मद्दत:If

Also see Meta page: mw:Help:Extension:ParserFunctions.

This page, Help:Conditional expressions, describes ways to display different results based on checking conditions in a page or template. The parser functions used to evaluate conditional expressions include the function names: #ifexpr, #ifeq, #switch, #if, and #iferror or #ifexist. Each function name has been linked to the explanations below.

Using #ifeq can compare 2 strings or numbers, but #ifexpr can check a math formula or multiple conditions. The #switch function can branch to dozens or hundreds of different paths depending on a value, to act as a case statement to choose among alternatives. Using #if can check to see if a parameter has been passed, or if an expression evaluates as true. Using #iferror can check to see if an expression value triggers an error else shows the value, while #ifexist can check to see if a page name or image/media file exists yet.

Summary of conditional expressions

The quick format of each function is as follows:

• {{#if: test string | value if non-empty | value if empty }} (selects one of two values based on whether the test string is empty)
• {{#ifeq: string 1 | string 2 | value if equal | value if unequal }} (selects one of two values based on whether the test strings are equal – numerically if applicable)
• {{#iferror: test string | value if error | value if correct }} (selects value based on whether the test string generates a parser error)
• {{#ifexpr: expression | value if true | value if false }} (selects value based on evaluation of expression)
• {{#ifexist: page title | value if exists | value if doesn't exist }} (selects value depending on whether a page title exists)
• {{#switch: test | case1 = value for case 1 | ... | default }} (provides alternatives based on the value of the test string)

• {{#expr: expression }} (evaluates the given expression; see Help:Calculation)

The magic words can be used together, in nested combinations, to branch on complex conditions. Some combinations can use tricks based on the interactions between them.

Note that with #if expressions, using a parameter in the form "{{{1}}}" always requires a final bar/pipe "|" in the parameter: {{{1|}}}. If the bar/pipe is omitted, then whenever the parameter 1 is absent, instead of leaving the field blank, the page will use the literal text "{{{1}}}" (as 3 sets of curly braces around a "1"), and the #if will be true unless parameter 1 is passed as an empty string, such as "1=".

Using #ifeq

Using #ifeq can compare 2 strings or numbers (but not numeric expressions: 1+1). The parser function #ifeq compares two values and determines whether they are identical.

{{#ifeq: string 1 | string 2 | value if identical | value if different }}

If both strings are valid numerical values, the strings are compared as numbers, rather than literal text:

{{#ifeq: 01 | 1 | equal | not equal}} → equal

{{#ifeq: x01 | x1 | equal | not equal}} → not equal

{{#ifeq: 2.000 | 002 | equal | not equal}} → equal

{{#ifeq: 2.5 | 2+.5 | equal | not equal}} → not equal (use #ifexpr for arithmetic)

{{#ifeq: {{#expr:10^3}} | 1000 | equal | not equal}} → equal

The comparison is case-sensitive, checking to match capital letters:

{{#ifeq: King | king | equal | not equal}} → not equal

{{#ifeq: {{lc:TopCat}} | topcat |equal|not equal}} → equal

{{#ifeq: {{lc:{{{catname}}} }} | topcat |equal|not equal}}

So, when checking the value of a parameter named "{{{catname}}}" then the function {{lc:___}} can be used to instantly convert to lowercase text, during the comparison. The value of {{{catname}}} will not be changed for later use, instead it is only compared as lowercase letters.

Using #ifexpr

Using #ifexpr can check a math formula or multiple conditions. The parser function #ifexpr evaluates a mathematical or boolean expression and branches depending on the boolean true/false value of the result (where zero means false):

{{#ifexpr: expression | value if true | value if false }}

{{#ifexpr: ( {{{1}}}+{{{2}}} ) * 2.63 > 45 |above 45 |not above 45}}

{{#ifexpr: {{{1}}} > 0 and {{{1}}} < 1.0 or {{#ifeq:{{{decimal}}}| yes}} |is decimal |not decimal}}

The expression result is evaluated exactly in the same manner as for function #expr, with the same operators being available. The output is then evaluated as a boolean expression.

An empty input expression evaluates to false:

{{#ifexpr: | yes | no}} → no

As mentioned above, zero evaluates to false and any nonzero value (such as 6.7) evaluates to true.

Invalid data will diplay an error message. However, function #ifexpr is equivalent to using #ifeq with #expr inside, but flipping the true/false (then/else) clauses:

{{#ifeq: {{#expr: expression }} | 0 | value if false | value if true }}

An invalid or wrong input expression will trigger the true-value part (an error message is treated as an ordinary string; it is not equal to zero, so we get value if false).

{{#ifexpr: = | yes | no }} →

{{#ifeq: {{#expr: = }} |0 | yes | no }} → no

Either or both of the return values may be omitted; no output is given when the appropriate branch is left empty:

{{#ifexpr: 1 > 0 | yes }} → yes

{{#ifexpr: 0 = 0 | yes }} → yes

{{#ifexpr: 1 > 0 | | no}} →

Using #switch

The #switch function can branch to dozens or hundreds of different paths depending on a value, to act as a case statement which chooses among alternatives. A #switch expression is a quick way to handle multiple code values for a parameter; however, the performance slows when more than 100 branches, and common values should be listed higher among the choices, to run 3x-8x faster. In rare cases, a #switch could have over two thousand branches, but it takes time just to scan all the branches, even before comparing the values.

See: Help:Switch parser function, for a full description and examples.

Using #if

Using #if can check to see if a parameter has been passed.

This function evaluates a test string and determines whether or not it is empty. A test string containing only white space is considered to be empty.

{{#if: test string | value if test string is not empty | value if test string is empty (or only white space) }}

{{#if: {{{1|}}} |parameter 1 has data | parameter 1 is empty}}

{{#if: {{{xx|}}} |parameter xx passed | parameter xx omitted}}

Using #iferror

Using #iferror can check to see if an expression value triggers an error, to then do something for that condition, else it shows the value which was being tested.

Using #ifexist

The function #ifexist can check to see if a page name or image/media file exists yet. It is extremely fast, but has been limited to 500 instances per page.

Using #expr

Using #expr can evaluate a mathematical or boolean expression, to augment the comparisons, and handle error messages.

{{#expr: ( {{{1}}}+{{{xshift}}} - 6 ) * 18.4}}

{{#expr: ln(7)^3 - abs(-0.344) + floor(5/3) round 3 }}

{{#expr: {{{n}}}>0 and {{{n}}}<1.0 }}

For more examples, see: Help:Calculation.

For the use of these functions in tables, see: WP:Conditional tables.