A calculator is an electronic device for performing mathematical calculations, distinguished from a computer by a limited problem solving ability and an interface optimized for interactive calculation rather than programming. Calculators can be hardware or software, and mechanical or electronic, and are often built into devices such as PDAs or mobile phones. Modern electronic calculators are generally small, digital, and usually inexpensive. In addition to general purpose calculators, there are those designed for specific markets; for example, there are scientific calculators which focus on advanced math like trigonometry and statistics. Modern calculators are more portable than most computers, though most PDAs are comparable in size to handheld calculators.