4. Control

Sequence commands execute each command one after another.

• set v e command assigns a value e at the second argument to a variable v at the first argument.

• incr v n command increases variable value v by increment n. Its first argument is a variable name and its second optional argument is an increment integer. Negative increment will decrease a variable. Its default increment is 1 if there is no second argument.

• expr e command evaluates math expression with C expression syntax. It concatenates all the arguments to form an input string. The expression may be integer, double, or boolean. The return value is a numeric string. Many basic math functions in the standard C math library are available as built-in. In mCL x is equivalent to $x as a value. However you need to use $foo(0) as an element of an array foo to distinguish a function such as foo(x).

• puts s command shows its argument string to console. There is only one argument of puts. In many cases it is convenitnet to show a variable’s value for simple debugging with puts.

• comment of mCL starts with # in the beginning of a line. It is better to treate comment # as a command because there are some quirks prohibited you from writing comment anywhere. (i.e. no # inside the switch command.)

4.2 Condition

Condition control may select a command to execute according to the value of a variable.

• if command will execute truebody when the boolean-expression is true, otherwise it will execute elsebody. Its body is a group of commands. The else and elseif keyword are optional for if command. boolexpr is an expression returned a boolean value. You can use a variable to represent its value without using the $ substitution.

if {boolexpr} {
	truebody
} else {
	elsebody
}

• The format of a switch command is:

switch option value pattern body ...

4.3 Loop

Loop commands execute a group of commands in iteration. The iteration may terminate after all the elements are traversed or a condition expression becomes true.

• foreach command repeatly executes its body until all the elements in a list have been traversed. Its form is,
  

foreach var list body

foreach {v1 v2} {a b c d e f} {
	puts "($v1 $v2)"

}

foreach v {a b c d} {v1 v2} {1 2 3 4 5 6} {
	puts "($v) ($v1 $v2)"
}

• while command evaluates the boolean-expression, if it is true then execute the body, and then evaluates the boolean-expression again until the boolean-expression is not true. Its syntax is like while statement of C language.
  

while boolexpr body 

• The for command format is,

for initial boolexpr increment body

4.4 Return

return command comes back from a procedure with a value. break command exits from a loop. And continue command will goto the start of a loop to execute the next iteration. For example,

4.5 Exception

Exceptions raise abnormal conditions during the execution of commands.

• catch command caught the exceptions of a command during its execution. Its format is,

catch {command args ... } result

• error command generates an error code. Its first argument is a string that indicates the reason of an error.

catch {...} errmsg
error $errmsg