#### 12.2.1 Issuing Warnings

It is possible to issue warnings from any code using the `warning` function. In its most simple form, the `warning` function takes a string describing the warning as its input argument. As an example, the following code controls if the variable ‘`a`’ is non-negative, and if not issues a warning and sets ‘`a`’ to zero.

    a = -1;
    if (a < 0)
      warning ("'a' must be non-negative.  Setting 'a' to zero.");
      a = 0;
    endif
         -| 'a' must be non-negative.  Setting 'a' to zero.

Since warnings aren’t fatal to a running program, it is not possible to catch a warning using the `try` statement or something similar. It is however possible to access the last warning as a string using the `lastwarn` function.

It is also possible to assign an identification string to a warning. If a warning has such an ID the user can enable and disable this warning as will be described in the next section. To assign an ID to a warning, simply call `warning` with two string arguments, where the first is the identification string, and the second is the actual warning. Note that warning IDs are in the format `"NAMESPACE:WARNING-NAME"`. The namespace `"Octave"` is used for Octave’s own warnings. Any other string is available as a namespace for user’s own warnings.

Built-in Function: warning (template, …)
Built-in Function: warning (id, template, …)
Built-in Function: warning ("on", id)
Built-in Function: warning ("off", id)
Built-in Function: warning ("query", id)
Built-in Function: warning ("error", id)
Built-in Function: warning (state, "backtrace")
Built-in Function: warning (state, id, "local")

  Format the optional arguments under the control of the template string template using the same rules as the `printf` family of functions (see Formatted Output) and print the resulting message on the `stderr` stream. The message is prefixed by the character string ‘`warning:` ’. You should use this function when you want to notify the user of an unusual condition, but only when it makes sense for your program to go on.

  The optional message identifier allows users to enable or disable warnings tagged by id. A message identifier is of the form "NAMESPACE:WARNING-NAME". Octave’s own warnings use the `"Octave"` namespace (see XREFwarning_ids). The special identifier `"all"` may be used to set the state of all warnings.

  If the first argument is `"on"` or `"off"`, set the state of a particular warning using the identifier id. If the first argument is `"query"`, query the state of this warning instead. If the identifier is omitted, a value of `"all"` is assumed. If you set the state of a warning to `"error"`, the warning named by id is handled as if it were an error instead. So, for example, the following handles all warnings as errors:

      warning ("error");

  If the state is `"on"` or `"off"` and the third argument is `"backtrace"`, then a stack trace is printed along with the warning message when warnings occur inside function calls. This option is enabled by default.

  If the state is `"on"`, `"off"`, or `"error"` and the third argument is `"local"`, then the warning state will be set temporarily, until the end of the current function. Changes to warning states that are set locally affect the current function and all functions called from the current scope. The previous warning state is restored on return from the current function. The `"local"` option is ignored if used in the top-level workspace.

  Implementation Note: For compatibility with MATLAB, escape sequences in template (e.g., "\n" => newline) are processed regardless of whether template has been defined with single quotes, as long as there are two or more input arguments. To disable escape sequence expansion use a second backslash before the sequence (e.g., "\\n") or use the `regexptranslate` function.

  See also: warning_ids, lastwarn, error.

Built-in Function: [msg, msgid] = lastwarn ()
Built-in Function: lastwarn (msg)
Built-in Function: lastwarn (msg, msgid)

  Query or set the last warning message. When called without input arguments, return the last warning message and message identifier. With one argument, set the last warning message to msg. With two arguments, also set the last message identifier.

  See also: warning, lasterror, lasterr.

The functions distributed with Octave can issue one of the following warnings.

`Octave:abbreviated-property-match`

  By default, the `Octave:abbreviated-property-match` warning is enabled.

`Octave:array-to-scalar`

  If the `Octave:array-to-scalar` warning is enabled, Octave will warn when an implicit conversion from an array to a scalar value is attempted. By default, the `Octave:array-to-scalar` warning is disabled.

`Octave:array-to-vector`

  If the `Octave:array-to-vector` warning is enabled, Octave will warn when an implicit conversion from an array to a vector value is attempted. By default, the `Octave:array-to-vector` warning is disabled.

