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Conditional Statements: if and switch

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• IF-statement

  • The syntax of the IF-statement is:

    IF: if expression1 [then] THEN-body IF-ELSE: if expression1 THEN-body [else] ELSE-body Cascaded IF-ELSE: if expression1 THEN-body elseif expression2 ELSE-IF-body [else] ELSE-body

    • The words then and else is optional !!!

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  • Execution of the IF statement
    1. The expression expression₁ (following if) is evaluated.

    2. If result is true, the THEN-body is evaluated

    3. Otherwise, (if available) the ELSE-body is evaluated

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  • NOTE:

    Always use { ... } to enclose all expressions and bodies of the if statement !!! (Because, you need to make a single word and more importantly, you must learn to prevent evaluation before passing in the arguments to be evaluated - this will become important in the while statement that repeatedly evaluates an expression !) )

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  • Examples

    set x 4 if { $x == 4 } {puts {x is 4}} {puts {x is not 4}} Error: need continuation char ! if { $x == 4 } {puts {x is 4}} {puts {x is not 4}} Solution using continuation char: if { $x == 4 } \ {puts {x is 4}} \ {puts {x is not 4}} This is OK, open brace will make NS keep on reading input... if { $x == 4 } { puts {x is 4} } { puts {x is not 4} } Multiple statments in body if { $x == 4 } \ {puts {x is 4} puts {One more statement}} \ {puts {x is not 4} puts {One more statement}}

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• SWITCH-statement

  • The syntax of the SWITCH-statement is:

    Syntax 1: unquoted switch STRING match-pattern1 body1 .... match-patternn bodyn Syntax 2: quoted switch STRING { match-pattern1 body1 .... match-patternn bodyn }

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  • Execution of the IF statement
    1. The string STRING is compared to a set of patterns - one at a time.

    2. When a pattern matches the string STRING, the code associated with that pattern will be evaluated.

    3. If the last pattern argument is the string default, that pattern will match any string.

       This guarantees that some set of code will be executed no matter what the contents of string are.

    4. If there is no default argument, and none of the patterns match string, then the switch command will return an empty string.

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  • The difference of the quoted and unquoted version is:

    In the quoted version, no substitutions are performed on the matching strings (because { ... } prevents substitution) Substitutions will be performed in the matching strings when you use the UNquoted version

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  • Example: quoted version of switch (this is probably the easiest syntax to use)

    set z abc switch $x { $z { puts "MATCH \$z." } ONE { puts "MATCH ONE." } default { puts "$x is NOT A MATCH" } } Try: set x {$z} --> MATCH $z set x "ONE" --> MATCH ONE set x "TWO" --> NOT A MATCH set x "abc" --> NOT A MATCH

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  • Example: UNQUOTED version of switch (the matching patterns are substituted !)

    set z abc switch $x \ $z { puts "MATCH case \$z ($z)." } \ {$z} { puts "MATCH case {\$z} (\$z literally)." } \ ONE { puts "MATCH case ONE." } \ default { puts "$x is NOT A MATCH" } Try: set x {$z} --> MATCH case {$z} ($z literally) set x "ONE" --> MATCH case ONE set x "TWO" --> NOT A MATCH set x "abc" --> MATCH case $z (abc)

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• Further reading
  • If: click here
  • Switch: click here