C++ Loops and Repetition

Before writing syntax, state what repeats, what changes, and what ends the repetition. A loop that does not move toward its stopping condition can run forever. A loop that stops one step late can access invalid memory or count an extra item.

Trace a small input by hand. Record the condition, current value, body effect, and update for each iteration. This exposes whether the boundary is inclusive and whether the final state matches the requirement.

A traditional for loop places initialization, continuation, and update together. Use it when the progression is central, such as visiting positions or generating a known range. Keep the body focused on the work for one iteration.

#include <iostream>

int main()
{
  int sum{};
  for (int number{1}; number <= 5; ++number)
  {
      sum += number;
  }
  std::cout << "Sum: " << sum << '\n';
}

Sum: 15

while checks before each iteration, so the body may run zero times. This matches input loops and state machines. Initialize the state before the loop, update it on every path that continues, and keep the sentinel out of ordinary data processing.

Failed input is another stopping state. while (std::cin >> value) reads values until extraction fails or input ends. If interactive retry is required, recovery belongs inside a deliberate loop rather than after using an invalid value.

A range-based loop says “for each element” without managing an index. Use const auto& for read-only access to elements that may be expensive to copy, and auto& only when the loop intentionally modifies them. A plain auto copies each element.

Index-based loops remain useful when the position is part of the output or neighboring elements are compared. Do not introduce an index solely from habit.