Basics of Python

Iteration

Lecture #5

Iteration

Iteration

• Iteration repeats the execution of a sequence of code.
• Iteration is useful for solving many programming problems.
• Iteration and conditional execution form the basis for algorithm construction.

The while Statement

The while Statement

The while Statement

• The execution of while, checks the condition before executing the while block and then checks the condition again after executing the while block.
• As long as the condition remains truth, the program repeatedly executes the code in the while block.
• If the condition initially is false, the program will not execute the code within the while block.
• If the condition initially is true, the program executes the block repeatedly until the condition becomes false, at which point the loop terminates.

The while Statement

The while Statement

Definite Loops vs. Indefinite Loops

The for Statement

• The while loop is ideal for indefinite loops.
• Python provides a more convenient way to express a definite loop.
• The for statement iterates over a sequence of values.
• One way to express a sequence is via a tuple, as shown here:

The for Statement

The general form of the range expression is:

The for Statement

• Initially emphasized the for loop’s ability to iterate over integer sequences because this is a useful and common task in software construction.
• The for loop, however, can iterate over any iterable object.
• A tuple is an iterable object, and a range object is an iterable object. A string also is an iterable object.
• We can use a for loop to iterate over the characters that comprise a string.

Nested Loops

• Just like with if statements, while and for blocks can contain arbitrary Python statements, including other loops.
• A loop can therefore be nested within another loop.

Abnormal Loop Termination

• Normally, a while statement executes until its condition becomes false.
• A running program checks this condition first to determine if it should execute the statements in the loop’s body.
• It then re-checks this condition only after executing all the statements in the loop’s body.
• Ordinarily a while loop will not immediately exit its body if its condition becomes false before completing all the statements in its body.
• The while statement is designed this way because usually the programmer intends to execute all the statements within the body as an indivisible unit.
• Sometimes, however, it is desirable to immediately exit the body or recheck the condition from the middle of the loop instead.

Abnormal Loop Termination

The break statement

• Python provides the break statement to implement middle-exiting loop control logic.
• The break statement causes the program’s execution to immediately exit from the body of the loop.

The continue Statement

• The continue statement is similar to the break statement, except the continue statement does not necessarily exit the loop.
• The continue statement skips the rest of the body of the loop and immediately checks the loop’s condition.
• If the loop’s condition remains true, the loop’s execution resumes at the top of the loop.

Infinite Loops

• An infinite loop is a loop that executes its block of statements repeatedly until the user forces the program to quit.
• Once the program flow enters the loop’s body it cannot escape. Infinite loops sometimes are by design.
• For example, a long-running server application like a Web server may need to continuously check for incoming connections.
• The Web server can perform this checking within a loop that runs indefinitely.

Q&A

Q&A