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SECTION 04 / 31
Check whether the key exists before reading it
REAL COURSE DATA · KEYS, COUNTS AND CONNECTIONS
Check keys, including a missing key
Follow the dictionary as its keys and values change. Predict each lookup or update before revealing the next step.
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Predict the next key, whether it already exists, and the value that will be stored.
if (isKey(d,k)) {
value = d[k]
}
else {
# No value is stored for this key.
# Decide whether to create an entry or report a missing key.
}With d={7:0}, isKey(d,7) is True and d[7] is 0. isKey(d,99) is False. Do not assume a missing lookup automatically returns 0 or False in this pseudocode. The lab’s key 7 has a constructed zero specifically to demonstrate presence; it is not Aditya’s actual Chemistry mark.
keys(d) tells us the existing keys, not all possible keys. A present key may hold False, 0, [] or {}. None of these values makes the key disappear.
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For d={7:False}, isKey(d,7) True or False?
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True. The key exists even though its stored Boolean is False.
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Does isKey({7:76},76) test whether 76 is a stored value?
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No. It tests for a KEY 76, which is absent here.
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What happens on an empty dictionary membership search?
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No loop body executes and found remains False.