untitled

210 10 Transforming with Traditional Programming Languages

program, and modified for each record in the health study file. The mean,

variance, and standard deviation are computed from these three values by

well-known formulas.

It is not actually necessary to initialize the three variables to 0. In other

words these three lines in the first part of the program could have been omit-

ted:

$count = 0;

$bmisum = 0;

$bmisumsq = 0;

Perl will automatically set any variable to a standard default value the first

time it is used. For numbers the default initial value is 0. For strings it is the

empty string.

Many commonly occurring statements can be abbreviated. For example,

the statement

$bmisum = $bmisum + $bmi;

can be abbreviated to the more succinct statement

$bmisum += $bmi;

Similar abbreviations are available for all the arithmetic operations such as

subtraction, multiplication, and division, as well as for other kinds of opera-

tion. Incrementing a variable (i.e., increasing it by 1) is so common that it has

its own special operator. As a result, one can abbreviate

$count = $count + 1;

to the more succinct

$count++;

So far the transformation tasks have been on data files in a fixed-width

format. The transformed data have used fields that vary in length. The next

task is to transform a data file with variable-length fields separated from one

another by spaces. This requires a new kind of variable as well as operations

for them. Anarrayorlistis a sequence of values. Perl indicates an array

variable with an initial@character. To transform a data file it must first be

splitinto fields. The result of the splitting operation is an array.