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    <p>It's easiest if you decompose the problem into the small parts (as said above). That is an important part of engineering in general (whether software or otherwise).</p> <p>The problems are roughly:</p> <ul> <li>Read the file</li> <li>output the result</li> <li>count the number of items in a line</li> <li>extract the number at the end of the line</li> <li>Store the file</li> <li>sort the file by number of items in a line</li> <li>sort the file by number at the end of the file</li> </ul> <p>After that it is simple!</p> <pre><code>#include &lt;string.h&gt; #include &lt;stdlib.h&gt; #include &lt;stdio.h&gt; #define BUFFER_SIZE 100 #define MAX_NODES 1000 int count_spaces( char * s ) { int spaces = 0; int i; for( i = 0; i &lt; strlen( s ) ; i ++ ) { if( s[ i ] == ' ' ) { spaces++; } i++; } return spaces; } /* sort by number of spaces */ int sort1( char *a, char * b ) { int spaces_a = count_spaces( a ); int spaces_b = count_spaces( b ); if( spaces_a &lt; spaces_b ) return -1; if( spaces_a &gt; spaces_b ) return 1; return 0; } int get_digits( char *s ) { char buffer[ 10 ]; memset( buffer, 0, 10 ); int state = 0; int i = 0, j = 0; int number = 0; for( i = 0; i &lt; strlen( s ) ; i ++ ) { if( state == 0 &amp;&amp; s[ i ] == '[' ) { state = 1; } if( state == 1 &amp;&amp; s[ i ] != ']' ) { buffer[ j ] = s[ i ]; j++; } if( state == 1 &amp;&amp; s[ i ] == ']' ) { break; } } sscanf( buffer, "%d", &amp;number ); } /* sort by digits in brackets at the end of the line */ int sort2( char *a, char * b ) { int a_num = get_digits( a ); int b_num = get_digits( b ); if( a_num &lt; b_num ) return -1; if( a_num &gt; b_num ) return 1; return 0; } int main() { FILE *infile; char line[ BUFFER_SIZE ]; int i = 0; char *nodes[ MAX_NODES ]; if( (infile = fopen( "data.txt", "r" )) == NULL ) { printf( "Error opening file\n" ); return -1; } while( fgets( line, BUFFER_SIZE, infile ) != NULL ) { if( (nodes[ i ] = malloc( strlen( line ) + 1 )) == NULL ) { printf( "Malloc failed\n" ); return -1; } strcpy( nodes[ i ], line ); i++; } // sort by # of items in a line qsort( nodes, i, sizeof( char *), (void *)(&amp;sort1) ); // sort by number qsort( nodes, i, sizeof( char *), (void *)(&amp;sort2) ); /* output results */ i--; do { printf( "%s", nodes[i] ); i--; } while( i &gt;= 0 ); } </code></pre>
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