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    <p>There are probably other ways to do it, but this code works. You did a good job on the algorithm in <code>reverse()</code>. In the <code>reverse()</code> function, the only changes were formatting of the code and adding the more complete diagnostic printing. The <code>main()</code> function was rewritten to read a line using <code>fgets()</code> and then parse that line. If nothing else, doing that makes it easier to report erroneous input (though the code does not report properly on lines that do not contain a semicolon, for example, and the code assumes that the <code>k</code> items at a time is always a single digit, and it does not insist on single digits separated by commas, and ...).</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;string.h&gt; static void reverse(char str[], int p) { int i, j, k, a, b; int len = strlen(str); printf("Input: %d: %s (%d)\n", len, str, p); a = 0; b = p - 1; while (1) { i = a; j = b; while (j &gt; i) { k = str[i]; str[i] = str[j]; str[j] = k; i++; j--; } a = a + p; b = b + p; if (b &gt;= len) break; } printf("Output: "); for (i = 0; i &lt; len - 1; i++) printf("%c, ", str[i]); if (i == len - 1) printf("%c", str[i]); printf("\n"); } int main(int argc, char* argv[]) { if (argc != 2) { printf("Wrong number of arguments\n"); exit(1); } FILE *fp = fopen(argv[1], "r"); if (fp == NULL) { printf("File can't be openend:\n"); exit(1); } char line[100]; while (fgets(line, sizeof(line), fp) != 0) { printf("Read: %s", line); fflush(0); char str[100]; int k = 0; int i = 0; char c; while ((c = line[i++]) != '\0') { if (c == ';') break; else if (c == ',') continue; else str[k++] = c; } str[k] = '\0'; printf("String: %s\n", str); if (c == ';') { c = line[i]; int p = c - '0'; reverse(str, p); } } fclose(fp); return 0; } </code></pre> <h3>Sample input data</h3> <pre><code>1,2,3,4,5;2 1,2,3,4,5,6,7;3 1,2,3,4,5,6,7,8,9;2 1,2,3,4,5,6,7,8,9;3 1,2,3,4,5,6,7,8,9;4 1,2,3,4,5,6,7,8,9;5 </code></pre> <h3>Sample output</h3> <pre><code>Read: 1,2,3,4,5;2 String: 12345 Input: 5: 12345 (2) Output: 2, 1, 4, 3, 5 Read: 1,2,3,4,5,6,7;3 String: 1234567 Input: 7: 1234567 (3) Output: 3, 2, 1, 6, 5, 4, 7 Read: 1,2,3,4,5,6,7,8,9;2 String: 123456789 Input: 9: 123456789 (2) Output: 2, 1, 4, 3, 6, 5, 8, 7, 9 Read: 1,2,3,4,5,6,7,8,9;3 String: 123456789 Input: 9: 123456789 (3) Output: 3, 2, 1, 6, 5, 4, 9, 8, 7 Read: 1,2,3,4,5,6,7,8,9;4 String: 123456789 Input: 9: 123456789 (4) Output: 4, 3, 2, 1, 8, 7, 6, 5, 9 Read: 1,2,3,4,5,6,7,8,9;5 String: 123456789 Input: 9: 123456789 (5) Output: 5, 4, 3, 2, 1, 6, 7, 8, 9 </code></pre> <h3>Debugging the original</h3> <p>If you have to debug the original code, then the diagnostic printing in <code>reverse()</code> that I added would help there. In the <code>main()</code> code, you'd need to think about echoing each character as it arrives, and perhaps printing the <code>str</code> array. You'd need to be careful there because you filled it with zeros. You might use <code>printf("String: %.*s\n", k, str);</code> to limit the output to the first <code>k</code> characters of <code>str</code>.</p>
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