Is there an equivalent to the “for … else” Python loop in C++?

Question

Python has an interesting for statement which lets you specify an else clause.

In a construct like this one:

for i in foo:
  if         


        

Answer 1

I would accomplish this with a simple helper variable:

#include <stdio.h>
#include <stdbool.h>

int main()

{
    bool b;
    printf("Numbers which are multiples of 7:\n");

    for (int i=8; b=(i<12); i++)
    {
        if (i%7==0)
        {
            printf("%d", i);
            break;
        }
    }
    if (!b)
    {
        printf("no numbers found\n");
    }
    return 0;
}

This way, you need to implement the condition (in the above examplei<12) only at one place.

Answer 2

This is my rough implementation in C++:

bool other = true;
for (int i = 0; i > foo; i++) {
     if (bar[i] == 7) {
          other = false;
          break;
     }
} if(other)
     baz();

Answer 3

Something like:

auto it = foo.begin(), end = foo.end();
while ( it != end && ! bar( *it ) ) {
    ++ it;
}
if ( it != foo.end() ) {
    baz();
}

should do the trick, and it avoids the unstructured break.

Answer 4

You could use a lambda function for this:

[&](){
  for (auto i : foo) {
    if (bar(i)) {
      // early return, to skip the "else:" section.
      return;
    }
  }
  // foo is exhausted, with no item satisfying bar(). i.e., "else:"
  baz();
}();

This should behave exactly like Python's "for..else", and it has some advantages over the other solutions:

• It's a true drop-in replacement for "for..else": the "for" section can have side effects (unlike none_of, whose predicate must not modify its argument), and it has access to the outer scope.
• It's more readable than defining a special macro.
• It doesn't require any special flag variables.

But... I'd use the clunky flag variable, myself.

Answer 5

I am not aware of an elegant way to accomplish this in C/C++ (not involving a flag variable). The suggested other options are much more horrible than that...

To answer @Kerrek SB about real life usages, I found a few in my code (simplified snippets)

Example 1: typical find/fail

for item in elements:
    if condition(item):
        do_stuff(item)
        break
else: #for else
    raise Exception("No valid item in elements")

Example 2: limited number of attempts

for retrynum in range(max_retries):
    try:
        attempt_operation()
    except SomeException:
        continue
    else:
        break
else: #for else
    raise Exception("Operation failed {} times".format(max_retries))

Answer 6

I came here because I had the same question, in C though. The best thing I came out with is

bool notTerminated = true;
for (int i = 0; i < 50 || (notTerminated = false); i++)
    if (bar(i))
        break;
if (! notTerminated)
    baz();

Explanation: the (notTerminated = false) is an assignment that will always return the false value, it will never affect the condition and will be evaluated iif the condition if true.