C++

Loops

Introduction#

A loop statement executes a group of statements repeatedly until a condition is met. There are 3 types of primitive loops in C++: for, while, and do…while.

Syntax#

  • while (condition) statement ;
  • do statement while (expression) ;
  • for (for-init-statement ; condition ; expression) statement ;
  • for (for-range-declaration : for-range-initializer) statement ;
  • break ;
  • continue ;

Remarks#

algorithm calls are generally preferable to hand-written loops.

If you want something an algorithm already does (or something very similar), the algorithm call is clearer, often more efficient and less error prone.

If you need a loop that does something fairly simple (but would require a confusing tangle of binders and adapters if you were using an algorithm), then just write the loop.

Range-Based For

for loops can be used to iterate over the elements of a iterator-based range, without using a numeric index or directly accessing the iterators:

vector<float> v = {0.4f, 12.5f, 16.234f};

for(auto val: v)
{
    std::cout << val << " ";
}

std::cout << std::endl;

This will iterate over every element in v, with val getting the value of the current element. The following statement:

for (for-range-declaration : for-range-initializer ) statement

is equivalent to:

{
    auto&& __range = for-range-initializer;
    auto __begin = begin-expr, __end = end-expr;
    for (; __begin != __end; ++__begin) {
        for-range-declaration = *__begin;
        statement
    }
}
{
    auto&& __range = for-range-initializer;
    auto __begin = begin-expr;
    auto __end = end-expr; // end is allowed to be a different type than begin in C++17
    for (; __begin != __end; ++__begin) {
        for-range-declaration = *__begin;
        statement
    }
}

This change was introduced for the planned support of Ranges TS in C++20.

In this case, our loop is equivalent to:

{
    auto&& __range = v;
    auto __begin = v.begin(), __end = v.end();
    for (; __begin != __end; ++__begin) {
        auto val = *__begin;
        std::cout << val << " ";
    }
}

Note that auto val declares a value type, which will be a copy of a value stored in the range (we are copy-initializing it from the iterator as we go). If the values stored in the range are expensive to copy, you may want to use const auto &val. You are also not required to use auto; you can use an appropriate typename, so long as it is implicitly convertible from the range’s value type.

If you need access to the iterator, range-based for cannot help you (not without some effort, at least).

If you wish to reference it, you may do so:

vector<float> v = {0.4f, 12.5f, 16.234f};

for(float &val: v)
{
    std::cout << val << " ";
}

You could iterate on const reference if you have const container:

const vector<float> v = {0.4f, 12.5f, 16.234f};

for(const float &val: v)
{
    std::cout << val << " ";
}

One would use forwarding references when the sequence iterator returns a proxy object and you need to operate on that object in a non-const way. Note: it will most likely confuse readers of your code.

vector<bool> v(10);

for(auto&& val: v)
{
    val = true;
}

The “range” type provided to range-based for can be one of the following:

  • Language arrays:

    float arr[] = {0.4f, 12.5f, 16.234f};

for(auto val: arr)
{
    std::cout << val << " ";
}

    Note that allocating a dynamic array does not count:

    float *arr = new float[3]{0.4f, 12.5f, 16.234f};

for(auto val: arr) //Compile error.
{
    std::cout << val << " ";
}

  • Any type which has member functions begin() and end(), which return iterators to the elements of the type. The standard library containers qualify, but user-defined types can be used as well:

    struct Rng
{
    float arr[3];

    // pointers are iterators
    const float* begin() const {return &arr[0];}
    const float* end() const   {return &arr[3];}
    float* begin() {return &arr[0];}
    float* end()   {return &arr[3];}
};

int main()
{
    Rng rng = {{0.4f, 12.5f, 16.234f}};

    for(auto val: rng)
    {
        std::cout << val << " ";
    }
}

  • Any type which has non-member begin(type) and end(type) functions which can found via argument dependent lookup, based on type. This is useful for creating a range type without having to modify class type itself:

    namespace Mine
{
    struct Rng {float arr[3];};

    // pointers are iterators
    const float* begin(const Rng &rng) {return &rng.arr[0];}
    const float* end(const Rng &rng) {return &rng.arr[3];}
    float* begin(Rng &rng) {return &rng.arr[0];}
    float* end(Rng &rng) {return &rng.arr[3];}
}

int main()
{
    Mine::Rng rng = {{0.4f, 12.5f, 16.234f}};

    for(auto val: rng)
    {
        std::cout << val << " ";
    }
}

For loop

A for loop executes statements in the loop body, while the loop condition is true. Before the loop initialization statement is executed exactly once. After each cycle, the iteration execution part is executed.

