Why are digraphs still present in C and C ?

Exploring the Purpose of Digraphs in C and C

In the world of programming, digraphs hold a peculiar place in the annals of C99 and C . With syntax such as %:include and <::>, digraphs may strike some as an archaic and enigmatic feature. So, let's delve into the rationale behind their existence.

Digraphs and Keyboard Constraints

The origins of digraphs trace back to a time when programmers often lacked keyboards that supported the full ISO 646 character set. This limitation hindered their ability to represent certain symbols, including the percent sign (%) and the square brackets ([]).

To overcome this challenge, the creators of C devised a clever solution: digraphs. These two-character sequences represented the missing symbols, allowing programmers to work around their hardware constraints without resorting to hex codes or other cumbersome methods.

A Case in Point

Consider the following example in C99:

<code class="c">%:include <stdio.h>

%:ifndef BUFSIZE
 %:define BUFSIZE 512
%:endif

void copy(char d<::>, const char s<::>, int len)
<%
    while (len-- >= 0)
    <%
        d<:len:> = s<:len:>;
    %>
%></code>

In this snippet, the %: digraph stands in place of the percent sign, enabling the #include preprocessor directive. Similarly, <::> represents the square brackets, enclosing the s and d parameters within the copy function's definition.

Without digraphs, programmers using keyboards lacking ISO 646 support would have had to write:

<code class="c">#include <stdio.h>

#ifndef BUFSIZE
 #define BUFSIZE 512
#endif

void copy(char d[__$$__], const char s[__$$__], int len)
__$$__
    while (len-- >= 0)
    __$$__
        d[__$$__len__$$__] = s[__$$__len__$$__];
    __$$__
__$$__</code>

As you can see, this alternative notation is significantly more cumbersome and less readable. Digraphs provide a concise and convenient solution to this problem.

Conclusion

While digraphs may seem like a relic of the past, their historical significance in the development of C99 and C is undeniable. They emerged as a pragmatic solution to hardware limitations, enabling programmers to write code without being constrained by the availability of specific symbols on their keyboards.

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