Exploring Expect

Lists

In the proc command, the second argument was a list of variables.

proc fib {ult pen n} {

The parameter list is just a string containing the characters, "u“, "l“, "t“, " “, "p“, "e“, "n“, " “, and "n“. Intuitively, the string can also be thought of as a list of three elements: ult, pen, and n. The whitespace just serves to separate the elements.

Lists are very useful, and Tcl provides many commands to manipulate them. For example, llength returns the length of a list.^[10]

tclsh> llength "a b c"
3
tclsh> llength ""
0
tclsh> llength [llength "a b c"]
1

In the next few sections, I will describe more commands to manipulate lists.

Selecting Elements Of Lists

The lindex and lrange commands select elements from a list by their index. The lindex command selects a single element. The lrange command selects a set of elements. Elements are indexed starting from zero.^[11]

tclsh> lindex "a b c d e" 0
a
tclsh> lindex "a b c d e" 2
c
tclsh> lrange "a b c d e" 0 2
a b c
tclsh> llength [lrange "a b c d e" 0 2]
3

You can step through the members of a list using an index and a for loop. Here is a loop to print out the elements of a list in reverse.

for {set i [expr [llength $list]−1]} {$i>=0} {incr $i −1} {
    puts [lindex $list $index]
}

Iterating from front to back is much more common than the reverse. In fact, it is so common, there is a command to do it called foreach. The first argument is a variable name. Upon each iteration of the loop, the variable is set to the next element in the list, provided as ...