A for loop is defined as follows:

for (/*initialization statement*/; /*condition*/; /*iteration execution*/)
{
    // body of the loop
}

Explanation of the placeholder statements:

  • initialization statement: This statement gets executed only once, at the beginning of the for loop. You can enter a declaration of multiple variables of one type, such as int i = 0, a = 2, b = 3. These variables are only valid in the scope of the loop. Variables defined before the loop with the same name are hidden during execution of the loop.
  • condition: This statement gets evaluated ahead of each loop body execution, and aborts the loop if it evaluates to false.
  • iteration execution: This statement gets executed after the loop body, ahead of the next condition evaluation, unless the for loop is aborted in the body (by break, goto, return or an exception being thrown). You can enter multiple statements in the iteration execution part, such as a++, b+=10, c=b+a.

The rough equivalent of a for loop, rewritten as a while loop is:

/*initialization*/
while (/*condition*/)
{
    // body of the loop; using 'continue' will skip to increment part below
    /*iteration execution*/
}

The most common case for using a for loop is to execute statements a specific number of times. For example, consider the following:

for(int i = 0; i < 10; i++) {
    std::cout << i << std::endl;
}

A valid loop is also:

for(int a = 0, b = 10, c = 20; (a+b+c < 100); c--, b++, a+=c) {
    std::cout << a << " " << b << " " << c << std::endl; 
}

An example of hiding declared variables before a loop is:

int i = 99; //i = 99
for(int i = 0; i < 10; i++) { //we declare a new variable i
    //some operations, the value of i ranges from 0 to 9 during loop execution
}
//after the loop is executed, we can access i with value of 99

But if you want to use the already declared variable and not hide it, then omit the declaration part:

int i = 99; //i = 99
for(i = 0; i < 10; i++) { //we are using already declared variable i
    //some operations, the value of i ranges from 0 to 9 during loop execution
}
//after the loop is executed, we can access i with value of 10

Notes:

  • The initialization and increment statements can perform operations unrelated to the condition statement, or nothing at all - if you wish to do so. But for readability reasons, it is best practice to only perform operations directly relevant to the loop.
  • A variable declared in the initialization statement is visible only inside the scope of the for loop and is released upon termination of the loop.
  • Don’t forget that the variable which was declared in the initialization statement can be modified during the loop, as well as the variable checked in the condition.

Example of a loop which counts from 0 to 10:

for (int counter = 0; counter <= 10; ++counter)
{
    std::cout << counter << '\n';
}
// counter is not accessible here (had value 11 at the end)

Explanation of the code fragments:

  • int counter = 0 initializes the variable counter to 0. (This variable can only be used inside of the for loop.)
  • counter <= 10 is a Boolean condition that checks whether counter is less than or equal to 10. If it is true, the loop executes. If it is false, the loop ends.
  • ++counter is an increment operation that increments the value of counter by 1 ahead of the next condition check.

By leaving all statements empty, you can create an infinite loop:

// infinite loop
for (;;)
    std::cout << "Never ending!\n";

The while loop equivalent of the above is:

// infinite loop
while (true)
    std::cout << "Never ending!\n";

However, an infinite loop can still be left by using the statements break, goto, or return or by throwing an exception.

The next common example of iterating over all elements from an STL collection (e.g., a vector) without using the <algorithm> header is:

std::vector<std::string> names = {"Albert Einstein", "Stephen Hawking", "Michael Ellis"};
for(std::vector<std::string>::iterator it = names.begin(); it != names.end(); ++it) {
    std::cout << *it << std::endl;
}

While loop

A while loop executes statements repeatedly until the given condition evaluates to false. This control statement is used when it is not known, in advance, how many times a block of code is to be executed.

For example, to print all the numbers from 0 up to 9, the following code can be used:

int i = 0;
while (i < 10)
{
    std::cout << i << " ";
    ++i; // Increment counter
}
std::cout << std::endl; // End of line; "0 1 2 3 4 5 6 7 8 9" is printed to the console

Note that since C++17, the first 2 statements can be combined

while (int i = 0; i < 10)
//... The rest is the same

To create an infinite loop, the following construct can be used:

while (true)
{
    // Do something forever (however, you can exit the loop by calling 'break'
}

There is another variant of while loops, namely the do...while construct. See the do-while loop example for more information.

Declaration of variables in conditions

In the condition of the for and while loops, it’s also permitted to declare an object. This object will be considered to be in scope until the end of the loop, and will persist through each iteration of the loop:

for (int i = 0; i < 5; ++i) {
    do_something(i);
}
// i is no longer in scope.

for (auto& a : some_container) {
    a.do_something();
}
// a is no longer in scope.

while(std::shared_ptr<Object> p = get_object()) {
   p->do_something();
}
// p is no longer in scope.