`Octave:assign-as-truth-value`

  If the `Octave:assign-as-truth-value` warning is enabled, a warning is issued for statements like

      if (s = t)
        …

  since such statements are not common, and it is likely that the intent was to write

      if (s == t)
        …

  instead.

  There are times when it is useful to write code that contains assignments within the condition of a `while` or `if` statement. For example, statements like

      while (c = getc ())
        …

  are common in C programming.

  It is possible to avoid all warnings about such statements by disabling the `Octave:assign-as-truth-value` warning, but that may also let real errors like

      if (x = 1)  # intended to test (x == 1)!
        …

  slip by.

  In such cases, it is possible suppress errors for specific statements by writing them with an extra set of parentheses. For example, writing the previous example as

      while ((c = getc ()))
        …

  will prevent the warning from being printed for this statement, while allowing Octave to warn about other assignments used in conditional contexts.

  By default, the `Octave:assign-as-truth-value` warning is enabled.

`Octave:associativity-change`

  If the `Octave:associativity-change` warning is enabled, Octave will warn about possible changes in the meaning of some code due to changes in associativity for some operators. Associativity changes have typically been made for MATLAB compatibility. By default, the `Octave:associativity-change` warning is enabled.

`Octave:autoload-relative-file-name`

  If the `Octave:autoload-relative-file-name` is enabled, Octave will warn when parsing autoload() function calls with relative paths to function files. This usually happens when using autoload() calls in PKG_ADD files, when the PKG_ADD file is not in the same directory as the .oct file referred to by the autoload() command. By default, the `Octave:autoload-relative-file-name` warning is enabled.

`Octave:built-in-variable-assignment`

  By default, the `Octave:built-in-variable-assignment` warning is enabled.

`Octave:deprecated-keyword`

  If the `Octave:deprecated-keyword` warning is enabled, a warning is issued when Octave encounters a keyword that is obsolete and scheduled for removal from Octave. By default, the `Octave:deprecated-keyword` warning is enabled.

`Octave:divide-by-zero`

  If the `Octave:divide-by-zero` warning is enabled, a warning is issued when Octave encounters a division by zero. By default, the `Octave:divide-by-zero` warning is enabled.

`Octave:fopen-file-in-path`

  By default, the `Octave:fopen-file-in-path` warning is enabled.

`Octave:function-name-clash`

  If the `Octave:function-name-clash` warning is enabled, a warning is issued when Octave finds that the name of a function defined in a function file differs from the name of the file. (If the names disagree, the name declared inside the file is ignored.) By default, the `Octave:function-name-clash` warning is enabled.

`Octave:future-time-stamp`

  If the `Octave:future-time-stamp` warning is enabled, Octave will print a warning if it finds a function file with a time stamp that is in the future. By default, the `Octave:future-time-stamp` warning is enabled.

`Octave:glyph-render`

  By default, the `Octave:glyph-render` warning is enabled.

`Octave:imag-to-real`

  If the `Octave:imag-to-real` warning is enabled, a warning is printed for implicit conversions of complex numbers to real numbers. By default, the `Octave:imag-to-real` warning is disabled.

`Octave:language-extension`

  Print warnings when using features that are unique to the Octave language and may be still be missing in MATLAB. By default, the `Octave:language-extension` warning is disabled. The –traditional or –braindead startup options for Octave may also be of use, see Command Line Options.

`Octave:load-file-in-path`

  By default, the `Octave:load-file-in-path` warning is enabled.

`Octave:logical-conversion`

  By default, the `Octave:logical-conversion` warning is enabled.

`Octave:md5sum-file-in-path`

  By default, the `Octave:md5sum-file-in-path` warning is enabled.

`Octave:missing-glyph`

  By default, the `Octave:missing-glyph` warning is enabled.

`Octave:missing-semicolon`

  If the `Octave:missing-semicolon` warning is enabled, Octave will warn when statements in function definitions don’t end in semicolons. By default the `Octave:missing-semicolon` warning is disabled.

`Octave:mixed-string-concat`

  If the `Octave:mixed-string-concat` warning is enabled, print a warning when concatenating a mixture of double and single quoted strings. By default, the `Octave:mixed-string-concat` warning is disabled.

`Octave:neg-dim-as-zero`

  If the `Octave:neg-dim-as-zero` warning is enabled, print a warning for expressions like

      eye (-1)

  By default, the `Octave:neg-dim-as-zero` warning is disabled.

