Programming by permutation

<h2 id="example">Example</h2>
<p>For example, the following code sample in <a href="/facts/C_(programming_language)/Ky2No763">C</a> (intended to find and copy a series of digits from a larger string) has several problems:
</p>
#include <stdio.h>
#include <string.h>
#include <ctype.h>

int main(void)
{
    const char* buffer = "123abc";
    char destination[10];
    int i = 0;
    int j = 0;
    int l = strlen(buffer);

while (i < l) {
        if (isdigit(buffer[i])) {
            destination[j++] = buffer[i++];
        }
        ++i;
    }

destination[j] = '\0';
    printf("%s\n", destination);
}

<p>First of all, it doesn't give the right answer. With the given starting string, it produces the output "13", when the correct answer is "123". A programmer who does not recognize the structural problems may seize on one statement, saying "ah, there's an extra increment". The line "++i" is removed; but testing the code results in an infinite loop. "Oops, wrong increment." The former statement is added back, and the line above it is changed to remove the post-increment of variable i:
</p>
    if (isdigit(buffer[i])) {
        destination[j++] = buffer[i];
    }

<p>Testing the code now produces the correct answer, "123". The programmer sighs in contentment: "There, that did it. All finished now." Additional testing with various other input strings bears out this conclusion.
</p><p>Of course, other problems remain. Because the programmer does not take the trouble to fully understand the code, they go unrecognized:
</p>
<ul><li>If the input contains several numbers separated by non-digit characters, such as "123ab456", the destination receives all the digits, concatenated together</li>
<li>If the input string is larger than the destination array, a buffer overflow will occur</li>
<li>If the input string is longer than INT_MAX, the behaviour is undefined as strlen() returns a value of type size_t which is an unsigned integer and may be wider than int.</li>
<li>If char is a signed type and the input string contains characters that are not in the range of 0..UCHAR_MAX after integer promotion, the call to isdigit() has undefined behaviour.</li></ul>
<p>While the solution is correct for a limited set of input strings, it is not fully correct, and since the programmer did not bother to understand the code, the errors will not be discovered until a later testing stage, if at all.
</p><p>Also known as "Trial and Error", "Generate and Test", "Poke and Hope",<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> "The Birdshot Method" and the "Million Monkeys Programming Style".
</p>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Catania, Anthony (1987). ""ever, although there are some algebraic skills, topics on factorization, rational."". Mathematics and Computer Education. 21. University of Michigan: 74. <a href="/wiki/Mathematics_and_Computer_Education" target="_blank">/wiki/Mathematics_and_Computer_Education</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
</ol>

Programming by permutation open-in-new

Programming by permutation