However, it is not permitted to do the same with a do...while loop; instead, declare the variable before the loop, and (optionally) enclose both the variable and the loop within a local scope if you want the variable to go out of scope after the loop ends:

//This doesn't compile
do {
    s = do_something();
} while (short s > 0);

// Good
short s;
do {
    s = do_something();
} while (s > 0);

This is because the statement portion of a do...while loop (the loop’s body) is evaluated before the expression portion (the while) is reached, and thus, any declaration in the expression will not be visible during the first iteration of the loop.

Do-while loop

A do-while loop is very similar to a while loop, except that the condition is checked at the end of each cycle, not at the start. The loop is therefore guaranteed to execute at least once.

The following code will print 0, as the condition will evaluate to false at the end of the first iteration:

int i =0;
do
{
    std::cout << i;
    ++i; // Increment counter
}
while (i < 0);
std::cout << std::endl; // End of line; 0 is printed to the console

Note: Do not forget the semicolon at the end of while(condition);, which is needed in the do-while construct.

In contrast to the do-while loop, the following will not print anything, because the condition evaluates to false at the beginning of the first iteration:

int i =0;
while (i < 0)
{
    std::cout << i;
    ++i; // Increment counter
}    
std::cout << std::endl; // End of line; nothing is printed to the console

Note: A while loop can be exited without the condition becoming false by using a break, goto, or return statement.

int i = 0;
do
{
    std::cout << i;
    ++i; // Increment counter
    if (i > 5) 
    {
        break;
    }
}
while (true);
std::cout << std::endl; // End of line; 0 1 2 3 4 5 is printed to the console

A trivial do-while loop is also occasionally used to write macros that require their own scope (in which case the trailing semicolon is omitted from the macro definition and required to be provided by the user):

#define BAD_MACRO(x) f1(x); f2(x); f3(x);

// Only the call to f1 is protected by the condition here
if (cond) BAD_MACRO(var);

#define GOOD_MACRO(x) do { f1(x); f2(x); f3(x); } while(0)

// All calls are protected here
if (cond) GOOD_MACRO(var);

Loop Control statements : Break and Continue

Loop control statements are used to change the flow of execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed. The break and continue are loop control statements.

The break statement terminates a loop without any further consideration.

for (int i = 0; i < 10; i++)
{
    if (i == 4)
        break; // this will immediately exit our loop
    std::cout << i << '\n';
}

The above code will print out:

1
2
3

The continue statement does not immediately exit the loop, but rather skips the rest of the loop body and goes to the top of the loop (including checking the condition).

for (int i = 0; i < 6; i++)
{
    if (i % 2 == 0) // evaluates to true if i is even
        continue; // this will immediately go back to the start of the loop
    /* the next line will only be reached if the above "continue" statement 
       does not execute  */
    std::cout << i << " is an odd number\n";
}

The above code will print out:

1 is an odd number
3 is an odd number
5 is an odd number

Because such control flow changes are sometimes difficult for humans to easily understand, break and continue are used sparingly. More straightforward implementation are usually easier to read and understand. For example, the first for loop with the break above might be rewritten as:

for (int i = 0; i < 4; i++)
{
    std::cout << i << '\n';
}

The second example with continue might be rewritten as:

for (int i = 0; i < 6; i++)
{
    if (i % 2 != 0) {
        std::cout << i << " is an odd number\n";
    }
}

Range-for over a sub-range

Using range-base loops, you can loop over a sub-part of a given container or other range by generating a proxy object that qualifies for range-based for loops.

template<class Iterator, class Sentinel=Iterator>
struct range_t {
  Iterator b;
  Sentinel e;
  Iterator begin() const { return b; }
  Sentinel end() const { return e; }
  bool empty() const { return begin()==end(); }
  range_t without_front( std::size_t count=1 ) const {
    if (std::is_same< std::random_access_iterator_tag, typename std::iterator_traits<Iterator>::iterator_category >{} ) {
      count = (std::min)(std::size_t(std::distance(b,e)), count);
    }
    return {std::next(b, count), e};
  }
  range_t without_back( std::size_t count=1 ) const {
    if (std::is_same< std::random_access_iterator_tag, typename std::iterator_traits<Iterator>::iterator_category >{} ) {
      count = (std::min)(std::size_t(std::distance(b,e)), count);
    }
    return {b, std::prev(e, count)};
  }
};

template<class Iterator, class Sentinel>
range_t<Iterator, Sentinel> range( Iterator b, Sentinal e ) {
  return {b,e};
}
template<class Iterable>
auto range( Iterable& r ) {
  using std::begin; using std::end;
  return range(begin(r),end(r));
}

template<class C>
auto except_first( C& c ) {
  auto r = range(c);
  if (r.empty()) return r;
  return r.without_front();
}

now we can do:

std::vector<int> v = {1,2,3,4};

for (auto i : except_first(v))
  std::cout << i << '\n';

and print out

2
3
4

Be aware that intermediate objects generated in the for(:range_expression) part of the for loop will have expired by the time the for loop starts.

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