v1.end(), v2.begin(), v1.begin(), std::plus<int>());

The last argument could also be an other iterator or a back inserter if you want.

answered Apr 6, 2017 at 16:00

Alexander Stante

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