09-24-2002
Yes it works. I actually stumbled upon this a few months ago.
I once pondered why this was. I get the same output whether I use > or >> to write out the file if it is outside the loop.
I believe that it is because the "> file.3" is outside of the loop so
ALL of the grepped data goes out together and is written ONCE and not each time the loop runs.
I believe that this is a little faster than writing each time the loop finishes, especially with 2 large files. I only had about 200 lines for file1, but file.2 had almost 1000 lines to be searched.
You would be right if I had put the > file.3 inside the loop, it would overwrite the file each time instead of appending as with >> file.3 would do.
So both of these are the same.
_________________________
for name in `cat file.1 `
do
grep $name file.2
done > file.3
_________________________
for name in `cat file.1 `
do
grep $name file.2 >> file.3
done
_________________________
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LEARN ABOUT DEBIAN
foreach
foreach(3tcl) Tcl Built-In Commands foreach(3tcl)
__________________________________________________________________________________________________________________________________________________
NAME
foreach - Iterate over all elements in one or more lists
SYNOPSIS
foreach varname list body
foreach varlist1 list1 ?varlist2 list2 ...? body
_________________________________________________________________
DESCRIPTION
The foreach command implements a loop where the loop variable(s) take on values from one or more lists. In the simplest case there is one
loop variable, varname, and one list, list, that is a list of values to assign to varname. The body argument is a Tcl script. For each
element of list (in order from first to last), foreach assigns the contents of the element to varname as if the lindex command had been
used to extract the element, then calls the Tcl interpreter to execute body.
In the general case there can be more than one value list (e.g., list1 and list2), and each value list can be associated with a list of
loop variables (e.g., varlist1 and varlist2). During each iteration of the loop the variables of each varlist are assigned consecutive
values from the corresponding list. Values in each list are used in order from first to last, and each value is used exactly once. The
total number of loop iterations is large enough to use up all the values from all the value lists. If a value list does not contain enough
elements for each of its loop variables in each iteration, empty values are used for the missing elements.
The break and continue statements may be invoked inside body, with the same effect as in the for command. Foreach returns an empty string.
EXAMPLES
This loop prints every value in a list together with the square and cube of the value:
set values {1 3 5 7 2 4 6 8} ;# Odd numbers first, for fun!
puts "Value Square Cube" ;# Neat-looking header
foreach x $values { ;# Now loop and print...
puts " $x [expr {$x**2}] [expr {$x**3}]"
}
The following loop uses i and j as loop variables to iterate over pairs of elements of a single list.
set x {}
foreach {i j} {a b c d e f} {
lappend x $j $i
}
# The value of x is "b a d c f e"
# There are 3 iterations of the loop.
The next loop uses i and j to iterate over two lists in parallel.
set x {}
foreach i {a b c} j {d e f g} {
lappend x $i $j
}
# The value of x is "a d b e c f {} g"
# There are 4 iterations of the loop.
The two forms are combined in the following example.
set x {}
foreach i {a b c} {j k} {d e f g} {
lappend x $i $j $k
}
# The value of x is "a d e b f g c {} {}"
# There are 3 iterations of the loop.
SEE ALSO
for(3tcl), while(3tcl), break(3tcl), continue(3tcl)
KEYWORDS
foreach, iteration, list, looping
Tcl foreach(3tcl)