`Octave:nested-functions-coerced`

  By default, the `Octave:nested-functions-coerced` warning is enabled.

`Octave:noninteger-range-as-index`

  By default, the `Octave:noninteger-range-as-index` warning is enabled.

`Octave:num-to-str`

  If the `Octave:num-to-str` warning is enable, a warning is printed for implicit conversions of numbers to their ASCII character equivalents when strings are constructed using a mixture of strings and numbers in matrix notation. For example,

      [ "f", 111, 111 ]
      ⇒ "foo"

  elicits a warning if the `Octave:num-to-str` warning is enabled. By default, the `Octave:num-to-str` warning is enabled.

`Octave:possible-matlab-short-circuit-operator`

  If the `Octave:possible-matlab-short-circuit-operator` warning is enabled, Octave will warn about using the not short circuiting operators `&` and `|` inside `if` or `while` conditions. They normally never short circuit, but MATLAB always short circuits if any logical operators are used in a condition. You can turn on the option

      do_braindead_shortcircuit_evaluation (1)

  if you would like to enable this short-circuit evaluation in Octave. Note that the `&&` and `||` operators always short circuit in both Octave and MATLAB, so it’s only necessary to enable MATLAB-style short-circuiting if it’s too arduous to modify existing code that relies on this behavior. By default, the `Octave:possible-matlab-short-circuit-operator` warning is enabled.

`Octave:precedence-change`

  If the `Octave:precedence-change` warning is enabled, Octave will warn about possible changes in the meaning of some code due to changes in precedence for some operators. Precedence changes have typically been made for MATLAB compatibility. By default, the `Octave:precedence-change` warning is enabled.

`Octave:recursive-path-search`

  By default, the `Octave:recursive-path-search` warning is enabled.

`Octave:remove-init-dir`

  The `path` function changes the search path that Octave uses to find functions. It is possible to set the path to a value which excludes Octave’s own built-in functions. If the `Octave:remove-init-dir` warning is enabled then Octave will warn when the `path` function has been used in a way that may render Octave unworkable. By default, the `Octave:remove-init-dir` warning is enabled.

`Octave:reload-forces-clear`

  If several functions have been loaded from the same file, Octave must clear all the functions before any one of them can be reloaded. If the `Octave:reload-forces-clear` warning is enabled, Octave will warn you when this happens, and print a list of the additional functions that it is forced to clear. By default, the `Octave:reload-forces-clear` warning is enabled.

`Octave:resize-on-range-error`

  If the `Octave:resize-on-range-error` warning is enabled, print a warning when a matrix is resized by an indexed assignment with indices outside the current bounds. By default, the ## `Octave:resize-on-range-error` warning is disabled.

`Octave:separator-insert`

  Print warning if commas or semicolons might be inserted automatically in literal matrices. By default, the `Octave:separator-insert` warning is disabled.

`Octave:shadowed-function`

  By default, the `Octave:shadowed-function` warning is enabled.

`Octave:single-quote-string`

  Print warning if a single quote character is used to introduce a string constant. By default, the `Octave:single-quote-string` warning is disabled.

`Octave:nearly-singular-matrix`
`Octave:singular-matrix`

  By default, the `Octave:nearly-singular-matrix` and `Octave:singular-matrix` warnings are enabled.

`Octave:sqrtm:SingularMatrix`

  By default, the `Octave:sqrtm:SingularMatrix` warning is enabled.

`Octave:str-to-num`

  If the `Octave:str-to-num` warning is enabled, a warning is printed for implicit conversions of strings to their numeric ASCII equivalents. For example,

      "abc" + 0
      ⇒ 97 98 99

  elicits a warning if the `Octave:str-to-num` warning is enabled. By default, the `Octave:str-to-num` warning is disabled.

`Octave:undefined-return-values`

  If the `Octave:undefined-return-values` warning is disabled, print a warning if a function does not define all the values in the return list which are expected. By default, the `Octave:undefined-return-values` warning is enabled.

`Octave:variable-switch-label`

  If the `Octave:variable-switch-label` warning is enabled, Octave will print a warning if a switch label is not a constant or constant expression. By default, the `Octave:variable-switch-label` warning is disabled.