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In the example in Segment 385, the value of the parameter,
index, becomes 1, 2, 3, and 4, with the
statement in the block evaluated each time:
| Iteration | Value of index | New value assigned to sum |
| 1 | 1 | 0 $+$ 570 $=$ 570 |
| 2 | 2 | 570 $+$ 720 $=$ 1290 |
| 3 | 3 | 1290 $+$ 640 $=$ 1930 |
| 4 | 4 | 1290 $+$ 640 $=$ 2650 |
You may be more comfortable with the index-based approach, because the
principal feature of Array instances is that you can get at them
using a parameter as an index. On the other hand, using a parameter as an
index, rather than as an element, produces a bulkier program, and hence a more
error-prone program.