Range for loop doesn't work when operating with an indexed element

Question

I was not sure how to frame the title but the explanation is simpler. I was playing with the c++11 range for loops (link).

I wrote the following:

for(auto &elem:vec){
    elem/=vec[0];
}

Which compiles but does not change anything in the vector 'vec'.

This, on the other hand, works:

elem0 = vec[0];
for(auto &elem:vec){
    elem/=elem0;
}

Is this expected? What is wrong with the first method? If it helps, my compiler version is g++ 4.6.3.

Answer 1

Actually it works just fine except that, in your example, vec[0] has been changed to 1 after the first run, which makes the remaining runs look like "not working" (remain the same value if divided by 1).

You will see it clearly by using the following example (will work "as expected"):

vector<float> vec = { 1, 2, 3, 4, 5 };
for (auto &elem : vec){
    elem /= vec[4];
}

// now vec={0.2, 0.4, 0.6, 0.8, 1.0}