Python – How to stop one or multiple for loop(s)

In Python, you can stop one or multiple for loops using various techniques depending on your use case. Here’s an explanation of each approach:

1. Breaking Out of a Single Loop

To exit a single for loop, use the break statement.

Example:

for i in range(10):
    if i == 5:
        print("Breaking out of the loop!")
        break
    print(i)

Output:

0
1
2
3
4
Breaking out of the loop!

2. Breaking Out of Nested Loops (One at a Time)

The break statement only stops the innermost loop. For nested loops, you need a break in each loop you want to exit.

Example:

for i in range(3):
    for j in range(3):
        if i == 1 and j == 1:
            print("Breaking inner loop!")
            break
        print(f"i={i}, j={j}")

Output:

i=0, j=0
i=0, j=1
i=0, j=2
i=1, j=0
Breaking inner loop!
i=2, j=0
i=2, j=1
i=2, j=2

3. Breaking Out of All Loops

To break out of multiple nested loops at once, you can:

1. Use a flag.
2. Use a function with return.
3. Use try–except with a custom exception.

3.1. Using a Flag

Set a flag variable to signal when to exit all loops.

found = False
for i in range(3):
    for j in range(3):
        if i == 1 and j == 1:
            found = True
            break
    if found:
        break
print("Exited all loops!")

3.2. Using a Function with return

Wrap the loops inside a function, and use return to exit all loops.

def nested_loops():
    for i in range(3):
        for j in range(3):
            if i == 1 and j == 1:
                print("Breaking all loops!")
                return
            print(f"i={i}, j={j}")

nested_loops()

3.3. Using try–except with a Custom Exception

Define a custom exception and raise it to exit all loops.

class BreakAllLoops(Exception):
    pass

try:
    for i in range(3):
        for j in range(3):
            if i == 1 and j == 1:
                raise BreakAllLoops
            print(f"i={i}, j={j}")
except BreakAllLoops:
    print("Exited all loops!")

4. Breaking Based on a Condition

You can include a condition in your loop to stop execution dynamically.

Example:

for i in range(3):
    for j in range(3):
        print(f"i={i}, j={j}")
        if i * j > 2:  # Condition to stop
            print("Stopping loops!")
            break

Which Approach to Use?

• Single loop: Use break.
• Nested loops:
  • Use a flag for clarity.
  • Use a function with return for modularity.
  • Use try–except for more advanced or reusable scenarios.

Let me know if you have a specific use case or need more clarification!