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In computer science, a flag is a value that acts as a signal for a function or process. The value of the flag is used to determine the next step of a program. Flags are often binary flags, which contain a boolean value (true or false). However, not all flags are binary, meaning they can store a range of values.

You can think of a binary flag as a small red flag that is laying flat when it is false, but pops up when it is true. A raised flag says to a program, "Stop - do something different." A common example of a flag in computer programming is a variable in a while loop. The PHP loop below will iterate until $flag is set to true.

$flag = false;
$i = 1;

while (!$flag) // stop when $flag is true
  echo "$i, ";
  $i++; // increment $i
  if ($i > 100) $flag = true;

The above code will print out numbers (1, 2, 3...) until 100. Then the loop will break because $flag will be set to true. Using a flag in this context is effective, but unnecessary. Instead, the while loop condition could have been while ($i < 101) instead of while (!$flag). This would produce the same result and eliminate the need for the $flag variable. Efficiently written programs rarely need explicit flags since an existing variable within a function can often be used as a flag.

A binary flag only requires one bit, which can be set to 0 or 1. However, bytes have eight bits, meaning seven bits are unused when a single byte stores a binary flag. While a single byte is still a very small amount of data, a programmer may choose to use a single byte to store multiple binary flags.

Non-Binary Flags

Non-binary flags use multiple bits and can store more than "yes or no" or "true or false." These types of flags require more than one bit, but not necessarily a full byte. For example, two bits can produce four possible options.

  1. 00 = option A
  2. 01 = option B
  3. 10 = option C
  4. 11 = option D

You can think of a non-binary flag as a flag with multiple colors. A program can check to see if 1) if the multi-bit flag is set and 2) what value it contains. Depending on the value (or "color") of the flag, the program will continue in the corresponding direction.

Updated: May 17, 2018

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