Умовні оператори

Останнє оновлення 2025-10-12 | Редагувати цю сторінку

Огляд

Питання

  • How can programs do different things for different data?

Цілі

  • Правильно писати програми, які використовують оператори if та else, та прості булеві вирази (без логічних операторів).
  • Trace the execution of unnested conditionals and conditionals inside loops.

Використовуйте команди if, щоб контролювати, чи виконується блок коду.

  • An if statement (more properly called a conditional statement) controls whether some block of code is executed or not.
  • Його синтаксис подібний до синтаксису оператора for:
    • Перший рядок починається з `if’ і закінчується двокрапкою
    • Body containing one or more statements is indented (usually by 4 spaces)

PYTHON 

mass = 3.54
if mass > 3.0:
    print(mass, 'is large')

mass = 2.07
if mass > 3.0:
    print (mass, 'is large')

ВИХІД 

3.54 is large

Умовні оператори часто використовуються всередині циклів.

  • Немає особливого сенсу використовувати умовний оператор, коли ми знаємо точне значення (як у прикладі вище).
  • Але це корисно, коли у нас є колекція для обробки.

PYTHON 

masses = [3.54, 2.07, 9.22, 1.86, 1.71]
for m in masses:
    if m > 3.0:
        print(m, 'is large')

ВИХІД 

3.54 is large
9.22 is large

Використовуйте else для виконання блоку коду, коли умова if не виконується.

  • else можна використовувати після if.
  • Це дозволяє вказати альтернативу для виконання, коли умова гілки if не виконується.

PYTHON 

masses = [3.54, 2.07, 9.22, 1.86, 1.71]
for m in masses:
    if m > 3.0:
        print(m, 'is large')
    else:
        print(m, 'is small')

ВИХІД 

3.54 is large
2.07 is small
9.22 is large
1.86 is small
1.71 is small

Використовуйте elif для визначення додаткових перевірок.

  • May want to provide several alternative choices, each with its own test.
  • Use elif (short for “else if”) and a condition to specify these.
  • Завжди асоціюється з if.
  • Must come before the else (which is the “catch all”).

PYTHON 

masses = [3.54, 2.07, 9.22, 1.86, 1.71]
for m in masses:
    if m > 9.0:
        print(m, 'is HUGE')
    elif m > 3.0:
        print(m, 'is large')
    else:
        print(m, 'is small')

ВИХІД 

3.54 is large
2.07 is small
9.22 is HUGE
1.86 is small
1.71 is small

Conditions are tested once, in order.

  • Python послідовно проходить гілки умовного оператора, перевіряючи кожну по черзі.
  • Отже, порядок має значення.

PYTHON 

grade = 85
if grade >= 90:
    print('grade is A')
elif grade >= 80:
    print('grade is B')
elif grade >= 70:
    print('grade is C')

ВИХІД 

grade is B
  • Does not automatically go back and re-evaluate if values change.

PYTHON 

velocity = 10.0
if velocity > 20.0:
    print('moving too fast')
else:
    print('adjusting velocity')
    velocity = 50.0

ВИХІД 

adjusting velocity
  • Often use conditionals in a loop to “evolve” the values of variables.

PYTHON 

velocity = 10.0
for i in range(5): # execute the loop 5 times
    print(i, ':', velocity)
    if velocity > 20.0:
        print('moving too fast')
        velocity = velocity - 5.0
    else:
        print('moving too slow')
        velocity = velocity + 10.0
print('final velocity:', velocity)

ВИХІД 

0 : 10.0
moving too slow
1 : 20.0
moving too slow
2 : 30.0
moving too fast
3 : 25.0
moving too fast
4 : 20.0
moving too slow
final velocity: 30.0

Створіть таблицю зі значеннями змінних для відстеження виконання програми.

i 0 . 1 . 2 . 3 . 4 .
velocity 10.0 20.0 . 30.0 . 25.0 . 20.0 . 30.0
  • The program must have a print statement outside the body of the loop to show the final value of velocity, since its value is updated by the last iteration of the loop.
Виноска

Compound Relations Using and, or, and Parentheses

Often, you want some combination of things to be true. You can combine relations within a conditional using and and or. Continuing the example above, suppose you have

