Thinking out of the Box: Comparing metaphors for variables in programming education

When teaching novices programming, misconceptions can occur. Misconception are incorrect beliefs about certain programming concept. For example, some novices think that a variable can hold multiple values, in the case of two consecutive assignment statements, such as x = 5; x = 7. While explaining variables introductory materials often use the metaphor of a box for a variable, which might contribute to the 'multiple values' hypothesis. To investigate this, we design and run a controlled experiment with 496 novice programmers, both children and adults. Half of our participants receive an introductory programming lesson in which we explain a variable as a box, while the other half of participants receive the explanation of a variable as being a label. They are subsequently questioned about their understanding of variables. Our results show that, for the simple questions involving one assignment, the box group performs better. However, for questions involving the misconception - with two consecutive assignment statements - the label group outperforms the box group. This however primarily occurs when considering variables of type string, for integers subjects interpret the statements as numeric values to be added.