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copied!<p>The problem with</p> <pre><code>while(scanf("%d%c",&num,&a) != 2 || a != '\n') { printf("Please enter an integer only : "); while(a != '\n') scanf("%c",&a); } </code></pre> <p>is that if <code>a</code> happens to contain <code>'\n'</code> before the scan, and the scan fails, the inner <code>while</code> loop doesn't run at all. So</p> <ul> <li><p>if the scan failed trying to parse an <code>int</code> from the input stream because the input was e.g. <code>"ab c\n"</code>, the offending input remains in the input stream, the next <code>scanf</code> in the outer <code>while</code> loop control fails parsing an <code>int</code> again, <code>a</code> remains <code>'\n'</code>, repeat.</p></li> <li><p>if an input error occurred before reading a character from the stream into <code>a</code>, the <code>scanf</code> in the outer loop control fails because of a corrupted stream, repeat.</p></li> </ul> <p>In the other version,</p> <pre><code>while(scanf("%d%c",&num,&a) != 2 || a != '\n') { printf("Please enter an integer only : "); if(a == '\n') scanf("%c",&a); else { while(a != '\n') scanf("%c",&a); } } </code></pre> <p>you make at least some progress as long as there is input to be read from the stream, since whatever <code>a</code> contains, you read at least one character from the input stream before attempting the next parsing of an <code>int</code>. It will also result in an infinite loop if the input stream is corrupted/closed/ends prematurely, e.g. if you redirect stdin from an empty file. You can have that loop also output multiple <code>"Please enter an integer only : "</code> messages by giving input like `"a\nb\nc\nd\n".</p> <p>So you should check whether <code>scanf</code> encountered the end of the stream or some other read error before converting anything from the input, and abort in that case:</p> <pre><code>int reads; while(((reads = scanf("%d%c", &num, &a)) != 2 && reads != EOF) || a != '\n') { printf("Please enter an integer only : "); // read at least one character until the next newline do { reads = scanf("%c", &a); }while(reads != EOF && a != '\n'); } </code></pre>

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