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PODynamic Array in C - realloc
 

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    <p>Before I start</p> <ol> <li><p>I searched the "Questions with similar titles" and although I found some very useful information I just can't seem to get it to work for me.</p></li> <li><p>This is related to homework. Although, not the actual project itself. I have that completed I'm just porting this from java to C so I can test with my professors unit testing framework.</p></li> </ol> <p>Ok, Dynamically allocated arrays. I know how to build them, but not how to grow them.</p> <p>for example I have the following interface..</p> <pre><code>void insertVertex( vertex p1, vertex out[], int *size); </code></pre> <p>This method takes a vertex and stores it into the out array. After storing the vertex I increase the count of length for future calls.</p> <p>p1 - is the vertex i'm going to add.</p> <p>out[] - is the array I need to store it in (which is always full)</p> <p>length - the current length</p> <p>Vertex is defined as..</p> <pre><code>typedef struct Vertex{ int x; int y; } Vertex; </code></pre> <p>This is what I'm using in java..</p> <pre><code>Vertex tempOut = new Vertex[size +1]; //Code to deep copy each object over tempOut[size] = p1; out = tempOut; </code></pre> <p>This is what I believed I could use in c..</p> <pre><code>out = realloc(out, (*size + 1) * sizeof(Vertex)); out[(*size)] = p1; </code></pre> <p>However, I keep on receiving an error message that the object was not allocated dynamically.</p> <p>I found a solution that will resolve this.. Instead of using Vertex* I was going to switch to Vertex** and store pointers vs. vertex. However, after switching everything over I found out that I over looked the fact that the unit test will be providing me a Vertex out[] that everything has to be stored in.</p> <p>I have also tried the following with no luck.</p> <pre><code>Vertex* temp = (Vertex *)malloc((*size + 1) * sizeof(Vertex)); for(int i = 0; i &lt; (*size); i++) { temp[i] = out[i]; } out = temp; </code></pre> <p>However, no matter what I do when I test after both of these the array returned has not changed.</p> <p>Any help is appreciated.</p> <p><strong>Update - Requested information</strong></p> <p>out - is defined as an array of Vertex (Vertex out[])</p> <p>It is originally built with the number of vertex in my polygon. For example.</p> <p>out = (Vertex *)malloc(vertexInPolygon * sizeof(Vertex))</p> <p>Where vertexInPolygon is an integer of the number of vertex in the polygon.</p> <p>length was a typo that should have been size. </p> <p>Size is an integer pointer</p> <pre><code>int *size = 0; </code></pre> <p>Each time a vertex is in the clipping plane we add it to the array of vertex and increase the size by one.</p> <p><strong>Update</strong></p> <p>Thanks everyone for the help. To better explain myself I came up with a short program to show what I'm trying to do.</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; typedef struct Vertex { int x, y; } Vertex; void addPointerToArray(Vertex v1, Vertex out[], int *size); void addPointerToArray(Vertex v1, Vertex out[], int *size) { int newSize = *size; newSize++; out = realloc(out, newSize * sizeof(Vertex)); out[(*size)] = v1; // Update Size *size = newSize; } int main (int argc, const char * argv[]) { // This would normally be provided by the polygon int *size = malloc(sizeof(int)); *size = 3; // Build and add initial vertex Vertex *out = (Vertex *)malloc((*size) * sizeof(Vertex)); Vertex v1; v1.x = 1; v1.y =1; Vertex v2; v2.x = 2; v2.y =2; Vertex v3; v3.x = 3; v3.y =3; out[0] = v1; out[1] = v2; out[2] = v3; // Add vertex // This should add the vertex to the last position of out // Should also increase the size by 1; Vertex vertexToAdd; vertexToAdd.x = 9; vertexToAdd.y = 9; addPointerToArray(vertexToAdd, out, size); for(int i =0; i &lt; (*size); i++) { printf("Vertx: (%i, %i) Location: %i\n", out[i].x, out[i].y, i); } } </code></pre>
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    1. COWhat is `out` defined as? What are `length` and `size`?
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    2. COYou said `realloc` complained that it wasn’t allocated dynamically. So... was it? You have to use `malloc` first in order to be able to use `realloc` later.
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    3. COCan you also show the definition of `length` or `size` before it's passed to `insertVertex()`
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