无符号数与有符号数常常出现在程序中的代码失误（一）

buggyCodes_000.c

/*
 * code来自《深入理解计算机系统》Page-52
 * The below function which called within sum_element(arr, 0) will occur a RAM's trouble to you
 * should be invoked within 2 arguments, an array and a unsigned integer "length".
 *
 * Analysis:
 * If the argument length is assigned with ZERO, namely return 0.0 under human's comprehension.
 * As an opposite ending, however in fact, length is unsigned. Such that length-1 will be treated
 * as length+((unsigned)(-1)), UMax. Even worse is the logical operator "<", whose operands are the
 * unsigned integer UMax and an integer i, it always return true - !
 * All right, the reason is found. RAM is occured with an overflow.
 *
 */
float sum_element(float arr[], unsigned length)
{
  int i;
  float result;

  for (i=0; i<=length-1; i++) {
    result += a[i];
  };
  return result;
}

/* Here is a correct version */
float sum_element_v1(float arr[], unsigned length)
{
  int i;
  float result;

  if (length == 0) {      // if length is 0, return 0.0
    return 0.0;
  }
  for (i=0; i<=length-1; i++) {
    result += a[i];
  };
  return result;
}

/* Here is 2nd correct version */
float sum_element_v2(float arr[], int length)   // let length be a integer encoded by two's-complement
{
  int i;
  float result;

  for (i=0; i<=length-1; i++) {
    result += a[i];
  };
  return result;
}

/* Here is 3rd correct version */
float sum_element_v3(float arr[], unsigned length)
{
  int i;        // "unsigned i;" is a better declaration for logical operators because of the unsigend argument length.
  float result;

  for (i=0; i<length; i++) {
    result += a[i];
  };
  return result;
}

// Any correct one?
// Welcome to submit and hava a discussion - !