Tutorial on if, else and else-if (elif) conditions

Based on the sources and our conversation, the if statement is a fundamental and powerful tool in Python programming used for decision making or selecting from alternative actions based on test results. It is considered the primary selection tool in Python and represents much of the logic a Python program possesses. The if statement evaluates expressions that produce TRUE or FALSE outcomes, and based on this outcome, the program determines which statements to execute.

The concept of if is part of a core triangle of powerful programming entities, along with for and while, which are crucial for understanding programming completely. The if statement is used in almost every piece of code and is found in 99 percent of code written.

Here’s a breakdown of the if statement in Python as described in the sources:

1. Basic if Statement

   • The simplest form consists of the word if, followed by an expression (the condition) that is interpreted as a true or false result.
   • The header line ends with a colon (:).
   • A block or suite of code follows the header. This block contains the statement(s) to run if the test is true.
   • All statements within this block must be indented the same amount. Indentation is how Python groups statements into blocks; there are no braces or “begin/end” delimiters like in C-like languages.
   • If the boolean expression evaluates to TRUE, the block of statements is executed. If it’s FALSE, nothing associated with that if block is executed, and the program continues after the if statement’s block.
   • There must be at least one statement in the body, or you can use the pass statement as a placeholder. pass is a no-operation placeholder used when syntax requires a statement but you have nothing to do.
   • Example syntax:

     if expression:
         statement(s) # indented block

2. Alternative Execution (else)

   • The if statement can be followed by an optional else statement.
   • The else statement contains a block of code that executes if the conditional expression in the if statement resolves to FALSE.
   • The else part also has an associated block of nested statements, indented under the else: header.
   • Exactly one of the alternatives (if block or else block) will run.
   • There can be at most one else statement following an if.
   • Example syntax:

     if expression:
         statement(s) # Executed if expression is TRUE
     else:
         statement(s) # Executed if expression is FALSE
   • An example given is checking if a number is even or odd or calculating a discount based on an amount.

3. Chained Conditionals (elif)

   • When there are more than two possibilities, you can use one or more optional elif (“else if”) tests.
   • The elif statement allows you to check another condition if the previous if or elif conditions were false.
   • Each elif also has an associated indented block of statements.
   • The conditions (if, then elifs) are checked in order from top to bottom. Python executes the block of code associated with the first test that evaluates to true.
   • If one condition is true, its corresponding block runs, and the entire if/elif/else statement ends.
   • If all if and elif tests are false, the optional else block (if present) is executed.
   • There is no limit on the number of elif statements.
   • This structure is the most straightforward way to code a multiway branch. Python does not have a switch or case statement like some other languages; multiway branching is typically done with if/elif/else or by using dictionaries.
   • Using elif can make code cleaner compared to nested if statements, especially when checking ranges of values (like for grading based on marks).
   • Example syntax:

     if <test1>:
         <statements1> # if test, associated block
     elif <test2>:
         <statements2> # Optional elif, associated block
     # ... more elifs ...
     else:
         <statements3> # Optional else, associated block
   • An example shows checking a variable x against different values (‘roger’, ‘bugs’) using if and elif.

4. Truth Tests and Boolean Values

   • if statements rely on truth tests or Boolean expressions.
   • Python interprets any non-zero and non-null values as TRUE, and any zero or null values as FALSE. For example, a non-empty string or list is True, while an empty string (""), empty list ([]), empty dictionary ({}), or the value None is False. A non-zero number (like 1) is True, and 0.0 is False.
   • This means you can test an object directly (if X:) instead of comparing it to an empty value (if X != '':).
   • The built-in values True and False are essentially predefined to have the same meanings as integer 1 and 0, respectively.
   • Comparison operators (>, >=, <, <=, ==, !=) are used to create boolean expressions. They return True or False.
   • Logical operators (and, or, not) are used to combine boolean expressions. In Python, these are typed as words. and and or perform short-circuit evaluation. For or, Python returns the first true operand it finds; for and, it returns the first false operand. The not operator negates a boolean expression.

5. Nested Conditionals

   • You can write an if statement within another if statement, or inside another if...elif...else construct.
   • When code is conditional or repeated, you simply indent it further to the right.
   • An example shows an if statement nested in the else clause of another if statement. While possible, deeply nested conditionals can become difficult to read.

6. Single Statement Suites

   • If the block of an if clause is only a single line, it can sometimes go on the same line as the header statement after the colon. Example: if var == 100 : print ("Value of expression is 100"). This saves an extra line.

7. Conditional Expression (Ternary Operator)

   • For simple cases, Python (since 2.5) offers a concise way to express a simple if/else that returns or assigns a value.
   • The syntax is Y if X else Z. It evaluates to Y if X is true, and Z otherwise.
   • It has the same effect as a four-line if statement (if X: A = Y else: A = Z) but is simpler to code for basic scenarios.
   • Like and and or, it short-circuits, evaluating only the necessary expression (Y or Z).
   • It can be used to simplify code, such as in recursive functions or handling default values.
   • The sources mention an older and/or combination that achieved a similar effect but was trickier and less preferred. The Y if X else Z form is recommended.

In summary, the if statement and its variations (else and elif) are fundamental control-flow statements in Python that allow programs to execute different code paths based on conditions evaluated as true or false. Indentation is critical for defining the blocks of code associated with each part of the statement.