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English
An empty string is a special string with a length of 0. When using strncmp to compare two strings, the presence of an empty string may affect the comparison results, especially in the following scenarios:
One string is empty
If strncmp is used to compare an empty string with a non-empty string, the return value is usually negative (because an empty string is considered lexicographically smaller than any non-empty string). For example:
<span><span><span class="hljs-variable">$result</span></span><span> = </span><span><span class="hljs-title function_ invoke__">strncmp</span></span><span>(</span><span><span class="hljs-string">''</span></span>, </span><span><span class="hljs-string">'hello'</span></span>, </span><span><span class="hljs-number">5</span></span>); </span><span><span class="hljs-comment">// The result will be negative</span></span><span>
</span></span>Therefore, when one string is empty, we need to carefully consider how to handle this situation.
Both strings are empty
When both strings are empty, strncmp will return 0 because they are considered equal in lexicographical order.
<span><span><span class="hljs-variable">$result</span></span> = </span><span><span class="hljs-title function_ invoke__">strncmp</span></span>(</span><span><span class="hljs-string">''</span></span>, </span><span><span class="hljs-string">''</span></span>, </span><span><span class="hljs-number">5</span></span>); </span><span><span class="hljs-comment">// The result is 0</span></span><span>
</span></span>The specified comparison length is 0
If the comparison length is set to 0, strncmp will compare “the first 0 characters” of the two strings. Even if the input strings are empty, strncmp will still consider them equal and return 0.
<span><span><span class="hljs-variable">$result</span></span> = </span><span><span class="hljs-title function_ invoke__">strncmp</span></span>(</span><span><span class="hljs-string">'abc'</span></span>, </span><span><span class="hljs-string">'def'</span></span>, </span><span><span class="hljs-number">0</span></span>); </span><span><span class="hljs-comment">// The result is 0</span></span><span>
</span></span>This is something that requires special attention in certain cases.
To avoid potential issues caused by empty strings, we can adopt the following strategies:
Check for empty strings
Before performing a string comparison, first check whether the string is empty. If it is, you can decide—based on business logic—whether to return a default value or skip the comparison entirely. For example:
<span><span><span class="hljs-keyword">if</span></span> (</span><span><span class="hljs-keyword">empty</span></span>(</span><span><span class="hljs-variable">$str1</span></span>) || </span><span><span class="hljs-keyword">empty</span></span>(</span><span><span class="hljs-variable">$str2</span></span>)) {
</span><span><span class="hljs-comment">// Special logic for handling empty strings</span></span><span>
</span><span><span class="hljs-keyword">echo</span></span> </span><span><span class="hljs-string">"One of the strings is empty\n"</span></span><span>;
} </span><span><span class="hljs-keyword">else</span></span> {
</span><span><span class="hljs-variable">$result</span></span> = </span><span><span class="hljs-title function_ invoke__">strncmp</span></span>(</span><span><span class="hljs-variable">$str1</span></span>, </span><span><span class="hljs-variable">$str2</span></span>, </span><span><span class="hljs-number">5</span></span>);
</span><span><span class="hljs-keyword">echo</span></span> </span><span><span class="hljs-variable">$result</span></span><span>;
}
</span></span>Set default values
If a string may be empty, you can assign it a default value before comparison to avoid issues caused by empty strings. For example:
<span><span><span class="hljs-variable">$str1</span></span> = </span><span><span class="hljs-variable">$str1</span></span> ?: </span><span><span class="hljs-string">'default'</span></span>;
</span><span><span class="hljs-variable">$str2</span></span> = </span><span><span class="hljs-variable">$str2</span></span> ?: </span><span><span class="hljs-string">'default'</span></span>;
</span><span><span class="hljs-variable">$result</span></span> = </span><span><span class="hljs-title function_ invoke__">strncmp</span></span>(</span><span><span class="hljs-variable">$str1</span></span>, </span><span><span class="hljs-variable">$str2</span></span>, </span><span><span class="hljs-number">5</span></span>);
</span></span>In this example, if $str1 or $str2 is empty, they will be replaced with the string 'default' before comparison.
Choose an appropriate comparison length
In real-world development, choose a reasonable comparison length when comparing strings. If the length is too short, the comparison may stop early and lead to inaccurate results. This is particularly important when empty strings or partially empty strings are involved, where the length should be chosen based on actual needs.
Use the strlen function to avoid unnecessary comparisons
Sometimes we only care about whether the strings actually contain content, rather than comparing them entirely. In such cases, we can first check the string length before deciding whether to proceed with comparison:
<span><span><span class="hljs-keyword">if</span></span> (</span><span><span class="hljs-title function_ invoke__">strlen</span></span>(</span><span><span class="hljs-variable">$str1</span></span>) > </span><span><span class="hljs-number">0</span></span> && </span><span><span class="hljs-title function_ invoke__">strlen</span></span>(</span><span><span class="hljs-variable">$str2</span></span>) > </span><span><span class="hljs-number">0</span></span>) {
</span><span><span class="hljs-variable">$result</span></span> = </span><span><span class="hljs-title function_ invoke__">strncmp</span></span>(</span><span><span class="hljs-variable">$str1</span></span>, </span><span><span class="hljs-variable">$str2</span></span>, </span><span><span class="hljs-number">5</span></span>);
</span><span><span class="hljs-keyword">echo</span></span> </span><span><span class="hljs-variable">$result</span></span><span>;
} </span><span><span class="hljs-keyword">else</span></span> {
</span><span><span class="hljs-keyword">echo</span></span> </span><span><span class="hljs-string">"One of the strings is empty, skipping comparison.\n"</span></span><span>;
}
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