Exactly how range for works?

Question

I was reading about "range for" statements when I got confused how it exactly works.

Below is a program to convert a string to Uppercase.

string s("Hello World!!!");

//convert s to uppercase

for( auto &c :s )  // for every char in s
   c= topper(c);   //  c is a reference,so the assignment changes the 
                   //  char in s
cout<< s << endl;

How is the reference to the string (that is, c) is changing elements to upppercase?

I've searched about how iteration could work here, but I couldn't find the answer.

[here](https://en.cppreference.com/w/cpp/language/range-for) you can find a detailed explanation — 463035818_is_not_an_ai, Oct 11 '18 at 13:19
@FrançoisAndrieux - To the answer section please if you intend to correct the OP. — StoryTeller - Unslander Monica, Oct 11 '18 at 13:19
related/dupe: https://stackoverflow.com/questions/7939399/how-does-the-range-based-for-work-for-plain-arrays — NathanOliver, Oct 11 '18 at 13:20
The explanation is in the code. If you don't follow it, please tell us which part you don't follow. Otherwise we are simply doomed to repeat the explanation... — Lightness Races in Orbit, Oct 11 '18 at 13:32

score 6 · Answer 1 · answered Oct 11 '18 at 13:22

This piece of code

for (auto& c : s)
{
    c = toupper(c); 
}

roughly translates to this

for (auto it = std::begin(s); it != std::end(s); ++it)
{
    auto& c = *it;
    c = toupper(c);
}

which is a basic iterator loop, covered in any beginner C++ book.

cppreference has a more detailed and precise explanation.

score 2 · Answer 2 · answered Oct 11 '18 at 13:27

2

"c" is not an ordinary variable, it acts as a proxy (or reference) to each element (character) in the string.

By changing "c" you are in fact changing the value which "c" is refering to.

answered Oct 11 '18 at 13:27

groovyspaceman

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@user463035818 It's not unfair to call references "not ordinary variables" at this level – Lightness Races in Orbit Oct 11 '18 at 13:32
@LightnessRacesinOrbit well on second read I got it. Sometimes I am faster in writing than thinking ;) – 463035818_is_not_an_ai Oct 11 '18 at 13:33

Exactly how range for works?

2 Answers2