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From: "Robert Martin" <rmartin@oma.com>
Subject: Re: Software landmines (loops)
Date: 1998/09/02
Message-ID: <6skgn4$3gq$1@hirame.wwa.com>#1/1
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Matthew Heaney wrote in message ...
>"Robert Martin" <rmartin@oma.com> writes:
>
>> >So it's the structured approach which causes the redundancy, becuase Get
>> >must be called twice.
>>
>> Consider:
>>
>>    do {
>>       Get(N);
>>       if (N)
>>         <process N>
>>    } while (N);
>>
>> Structured programming allows the loop condition to be either at the top
or
>> the bottom.
>
>The thing I dislike about the above code fragment is that N gets tested
>twice.

For good reason.  There are two independent program states that depend upon
N.  One is whether or not we call process, and the other is whether or not
the loop exits.

Now, we could combine them as:

    for(;;)
    {
       Get(N);
       if (N)
         <process N>
       else
         break;
    }

This is certainly more efficient; and if efficiency is what you need then
you'd better do this.

However, the efficiency costs us something.  It couples the two independent
program states.  This increased coupling makes it difficult to make changes
that are independent of N.

For example, suppose I need to flush some buffers at the end of each
iteration.  I do this by calling 'flush'.   I must write this as:

    for(;;)
    {
       Get(N);
       if (N)

         <process N>
         flush();
       }
       else {
         break;
         flush();
       }
    }

Whereas if I had lived with the double test:

    do {
       Get(N);
       if (N) {
         <process N>
       }
       flush();
    } while (N);


Robert C. Martin    | Design Consulting   | Training courses offered:
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