Conditional Statement – If… Else

Conditional Statements (Decision Making)

The basic decision statements in the computer are selection structure. The decision is described to computer as a conditional statement that can be answered True or False Python language provides the following conditional (decision making) statements.

• If statement
• If….. else statement
• If……elif…..else statement
• Nested if …. else statement

if condition / test expression :  statement(s)

 : at the end of if indicates continuation of statement
 Statement(s) belonging to if are indented to indicate block of code to be executed, if condition is satisfied
 Indentation is used to form a block of statement in Python. All statement at same indent level are considered in same block.
 Usually four spaces are used to start a new block, but it can be any number of spaces or even tab(s) may be used.
 First unindented statement marks end of block.

 

if condition / test expression :
statement(s)
else :
statement(s)

if condition / test expression :
statement(s)
elif condition / test expression :
statement(s)
:
:
else :

statement(s)

 There will only be one else in an if statement.
 Nested condition – An if statement can be placed inside if, just like any other
statement.
 There can be chain of elif statement in one if.

Iterative construct
Python provide two types of looping construct
 while
 for

While loop
while test expression / condition :
statement(s)
[else:
statement(s)] optional

• When test expression written in while returns false then the else statement if present, gets executed.

For loop

for var(iteratorVar) in sequence :
statement(s)
[ else :
statement(s) ] optional

• Sequence is any sequence data type viz list, tuple, dictionary.
• Else statement, if given, will execute immediately after for block, always.
• Apart from sequence type, for loop can iterate over range() function, file, etc.
• range function

➢ range(start, stop, step)

➢ Generates a sequence of numbers beginning from start till stop-1
➢ Step signifies the interval between the terms
➢ We can have negative value for step when start> stop
➢ First and third arguments are optional
➢ Default value of start is 0 and step is -1

 

Unconditional transfer of control

break statement
                  Transfers control out of the loop
continue statement
                 Skips current iteration in the loop
pass statement
                 Valid executable statement. Used to replace the section of code to be defined later