#include <stdlib.h>
  int atoi( const char *str );

The atoi() function converts str into an integer, and returns that integer. str should start with some sort of number, and atoi() will stop reading from str as soon as a non-numerical character has been read. For example,

   i = atoi( "512.035" );             

would result in i being set to 512.

You can use (Standard C I/O) sprintf() to convert a number into a string.