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Nothing to be amazed.
It works since scanf() %d stops when encountering letter on next line, and it pushes back (ungetc()). the character.
If next line is allowed to start with number(not OP...

Is it? But i don't think so.

Read a word. scanf %s does that.
Depending on the word(RUN,SET), you know how many more input to read.

Eg.


char command[21];
while( scanf("%20s",command) != EOF ) {
if(...