Why Does My Recursive PHP Function Sometimes Return 'Not Set' Instead of a Unique Barcode?

Utilizing Return within Recursive Functions in PHP

In PHP, recursive functions can be a powerful tool for solving complex problems. When writing recursive functions, properly utilizing the return statement is crucial to ensure correct logic and avoid infinite loops.

One case where such utilization is commonly overlooked is when generating unique values, as exemplified in the following scenario:

Question:

The goal is to generate random barcodes that are not already present in a database column. The function uses recursion to repeatedly generate numbers until a unique one is found. However, the function sometimes returns "Not Set" instead of producing a unique number.

Code:

function generate_barcode() {
  $barcode = rand(1, 10);
  $bquery = mysql_num_rows(mysql_query("SELECT * FROM stock_item WHERE barcode='$barcode'"));
  if ($bquery == 1) {
    generate_barcode();
  } else {
    return $barcode;
  }
}

Answer:

The issue in the provided code lies in the recursive call. To ensure that the function correctly returns a unique number, the newly generated number needs to be returned from the recursive call itself:

function generate_barcode() {
  $barcode = rand(1, 10);
  $bquery = mysql_num_rows(mysql_query("SELECT * FROM stock_item WHERE barcode='$barcode'"));
  if ($bquery == 1) {
    return generate_barcode(); // updated
  } else {
    return $barcode;
  }
}

By returning the generated number from the recursive call, the function ensures that the caller receives the unique number.

Additional Considerations:

It is essential to include an exit condition for the case where all numbers are taken. If there is no such condition, the function will call itself recursively until the PHP recursion limit is reached, causing an error.

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