PYTHON 

mass     = [ 3.54,  2.07,  9.22,  1.86,  1.71]
velocity = [10.00, 20.00, 30.00, 25.00, 20.00]

i = 0
for i in range(5):
    if mass[i] > 5 and velocity[i] > 20:
        print("Fast heavy object.  Duck!")
    elif mass[i] > 2 and mass[i] <= 5 and velocity[i] <= 20:
        print("Normal traffic")
    elif mass[i] <= 2 and velocity[i] <= 20:
        print("Slow light object.  Ignore it")
    else:
        print("Whoa!  Something is up with the data.  Check it")

Just like with arithmetic, you can and should use parentheses whenever there is possible ambiguity. A good general rule is to always use parentheses when mixing and and or in the same condition. That is, instead of:

PYTHON 

if mass[i] <= 2 or mass[i] >= 5 and velocity[i] > 20:

write one of these:

PYTHON 

if (mass[i] <= 2 or mass[i] >= 5) and velocity[i] > 20:
if mass[i] <= 2 or (mass[i] >= 5 and velocity[i] > 20):

so it is perfectly clear to a reader (and to Python) what you really mean.

Вправа

Відстеження виконання

Що надрукує ця програма?

PYTHON 

pressure = 71.9
if pressure > 50.0:
    pressure = 25.0
elif pressure <= 50.0:
    pressure = 0.0
print(pressure)

ВИХІД 

25.0
Вправа

Обрізання значень

Fill in the blanks so that this program creates a new list containing zeroes where the original list’s values were negative and ones where the original list’s values were positive.

PYTHON 

original = [-1.5, 0.2, 0.4, 0.0, -1.3, 0.4]
result = ____
for value in original:
    if ____:
        result.append(0)
    else:
        ____
print(result)

ВИХІД 

[0, 1, 1, 1, 0, 1]

PYTHON 

original = [-1.5, 0.2, 0.4, 0.0, -1.3, 0.4]
result = []
for value in original:
    if value < 0.0:
        result.append(0)
    else:
        result.append(1)
print(result)
Вправа

Обробка малих файлів

Modify this program so that it only processes files with fewer than 50 records.

PYTHON 

import glob
import pandas as pd
for filename in glob.glob('data/*.csv'):
    contents = pd.read_csv(filename)
    ____:
        print(filename, len(contents))

PYTHON 

import glob
import pandas as pd
for filename in glob.glob('data/*.csv'):
    contents = pd.read_csv(filename)
    if len(contents) < 50:
        print(filename, len(contents))
Вправа

Ініціалізація

Modify this program so that it finds the largest and smallest values in the list no matter what the range of values originally is.

PYTHON 

values = [...деякі тестові дані...]
smallest, largest = None, None
for v in values:
    if ____:
        smallest, largest = v, v
    ____:
        smallest = min(____, v)
        largest = max(____, v)
print(smallest, largest)

What are the advantages and disadvantages of using this method to find the range of the data?

PYTHON 

values = [-2,1,65,78,-54,-24,100]
smallest, largest = None, None
for v in values:
    if smallest is None and largest is None:
        smallest, largest = v, v
    else:
        smallest = min(smallest, v)
        largest = max(largest, v)
print(smallest, largest)

If you wrote == None instead of is None, that works too, but Python programmers always write is None because of the special way None works in the language.

It can be argued that an advantage of using this method would be to make the code more readable. However, a disadvantage is that this code is not efficient because within each iteration of the for loop statement, there are two more loops that run over two numbers each (the min and max functions). It would be more efficient to iterate over each number just once:

PYTHON 

values = [-2,1,65,78,-54,-24,100]
smallest, largest = None, None
for v in values:
    if smallest is None or v < smallest:
        smallest = v
    if largest is None or v > largest:
        largest = v
print(smallest, largest)

Now we have one loop, but four comparison tests. There are two ways we could improve it further: either use fewer comparisons in each iteration, or use two loops that each contain only one comparison test. The simplest solution is often the best:

PYTHON 

values = [-2,1,65,78,-54,-24,100]
smallest = min(values)
largest = max(values)
print(smallest, largest)
Ключові моменти
  • Use if statements to control whether or not a block of code is executed.
  • Умовні оператори часто використовуються всередині циклів.
  • Use else to execute a block of code when an if condition is not true.
  • Використовуйте elif для визначення додаткових перевірок.
  • Conditions are tested once, in order.
  • Create a table showing variables’ values to trace a program’s execution.