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Programming-Idioms

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  • C++

Idiom #223 for else loop

Loop through list items checking a condition. Do something else if no matches are found.

A typical use case is looping through a series of containers looking for one that matches a condition. If found, an item is inserted; otherwise, a new container is created.

These are mostly used as an inner nested loop, and in a location where refactoring inner logic into a separate function reduces clarity.

#include <algorithm>
if (std::any_of(items.begin(), items.end(), condition))
    baz();
else
    DoSomethingElse();

Calls condition(x) on each element in items until one returns true
#include <list>
bool found = false;
for (const auto item : items) {
  if (item == "baz") {
    baz();
    found = true;
    break;
  }
}
if (!found) {
  DoSomethingElse();
}

c++ 11 implementation

c++ doesn't have for..else construction, so found variable can be used instead
(if (seq (filter odd? my-col))
  "contains odds"
  "no odds found")

(seq (filter pred-fn col)) returns nil when there are no matches, and nil is falsy.

New implementation...
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Ruien