Conditional Operator

Part of the Fundamentals section of Coddy's Verilog journey — lesson 28 of 90.

The conditional operator ? : selects between two values based on a condition. It is also called the ternary operator. The conditional operator evaluates a condition and returns one of two values:

condition ? value_if_true : value_if_false

If the condition is true (1), the first value is returned. If false (0), the second value is returned.

Why Use Conditional Operator

It is a compact way to write simple if-else logic in a single line. Common uses include:

Selecting between two signals
Creating multiplexers
Setting default values
Inline conditional assignments

Syntax and Examples

Basic example:

result = (a > b) ? a : b;   // result gets the larger of a and b

Select between values:

data_out = (enable) ? data_in : 8'b00000000;   // Output data if enabled, else 0

Invert signal conditionally:

out = (invert) ? ~in : in;   // If invert is 1, output inverted; else unchanged

Code Example

module conditional_demo;
  reg [3:0] a, b, max;
  reg enable;
  reg [3:0] data_in, data_out;
  
  initial begin
    a = 4'd7;
    b = 4'd9;
    enable = 1;
    data_in = 4'b1010;
    
    max = (a > b) ? a : b;           // 9
    data_out = (enable) ? data_in : 4'b0000;   // 1010
    
    $display("max = %d", max);
    $display("data_out = %b", data_out);
    $finish;
  end
endmodule

Output:

max = 9
data_out = 1010

Nested Conditional Operators

You can nest conditional operators for multiple selections:

result = (a > b) ? a : (b > c) ? b : c;   // Find the largest of three values

Use parentheses to make nested conditions clear.

Important Notes

The condition must be a single bit (or expression that evaluates to 0 or 1)
Both value options must have the same bit width
The operator can be used in continuous assignments (assign) and procedural blocks (always, initial)

Challenge

Write the correct conditional expressions for each task.

What to do:

Set max to the larger of x and y
Set abs to the absolute value of val (if negative, make positive)
Set output_data to data if enable is 1, otherwise to 8'b0

Cheat sheet

The conditional (ternary) operator ? : selects between two values based on a condition:

condition ? value_if_true : value_if_false

Examples:

max      = (a > b) ? a : b;                  // larger of a and b
data_out = (enable) ? data_in : 8'b0;        // output data if enabled, else 0
out      = (invert) ? ~in : in;              // conditionally invert signal

Nested conditional operators:

result = (a > b) ? a : (b > c) ? b : c;     // largest of three values

Notes:

Condition must evaluate to 0 or 1
Both value options must have the same bit width
Usable in assign, always, and initial blocks

Try it yourself

module conditional_challenge;
  reg [3:0] x, y;
  reg [3:0] max;
  reg signed [3:0] val;
  reg [3:0] abs;
  reg enable;
  reg [7:0] data;
  reg [7:0] output_data;
  
  initial begin
    x = 4'd12;
    y = 4'd8;
    val = -4'sd5;
    enable = 1;
    data = 8'b10101010;
    
    max = ______;          // larger of x and y
    abs = ______;          // absolute value of val
    output_data = ______;  // data if enable, else 0
    
    $display("max = %d", max);
    $display("abs = %d", abs);
    $display("output_data = %b", output_data);
    $finish;
  end
endmodule