The in_mm() function is used to convert inches to millimeters. It first prompts the user to input a decimal value in inches and performs numerical validation. Then, it multiplies the inch value by 25.4 for conversion and prints the result. The mm_in() function is used to convert millimeters to inches. It's similar to the in_mm() function but performs conversion from millimeters to inches.
The select_conversion() function provides an option menu for the user to select the type of conversion to perform. The user can choose to convert inches to millimeters or millimeters to inches. Depending on the user's selection, the corresponding conversion function is called. The entire program begins execution by calling the select_conversion() function, allowing the user to choose the conversion type as needed.