Conditional execution
In order to write useful programs, we almost always need the ability to check conditions and change the behavior of the program accordingly. Conditional statements give us this ability. The simplest form is the if statement:
if x > 0 :
print('x is positive')
The if statement contains a logical expression using which the data is compared and a decision is made based on the result of the comparison.
The boolean expression after the if statement is called the condition. We end the if statement with a colon character (:) and the line(s) after the if statement are indented.
If the logical condition is true, then the indented statement gets executed. If the logical condition is false, the indented statement is skipped.
if statement consists of a header line that ends with the colon character (:) followed by an indented block. Statements like this are called compound statements because they stretch across more than one line.
There is no limit on the number of statements that can appear in the body, but there must be at least one. Occasionally, it is useful to have a body with no statements (usually as a place holder for code you haven’t written yet). In that case, you can use the pass statement, which does nothing.
if x < 0 :
pass # need to handle negative values!
Alternative execution
A second form of the if statement is alternative execution, in which there are two possibilities and the condition determines which one gets executed.
The syntax looks like this:
if x%2 == 0 :
print('x is even')
else :
print('x is odd')
If the remainder when x is divided by 2 is 0, then we know that x is even, and the program displays a message to that effect. If the condition is false, the second set of statements is executed
An else statement can be combined with an if statement. An else statement contains a block of code that executes if the conditional expression in the if statement resolves to 0 or a FALSE value.
The else statement is an optional statement and there could be at the most only one else statement following if.
Since the condition must either be true or false, exactly one of the alternatives will be executed. The alternatives are called branches, because they are branches in the flow of execution.
Chained conditionals
Sometimes there are more than two possibilities and we need more than two branches. One way to express a computation like that is a chained conditional:
iframe if else elif statement
if x < y:
print('x is less than y')
elif x > y:
print('x is greater than y')
else:
print('x and y are equal')
elif is an abbreviation of “else if.” Again, exactly one branch will be executed. There is no limit on the number of elif statements. If there is an else clause, it has to be at the end, but there doesn’t have to be one.
The elif statement allows you to check multiple expressions for TRUE and execute a block of code as soon as one of the conditions evaluates to TRUE.
Similar to the else, the elif statement is optional.
if choice == 'a':
print('Bad guess')
elif choice == 'b':
print('Good guess')
elif choice == 'c':
print('Close, but not correct')
Each condition is checked in order. If the first is false, the next is checked, and so on. If one of them is true, the corresponding branch executes, and the statement ends. Even if more than one condition is true, only the first true branch executes.
Nested conditionals
There may be a situation when you want to check for another condition after a condition resolves to true. In such a situation, you can use the nested if construct.
One conditional can also be nested within another. We could have written the three-branch example like this:
if expression1:
statement(s)
if expression2:
statement(s)
elif expression3:
statement(s)
else
statement(s)
elif expression4:
statement(s)
else:
statement(s)
The outer conditional contains two branches. The first branch contains a simple statement. The second branch contains another if statement, which has two branches of its own. Those two branches are both simple statements, although they could have been conditional statements as well.
Although the indentation of the statements makes the structure apparent, nested conditionals become difficult to read very quickly. In general, it is a good idea to avoid them when you can.
Logical operators often provide a way to simplify nested conditional statements.
For example, we can rewrite the following code using a single conditional:
x = 5
if 0 < x:
if x < 10:
print('x is a positive single-digit number.')
Single Statement Suites
If the suite of an if, elif, else clause consist only of a single line, it may go on the same line as the header statement.
a = 5
b = 10
if (a > b) : print("a greater than b")
elif (a == b) : print("a equals to b")
else : print(" a less than b")
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