Question

(Dangling-Else Problem) State the output for each of the following when x is 9 and y is 11 and when x is 11 and y is 9. Note that the compiler ignores the indentation in a C++ program. The C++ compiler always associates an else with the previous if unless told to do otherwise by the placement of braces {}. On first glance, the programmer may not be sure which if and else match, so this is referred to as the "dangling-else" problem. We eliminated the indentation from the following code to make the problem more challenging. [Hint: Apply indentation conventions you have learned.] a. if ( x < 10 ) if ( y > 10 ) cout << "*****" << endl; else cout << "#" << endl; cout << "$$$$$" << endl; b. if ( x < 10 ) { if ( y > 10 ) cout << "*****" << endl; } else { cout << "#" << endl; cout << "$$$$$" << endl; }

Short answer

(a) For (x=9, y=11): ***** $$$$$; For (x=11, y=9): $$$$$. (b) For (x=9, y=11): *****; For (x=11, y=9): # $$$$.

Step-by-step solution

Analyze the expression for case (a) when x=9 and y=11

In the given block of code, check if `x < 10`. Since `x` is 9, this condition is true. Now check the next condition `y > 10`. Since `y` is 11, this condition is also true. Therefore, the output will be `*****`, followed by `$$$$$` since the last `cout` belongs to the main block.

Evaluate case (a) for x=11 and y=9

Check if `x < 10`. Since `x` is 11, this condition is false, so we skip the nested `if ( y > 10 )` entirely. However, `cout << "$$$$$" << endl;` line is executed regardless of the previous conditions, leading to the output `$$$$$`.

Analyze the expression for case (b) when x=9 and y=11

Check the condition `x < 10`. Since `x` is 9, it is true. The next block is executed, where `if (y > 10)` checks `y=11`, which is also true. Therefore, `*****` is output. The else part is not reached, resulting in no further output.

Evaluate case (b) for x=11 and y=9

Check if `x < 10`. Since `x` is 11, this condition is false. Thus, move to the `else` block, outputting `#` and `$$$$$`.

Key concepts

C++ Conditional Statements

In C++, conditional statements are used to make decisions in the code and alter the flow based on certain conditions. The basic conditional statements in C++ are the `if`, `else if`, and `else` statements. The `if` statement checks for a condition, and if it evaluates to true, the block of code inside the `if` is executed.
If the condition is false, the program skips the `if` block and moves on to the `else if` (if present) or `else` block. Consider this basic pattern for `if` and `else` statements:

• if (condition) {
// execute this block if condition is true
}
• else {
// execute this block if condition is false
}

The `else if` statement allows multiple checks:

• else if (another_condition) {
// execute this block if the previous conditions were false and this condition is true
}

This flow control structure can aid in addressing more complex program logic, influencing the output based on dynamic input. Understanding these basics can also help resolve the dangling-else problem by clarifying which `else` corresponds to which `if`. Always use braces `{}` to clarify the intended blocks and avoid confusion.

Code Indentation in Programming

Code indentation is critical in programming for improving readability and understanding of code. While C++ compilers do not require specific indentation and treat the code as a continuous stream of characters, programmers use indentation to visually organize code.
Proper indentation helps to understand the structure and flow of complex logical statements and is especially crucial in `if-else` constructs. A common guideline is to indent the code block within braces or associated with conditional statements.

• Align all `if`, `else if`, and `else` keywords with the same indentation level.
• Indent the code inside an `if` or `else` block further to distinguish it from the main flow.
• Consistently use either spaces or tabs for indentation to maintain uniformity throughout the code.

For instance, the indentation in conditional statements helps in associating the `else` with its respective `if`, preventing the potential confusion seen in the dangling-else problem. Always ensure the indentation reflects the logical structure of the code, aiding in easy maintenance and collaboration between developers.

Control Flow

Control flow refers to the order in which individual statements, instructions, or function calls are executed or evaluated in a program. In C++, control flow is altered using statements that jump (such as `goto`), loop (such as `for`, `while`), and make decisions (such as `if`, `switch`).
In the case of conditional statements, control flow depends on the evaluation of boolean expressions.

• For true conditions, the `if` block is executed, possibly leading to nested execution depending on the program design.
• False evaluations cause the control to skip blocks or activate `else`/`else if` branches.

C++ automatically associates any `else` with the closest preceding unmatched `if`. This automatic control handling emphasizes the need for clear logical positioning using braces to dictate flow explicitly. Understanding control flow is crucial for debugging and optimizing code performance, ensuring that logic is consistently applied and outcomes are predictable. By actively managing control flow, developers can introduce more complex patterns and manage logical dependencies within their programs.