Range-based for loops to iterate over a range of numbers or values
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Re-imagining the for
loop
C++11 now knows two distinct types of
forloops: the classic loop over an “index” and the range-based
forloop which vastly simplifies the iteration over a range specified by a pair of iterators.
By contrast, Python knows only one loop type – roughly equivalent to the range-based for loop. In fact, loops over indices are exceedingly rare, but made possible by the use of the
rangemethod:
for i in range(10):
print i
Which does what it promises – although Python version < 3.0 does the “wrong” thing and actually instantiates the whole collection in memory at once; a remedy is
xrangewhich yields values lazily as they are consumed by the loop.
C++11 effortlessly allows the same but there is no standard library function to provide this. Boost.Range provides part of the functionality via
irangewhich only works on integers, and not for unlimited ranges (this will make sense in a second).
The header
range.hppprovides a very basic implementation for this. It allows running the following code:
for (auto i : range(1, 5))
cout << i << "\n";
for (auto u : range(0u))
if (u == 3u) break;
else cout << u << "\n";
for (auto c : range('a', 'd'))
cout << c << "\n";
for (auto i : range(100).step(-3))
if (i < 90) break;
else cout << i << "\n";
rangewith a single argument deviates from the Python semantic and creates an endless loop, unless it’s interrupted manually. This is an interesting use-case that cannot be modelled in Python using
range.
Iterating over container indices
In Python, the one-argument version of
rangeis often used to iterate over the indices of a container via
range(len(container)). Because that overload creates an infinite range in our C++ library, we cannot use this idiom.
But we can do better anyway. For those few cases where we actually want to iterate over a container’s indices, we just use the
indicesfunction:
std::vector x{1, 2, 3};
for (auto i : indices(x))
cout << i << '\n';
This works as expected for any type which has a member function
size() constthat returns some integral type. It also works with
initializer_lists and C-style fixed-size arrays.1
Adding
.step(…)to the end of either
rangeor
indicesspecifies a step size instead of the default, 1.
The construct works for arbitrary types which fulfil the interface requirements (incrementing, copying, equality comparison, default construction in the case of infinite ranges).
1 This includes string literals, which are C-style strings that include null termination; this may lead to surprising results, because
indices("test") results in 0, 1, 2, 3, 4, whereas indices(std::string{"test"}) results in 0, 1, 2, 3. ↩
Performance (the cost of beauty)
When compiling with optimisations enabled (and why wouldn’t you?), using the
rangefunction yield very similar output compared with a manual
forloop. In fact, on g++ 4.8 with
-O2or higher, the following two loops yield identical assembly.
for (int i = 0; i < n; ++i)
cout << i;
for (int i : range(0, n))
cout << i;
Even though the
rangefunction creates a proxy container and an iterator wrapper, those are completely elided from the resulting code.
☞ Beauty is free.