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SECTION 05 / 24
foreach visits values in list order
REAL COURSE DATA · BUILD AND UNPACK LISTS
Visit each list occurrence
Follow one occurrence at a time. Choose another input to test empty lists, repeated IDs and different parameters.
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Predict the next change, then step through the list. The original course records remain unchanged.
Count = 0
foreach x in MayIDs:
Count = Count + 1
# Count = 4; x took 3, then 13, then 23, then 25.x is the current element, not automatically a list index. In MayIDs, the first x is 3, even though it occupies the first position. The loop runs once per occurrence, including repeated values. A foreach loop does not need a manual move-to-next-card instruction in this list notation.
If the list is empty, the body runs zero times. Variables used after the loop must have meaningful initial values: Count = 0 works; a variable assigned only inside the loop may remain undefined. A return inside a function ends that function immediately, even if the foreach has more elements left.
The tutorial also permits foreach over rows or cards. The important idea is systematic traversal, not a rule that WHILE is only for cards and FOREACH is only for lists. Avoid changing the list being traversed unless the iteration contract explains how that change behaves.
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For [3,3,25], how many times does Count increase?
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Three times: once for each list position.
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If the input is [], what is Count after the displayed code?
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Zero. It was initialized before the loop, and no iteration changes it.
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Does foreach x in [3,13] make x equal to 1 and then 2?
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No. x becomes 3 and then 13. Positions and values are different.