Control structures in Python

Control structures in Python

Control structures control the direction of flow of the program. It determines how a computer will respond when given certain conditions and parameters. The three types of control structures in Python are Sequence, Selection (Branching), and Repetition (Looping / Iteration) Structures.

Sequence Structure:

The sequence structure is the default structure. It contains a sequence of statements placed one after the other and is executed in the same order.

Selection Structure:

Selection structure is used for choosing between two or more alternative blocks of code. It is a binary decision based on whether an expression (also called as test condition) evaluates to TRUE or FALSE. Given two alternative blocks, if the test condition evaluates to true, one of the two branches is explored; if the condition evaluates to false, the other alternative is taken. The commonly used selection structures in Python are ‘if, ‘if – else’ and ‘if – elif – else’

if structure: if the condition evaluates to true, the statements in the block will be executed. If the condition evaluates to false the statements will not be executed. Syntax:

if (test condition) :

(statements)

if – else structure: Binary condition where only one block of statements will be executed. If the condition evaluates to true, the statements in the if-block will be executed. If it evaluates to false, the statements in the else block will be executed. Syntax:

if (test condition) :

          (statements)

else:

          (other statements)

if – elif – else  Structure: By using if-elif-else, it is possible to combine several conditions. Only the statements following the first condition that is found to be true will be executed. All other statements will be skipped. Syntax:

if (test condition 1) :

                    (statements)

elif (test condition 2) :   

        (statements)

elif (test condition 3) :

                   (statements)

...

else:

                   (statements)

Repetition Structure:

Repetition structure is used for looping, i.e. repeating a block of code multiple times in a row. It is a construct where statements can be executed repeatedly until a condition evaluates to TRUE or FALSE. It is represented by the ‘while’ and ‘for’ constructs in Python.

while structure: The while construct consists of a block of code and a test condition. The test condition is evaluated, and if it evaluates to true, the code within the block is executed. This repeats until the test condition becomes false. Because the while loop checks the test condition before the block is executed, the while control structure is often also known as an entry-controlled loop. If the test condition evaluates to false the first time, the statement block may never be executed. Syntax:

while (test-condition) :

            // Statement block

for structure: While loop is used in situations where the number of times the loop is to be executed is unknown. For structure in contrast is used in situation where number of times the loop is to executed is deterministic. Syntax .

for <variable> in sequence :

// statement block