Shorthand for for-loop - syntactic sugar in C++(11) - c++

Question

Actually these are two related questions.
I know there is a new syntax in C++11 for range-based for loops of the form:
//v is some container
for (auto &i: v){
// Do something with i
}
First question: how can I infer at which iteration I am in this loop? (Say I want to fill a vector with value j at position j).
Second question: I wanted to know if there also is some other way to write a loop of the form
for (int i=0; i<100; i++) { ... }
I find this way of writing it a bit cumbersome, and I do this so often and I would love to have a more concise syntax for it.
Something along the lines:
for(i in [0..99]){ ... }
would be great.
For both questions I would like to avoid having to use additional libraries.

Answer 1

First answer: you don't. You've used a simple construct for a simple purpose; you'll need something more complicated if you have more complicated needs.
Second answer: You could make an iterator type that yields consecutive integer values, and a "container" type that gives a range of those. Unless you have a good reason to do it yourself, Boost has such a thing:
#include <boost/range/irange.hpp>
for (int i : boost::irange(0,100)) {
// i goes from 0 to 99 inclusive
}

Answer 2

Use this:
size_t pos = 0;
for (auto& i : v) {
i = pos;
++pos;
}
(Boost is good, but it is not universally accepted.)

Answer 3

For the first question, the answer is pretty simple: if you need the iteration count, don't use the syntactic construct which abstracts away the iteration count. Just use a normal for loop and not the range-based one.
For the second question, I don't think there's anything currently in the standard library, but you could use a boost::irange for it:
for (int i : boost::irange(0, 100))

Answer 4

For the second question - if Boost is too heavy, you could always use this library:
cppitertools
for(auto i : range(10, 15)) { cout << i << '\n'; } will print 10 11 12 13 14
for(auto i : range(20, 30, 2)) { cout << i << '\n'; } will print 20 22 24 26 28
Doubles and other numeric types are supported too.
It has other pythonic iteration tools and is header-only.

Answer 5

You can do both of these things with Boost.Range: http://boost.org/libs/range
boost::adaptors::indexed: element value & index
boost::irange: integer range
For brevity (and to spice things up a little, since boost::irange has been already demonstrated in isolation), here's a sample code demonstrating these features working together:
// boost::adaptors::indexed
// http://www.boost.org/doc/libs/master/libs/range/doc/html/range/reference/adaptors/reference/indexed.html
#include <boost/range/adaptor/indexed.hpp>
// boost::irange
// http://www.boost.org/doc/libs/master/libs/range/doc/html/range/reference/ranges/irange.html
#include <boost/range/irange.hpp>
#include <iostream>
#include <vector>
int main()
{
std::vector<int> input{11, 22, 33, 44, 55};
std::cout << "boost::adaptors::indexed" << '\n';
for (const auto & element : input | boost::adaptors::indexed())
{
std::cout << "Value = " << element.value()
<< " Index = " << element.index()
<< '\n';
}
endl(std::cout);
std::cout << "boost::irange" << '\n';
for (const auto & element : boost::irange(0, 5) | boost::adaptors::indexed(100))
{
std::cout << "Value = " << element.value()
<< " Index = " << element.index()
<< '\n';
}
return 0;
}
Sample output:
boost::adaptors::indexed
Value = 11 Index = 0
Value = 22 Index = 1
Value = 33 Index = 2
Value = 44 Index = 3
Value = 55 Index = 4
boost::irange
Value = 0 Index = 100
Value = 1 Index = 101
Value = 2 Index = 102
Value = 3 Index = 103
Value = 4 Index = 104

Answer 6

If v is a vector (or any std contiguous container), then
for(auto& x : v ) {
size_t i = &x-v.data();
x = i;
}
will set the ith entry to the value i.
An output iterator that counts is reasonably easy to write. Boost has one and has an easy-to-generate range of them called irange.
Extracting the indexes of a container is relatively easy. I have written a function called indexes that can take a container, or a range of integers, and produces random output iterators over the range in question.
That gives you:
for (size_t i : indexes(v) ) {
v[i] = i;
}
There probably is an equivalent container-to-index range function in Boost.
If you need both, and you don't want to do the work, you can write a zipper.
for( auto z : zip( v, indexes(v) ) ) {
auto& x = std::get<0>(z);
size_t i = std::get<1>(z);
x = i;
}
where zip takes two or more iterable ranges (or containers) and produces a range view over tuples of iterator_traits<It>::references to the elements.
Here is Boost zip iterator: http://www.boost.org/doc/libs/1_41_0/libs/iterator/doc/zip_iterator.html -- odds are there is a Boost zip range that handles syntax like the above zip function.

Answer 7

For the 2nd question:
There is another way, but I would not use or recommend it. However, for quickly setting up a test you could write:
if you do not want to use a library and you are fine with only providing the top bound of the range you can write:
for (auto i:vector<bool>(10)) {
cout << "x";
}
This will create a boolean vector of size 10, with uninitialized values. Looping through these unitialized values using i (so do not use i) it will print 10 times "x".

Answer 8

For the second question, if you are using the latest Visual Studio versions, type 'if' then Tab, Tab, and Tab to fill in init value, step-up and so on.