Loops in Python

Loops in Python

Introduction

Consider the following problem:

Problem

Print the sum of the first five positive integers.

Solution

print(1 + 2 + 3 + 4 + 5)

Now, how about the following problem?

Problem

Print the sum of the first 1,000,000 positive integers.

The earlier approach is not going to work. If it takes about five seconds on average to write a number followed by the + symbol, how much time will it take to find the sum of all 1 million numbers? Let us check:

Code

num = 1_000_000     # _ in a number is used when we have large numbers; improves readability
avg_time = 5
seconds = num * avg_time
minutes = seconds / 60
hours = minutes / 60
days = hours / 24
print('Approximate number of days =', round(days))

It will take nearly 58 days to sum all 1 million integers! This is assuming that we work like machines that don’t need food or sleep. All of this just to do something as trivial as finding the sum of numbers. This is where loops come in.

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while Loops

The “loopy” solution to this problem:

Code

total = 0
num = 0
while num < 1_000_000:
    num = num + 1
    total = total + num
print(total)

The while keyword is used in Python to create a loop. The expression adjacent to while is a boolean expression, called the while-condition. Lines 4 and 5 make up the body of the while loop. If the condition evaluates to True, control enters the body of the while. The lines in the body are sequentially executed. After the last line in the body is executed, the control loops back to the condition evaluation. As long as the condition is True, the body of the while keeps getting executed. The moment the condition becomes False, the body of the while is skipped, and control transfers to the next line. The body of the while loop must always be indented; this helps to separate it from the rest of the code.

Visual Representation

Let us consider another example:

Problem

Keep accepting integers as input from the user until the user enters a negative number. Print the sum of the positive numbers entered by the user. Print 0 if the user doesn’t enter any positive integer.

Code

total = 0
num = int(input())
while num >= 0:
    total += num
    num = int(input())
print(total)

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Another Example

Problem

Keep accepting integers as input from the user until the user enters a negative number. Print the maximum among the positive numbers entered by the user. Print 0 if the user doesn’t enter any positive integer.

Solution

# Initialize
num = int(input())
max_num = 0
# Loop
while num >= 0:
    if num > max_num:
        max_num = num
    num = int(input())
# Print output
print(max_num)

Note that lines 6-8 make up the body of the while loop and are indented. Lines 1, 4, and 9 have comments to help the reader understand what is happening in the code.

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break and continue

The break and continue keywords are associated with loops in Python.

break

The break statement is used to exit out of a loop without executing any code that comes below it. For example:

Code

num = 1
while True:
    if (num % 2 == 0) and (num % 3 == 0) and (num % 4 == 0):
        break
    num = num + 1
print(num)

The above code prints the smallest positive integer that is divisible by 2, 3, and 4, which is the same as the LCM of (2, 3, 4). The moment this number is found, the code breaks out of the loop.

continue

The continue statement is used to move to the next iteration of the loop, skipping whatever code comes below it. For example:

Code

x = 0
while x < 50:
    x = x + 1
    if x % 3 != 0:
        continue
    print(x)

The code given above prints all positive integers less than or equal to 50 that are divisible by 3. Whenever x is not divisible by 3, we do not want to print the number, so we continue to the next iteration.