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Lektion 5. While cycle
Schwierigkeit:
Aufgabe«Suche nach Vielfachem zweier Zahlen»
💻 Python
In der Baufirma "Stroyprogress" standen die Logistikingenieure vor einem Problem. Zement wird alle n Tage zur Baustelle gebracht und die Bewehrung — alle m Tage. Um die Arbeit zu optimieren und zu wissen, an welchen Tagen beide Materialien auf der Baustelle entladen werden, müssen sie den Tag finden, an dem dies das nächste Mal geschieht. Dieser Tag entspricht dem kleinsten gemeinsamen Vielfachen (kgV) der Lieferzeiträume.
rnIhre Aufgabe — Schreiben Sie ein Programm, das die Nummer des nächstgelegenen Tages ermittelt, an dem Zement und Bewehrung gleichzeitig auf der Baustelle eintreffen.
Eingabeformat
In einer Zeile werden zwei natürliche Zahlen N und M durch eine Lücke eingegeben; die Häufigkeit der Lieferung von Zement und Bewehrung. (1 ≤ n, m ≤ 100000).
Ausgabeformat
eine Zahl & mdash; Die Nummer des Tages, an dem die folgende gemeinsame Lieferung stattfinden wird (ihr kleinstes gemeinsames Vielfaches).
Beispiel
Eingabe
6
8
Ausgabe
Das kleinste gemeinsame Vielfache (kgV): 24
Hinweis
While in python
In the Python programming language, the while loop construction can have an optional else block. This block is executed only when the loop completes in its natural way, that is, when its condition becomes false, and not as a result of a forced interruption using the break operator.
Syntax:
while condition:
# the body of the loop
# ...the code that runs on each iteration...
else:
# else block
# ...the code that is executed if the loop ended without a break...
You can think of the else block as a "plan B": it is triggered if the main goal within the loop (which is usually completed through a break) has not been achieved.
Usage example: Searching for an element
Consider a classic problem where we search for a number in a list. If a number is found, we immediately break the loop using break. If we have checked all the elements and still haven't found what we are looking for, the else block is executed.
numbers = [1, 2, 3, 4, 5]
target = 8 # Let's try to find a number that is not in the list.
index = 0
while index < len(numbers):
if numbers[index] == target:
print(f"Success! {target} found at {index}.")
break # Goal reached, exit the loop
index += 1
else:
# This block will be executed because break was not called
print(f"The search is completed. {target} not found in the list.")
How it works:
- The
whileloop is executed as long as theindex < len(numbers)condition is true. - Inside the loop, we check whether the current element is equal to the desired one (
target). - If we find a value, a success message is displayed and the loop is immediately interrupted by the
breakstatement. Theelseblock is ignored in this case. - If the loop reaches the end (checked all the elements), it means that
breakhas never worked. Theindex < len(numbers)condition becomes false, and then theelseblock is executed, reporting a failed search.
A useful tip: Using while...else helps to avoid "flagged" variables. Without this block, the code would look like this:
# Alternative version with "flag"
numbers = [1, 2, 3, 4, 5]
target = 8
found = False # Flag variable
index = 0
while index < len(numbers):
if numbers[index] == target:
found = True
break
index += 1
if found:
print(f"Success! {target} found.")
else:
print(f"Search completed. {target} not found in the list.")
As you can see, the while...else construction makes the code cleaner, more concise, and more "Pythonic", eliminating the need to manually control the flag.
If break is missing
If there is no break operator in the body of the while loop, then the else block will always be executed after the loop ends.
count = 0
while count < 3:
print(f"Iteration number {count}")
count += 1
else:
print("The loop ended naturally, so the else block is executed.")
Here, the while loop runs until count is less than 3. As soon as count becomes 3, the loop condition becomes false, and control is transferred to the else block.
Important nuances
- What about
continue? Thecontinueoperator does not interrupt the loop, but only ends the current iteration ahead of schedule. It does not affect the execution of theelseblock in any way. - If the loop was never executed? If the
whilecondition is initially false, the loop body will not be executed even once. However, theelseblock will still run, since the loop ended "normally" (it was not interrupted viabreak).
value = 10
while value < 5: # The condition is immediately false
print("This text will never be printed")
else:
print("The cycle didn't even start, but it ended normally, so else worked.")
- This also works for
forloops! The same logic applies tofor...elseloops. Theelseblock afterforis executed if the cycle has passed through all the elements and has not been interrupted usingbreak.
Output
The while...else construction is a powerful and elegant Python tool. It is ideal for situations where you need to perform an action only if the loop has completed its work completely without being interrupted. This is especially useful in search algorithms, condition checking, or when working with data streams, where break signals a special event, and else indicates a regular termination without any.
main.py
Test 1
Test 2
Test 3
Test 4
Test 5
Test 6
Test 7
Test 8
Test 9
Test 10
Entwicklerlösung
# Считываем два числа N и M из одной строки
n = int(input())
m = int(input())
# Определяем большее из двух чисел, с него и начнем поиск
if n > m:
greater = n
else:
greater = m
# Запускаем бесконечный цикл, который будет прерван, когда найдется НОК
while True:
# Проверяем, делится ли текущее число 'greater' на n и m без остатка
if (greater % n == 0) and (greater % m == 0):
# Если да, то это и есть НОК. Запоминаем его.
lcm = greater
# Прерываем цикл
break
# Если нет, увеличиваем 'greater' и повторяем проверку на следующей итерации
greater += 1
# Выводим найденное значение НОК
print(f"Наименьшее общее кратное (НОК): {lcm}")
