PREP 2014 Question Authoring - Archived

evaluating a function at multiple values in an array

Re: evaluating a function at multiple values in an array

by Davide Cervone -
Number of replies: 0
Just for fun, I'm going to show you another way to generate this array (there is always more than one way to do something in Perl).

First, there is an easy way to generate consecutive integers, using the .. operator. So

    @a = (1..10);
is the list
     @a = (1,2,3,4,5,6,7,8,9,10);
(You can also do this for letters, so ('a'..'d') is the same as ('a','b','c','d').)

There is also a command called map that takes a code block and an array and performs the code on each element of the array. Within the code, the variable $_ refers to the array element. So

    @x = map {.5*$_} (1..20);
produces the same @x as Gavin's code more efficiently (but harder to read for those not familiar with map).

There is also a looping command foreach that is similar to a for loop, but instead of giving an initial condition, a termination test, and an increment function, you give it an array and it runs the variable through that array. This is similar to map, but you get to specify the name of the loop variable. So, for example, you could do

    @x = ();
    foreach $i (1..20) {
This can sometimes make your loops easier to specify. Note that this works for any array, with any type of entry, so
    foreach $c ('a'..'d') {
      ... do something with $c ...
would let you loop through the letters a through d. Similarly, if you have an array @a and want to do something to the entries in the array, use
    foreach $x (@a) {
      ... do something with $x ...
In situations where you don't actually need to index into the array, this can be very helpful. Also, note that if you set the value of $x within the loop, that will change the value within @a as well, so this is a way of modifying the entries in @a one at a time. Also very useful at times.