This week I was diving into some of the new features C++ has regarding iteration. This newfound interest stemmed from wanting to make cleaner, more understandable code. There have been many situations when doing LeetCode problems or personal projects where I felt like I could clean up or use more modern tools in certain situations. In this post, I will explore some new ways for iterating for loops. I am using C++20.
Basic Iteration
We all are familiar with the generic for loop like this:
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#include <iostream>
for (int i{ 0 }; i < 5; ++i) {
std::cout << i << ',';
}
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❯ /Users/kylehurd/Workplace/tmp/build/for-loops
0,1,2,3,4,
The above for loop will display to stdout the values from 0-4. For me, I do see two issues that have always bugged me (like my joke?). The first is, for each iteration, we may want to ensure that the value of i does not update within the for loop itself. With this implementation there is not a good way to do this. The second is it doesn’t look clean to me. Surely there can be a better way to use an iterator.
As I did some digging, I found a new function in the C++20 standard called std::views::iota. We can use that function to help try and clean up the for loop. In a basic setting, it works very similarly to a range for loop in Python. The first argument is the starting position and the second argument is the ending position. The start is inclusive where as the end is exclusive. [start, end)
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for (const int i : std::views::iota(0, 5)) {
std::cout << i << ',';
}
I personally find this easier to read, although the implementation is actually a bit lengthier compared to the generic for loop. Plus, our iterator i is now a const type. There are other reasons why this type of for loop would be ideal over the generic version. For example, if you wanted to filter the iterations by odd numbers only you can add a filter option:
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auto odd = [](const int value) { return value % 2 == 1; };
for (const int i : std::views::iota(0,10)
| std::views::filter(odd)) {
std::cout << i << ',';
}
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❯ /Users/kylehurd/Workplace/tmp/build/for-loops
1,3,5,7,9,
There are much more modifications you can make within the std::views namespace. You can check them out in the Microsoft Documentation.
Container Iteration
For this section, I focused on different ways you can iterate over containers. We can filter, transform, and perform other operations on the container in the same way as above. For example, given a std::vector object, we can do the following:
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#include <vector>
// ...
std::vector<int> v { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
const auto odd = [](const int number) { return number % 2 == 1; };
for (const int value : v | std::views::filter(odd)) {
std::cout << value << ',';
}
std::cout << std::endl;
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❯ /Users/kylehurd/Workplace/tmp/build/for-loops
1,3,5,7,9,
Shifting over to a different type, consider the following situation. Below we have an unordered map containing a string as the key and float as the value. The key is the store that was visited and the value is the amount spent at said store.
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std::unordered_map<std::string, float> transactions;
transactions.insert_or_assign("Safeway", 1.97);
transactions.insert_or_assign("Walmart", 18.81);
transactions.insert_or_assign("Apple", 119.03);
transactions.insert_or_assign("Sears", 32.43);
Given this map, if we wanted to iterate over each item and print each item, we could do this:
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for (const auto& transaction : transactions) {
std::cout << transaction.first << ": $" << transaction.second << '\n';
}
// or we could utilize a lambda
const auto print_transaction = [](const auto& node) {
std::cout << node.first << ": $" << node.second << '\n';
};
for (const auto& transaction : transactions) {
print_transaction(transaction)
}
The lambda helps the for loop become slightly more readable, but I do not like the variables .first and .second. To me, they make it unclear what first and second are. There is another way we can iterate over a pair object like this by unpacking the map before entering the for loop. This is one of my favorite ways to iterate over these types of objects at the moment. It allows us to give better names to our variables before entering the loop.
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#include <format>
// ...
for (const auto& [store, cost] : transactions) {
// gcc-13
std::cout << std::format("{}: ${}\n", store, cost);
// gcc-12
std::cout << store << ": $" << cost << '\n';
}
That’s my rundown on a few different ways to iterate! There are a lot of cool features released in C++20 and a lot of it helps improve readability of a program.