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More Iteration Control: break and continue

Python offers several statements for more subtle loop control. The point of these statements is to permit two common simplifications of a loop. In each case, these statements can be replaced with if statements; however, those if statement versions might be considered rather complex for expressing some fairly common situations.

The break statement terminates a loop prematurely. This allows a complex condition to terminate a loop. A break statement is always found within if statements within the body of a for or while loop. A break statement is typically used when the terminating condition is too complex to write as an expression in the while clause of a loop. A break statement is also used when a for loop must be abandoned before the end of the sequence has been reached.

The continue statement skips the rest of the loop's suite. Like a break statement, a continue statements is always found within an if statement within a for or while loop. The continue statement is used instead of deeply nested else clauses.

Examples break and continue Here's an example that has a complex break condition. We are going to see if we get six odd numbers in a row, or spin the roulette wheel 100 times.

We'll look at this in some depth because it pulls a number of features together in one program. This program shows both break and continue constructs. Most programs can actually be simplified by eliminating the break and continue statements. In this case, we didn't simplify, just to show how the statements are used.

Note that we have a two part terminating condition: 100 spins or six odd numbers in a row. The hundred spins is relatively easy to define using the range function. The six odd numbers in a row requires testing and counting and then, possibly, ending the loop. The overall ending condition for the loop, then, is the number of spins is 100 or the count of odd numbers in a row is six.

Example 8.1.

import random
oddCount= 0
for s in range(100):
    lastSpin= s
    n= random.randrange(38)
    # Zero
    if n == 0 or n == 37: # treat 37 as 00
        oddCount = 0
    # Odd
    if n%2 == 1:
        oddCount += 1
        if oddCount == 6: break
    # Even
    assert n%2 == 0 and 0 < n <= 36
    oddCount == 0
print oddCount, lastSpin

We import the random module, so that we can generate a random sequence of spins of a roulette wheel. We initialize oddCount, our count of odd numbers seen in a row. It starts at zero, because we haven't seen any add numbers yet.


The for statement will assign 100 different values to s, such that 0 ≤ s < 100. This will control our experiment to do 100 spins of the wheel.


Note that we save the current value of s in a variable called lastSpin, setting up part of our post condition for this loop. We need to know how many spins were done, since one of the exit conditions is that we did 100 spins and never saw six odd values in a row. This "never saw six in a row" exit condition is handled by the for statement itself.


We'll treat 37 as if it were 00, which is like zero. In Roulette, these two numbers are neither even nor odd. The oddCount is set to zero, and the loop is continued. This continue statement resumes loop with the next value of s. It restarts processing at the top of the for statement suite.


We check the value of oddCount to see if it has reached six. If it has, one of the exit conditions is satisfied, and we can break out of the loop entirely. We use the break statement will stop executing statements in the suite of the for statement. If oddCount is not six, we don't break out of the loop, we use the continue statement to restart the for statement suite from the top with a new value for s.


We threw in an assert (see the next section, the section called “The assert Statement”, for more information) that the spin, n, is even and not 0 or 37. This is kind of a safety net. If either of the preceding if statements were incorrect, or a continue statement was omitted, this statement would uncover that fact. We could do this with another if statement, but we wanted to introduce the assert statement.

At the end of the loop, lastSpin is the number of spins and oddCount is the most recent count of odd numbers in a row. Either oddCount is six or lastSpin is 99. When lastSpin is 99, that means that spins 0 through 99 were examined; there are 100 different numbers between 0 and 99.

  Published under the terms of the Open Publication License Design by Interspire