Duplicate values

• Array sum program:

  i := 0; s := 0;
  
  while i ? n do
  { s := s+b[i]; i := i+1 }

• b is unchanged inside loop

• Problem: at loop head

  –88 ? b[n – 1] ? 99
  
  –556 ? sum(b) ? 539

• Reason: more samples inside loop

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