Loops

Loops may be perceived as repeatable if statements, which again should be translated into CPU comparison and jump instructions. For example, we can calculate the sum of numbers from 0 to 10 using the while loop:

auto num = 0;
auto sum = 0;
while (num <= 10) {
  sum += num;
  ++num;
}

This will translate to the following assembly code (simplified):

mov rax, 0; the sum
mov rcx, 0; the num
LOOP:
  cmp rbx, 10
  jg END; jump to the END if num is greater than 10
  add rax, rcx; add to sum
  inc rcx; increment num
  jmp LOOP; repeat
END:
  ...

C++17 introduced init statements that can be used in conditionals and loops. The num variable declared outside of the while loop may now be moved into the loop:

auto sum = 0;
while (auto num = 0; num <= 10) {
  sum += num;
  ++num;
}

The same rule applies to the if statement, for example:

int get_absolute(int num) {
  if (int neg = -num; neg < 0) {
    return -neg;
  }
  return num;
}

C++11 introduced the range-based for loop, which makes the syntax much clearer. For example, let's call all the arithmetic operations we defined earlier using the new for loop:

for (auto& op: operations) {
  std::cout << op.second(num1, num2);
}

Iterating  unordered_map returns a pair with the first and second members, the first being the key, and the second being the value mapped to that key. C++17 moved us even further, allowing us to write the same loop as follows:

for (auto& [op, func]: operations) {
  std::cout << func(num1, num2);
}

Knowing what the compiler actually generates is key in designing and implementing efficient software. We touched on the low-level details of conditionals and loops, which are at the base of almost every program.