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copied!<p>For understanding the difference, we need to understand two different <em>contexts</em>.</p> <ul> <li>In <em>value</em> contexts, the name of an array of type <code>T</code> is equivalent to a pointer to type <code>T</code>, and is equal to a pointer to the array's first element.</li> <li>In <em>object</em> contexts, the name of an array of type <code>T</code> does not reduce to a pointer.</li> </ul> <p>What is object context?</p> <p>In <code>a = b;</code>, <code>a</code> is in object context. When you taken the address of a variable, it's used in object context. Finally, when you use <code>sizeof</code> operator on a variable, it's used in object context. In all other cases, a variable is used in value context.</p> <p>Now that we have this knowledge, when we do:</p> <pre><code>void f(int arr[4]); </code></pre> <p>It is <em>exactly</em> equivalent to</p> <pre><code>void f(int *arr); </code></pre> <p>As you found out, we can omit the size (4 above) from the function declaration. This means that you can't know the size of the "array" passed to <code>f()</code>. Later, when you do:</p> <pre><code>int a[]={1,2,3,4}; f(a); </code></pre> <p>In the function call, the name <code>a</code> is in value context, so it reduces to a pointer to <code>int</code>. This is good, because <code>f</code> expects a pointer to an <code>int</code>, so the function definition and use match. What is passed to <code>f()</code> is the pointer to the first element of <code>a</code> (<code>&a[0]</code>).</p> <p>In the case of</p> <pre><code>int a[]={1,2,3,4}; int b[4] = a; </code></pre> <p>The name <code>b</code> is used in a object context, and does not reduce to a pointer. (Incidentally, <code>a</code> here <em>is</em> in a value context, and reduces to a pointer.)</p> <p>Now, <code>int b[4];</code> assigns storage worth of 4 <code>int</code>s and gives the name <code>b</code> to it. <code>a</code> was also assigned similar storage. So, in effect, the above assignment means, "I want to make the storage location the same as the previous location". This doesn't make sense.</p> <p>If you want to <em>copy</em> the contents of <code>a</code> into <code>b</code>, then you could do:</p> <pre><code>#include <string.h> int b[4]; memcpy(b, a, sizeof b); </code></pre> <p>Or, if you wanted a pointer <code>b</code> that pointed to <code>a</code>:</p> <pre><code>int *b = a; </code></pre> <p>Here, <code>a</code> is in value context, and reduces to a pointer to <code>int</code>, so we can assign <code>a</code> to an <code>int *</code>.</p> <p>Finally, <em>when initializing an array</em>, you can assign to it explicit values:</p> <pre><code>int a[] = {1, 2, 3, 4}; </code></pre> <p>Here, a has 4 elements, initialized to 1, 2, 3, and 4. You could also do:</p> <pre><code>int a[4] = {1, 2, 3, 4}; </code></pre> <p>If there are fewer elements in the list than the number of elements in the array, then the rest of the values are taken to be 0:</p> <pre><code>int a[4] = {1, 2}; </code></pre> <p>sets <code>a[2]</code> and <code>a[3]</code> to 0.</p>

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