Call to length function:
The call to length function consists of the name of the function, followed by an argument in the parentheses. This function takes the argument, and returns an outcome. What happens when the call to the function is encountered is that control is passed to the function itself (in another words, the function starts executing). The arguments are also passed to the function. It executes its statements and does whatever it does (the real contents of the built-in functions are not commonly known or seen by the programmer) in order to determine the number of elements in the vector. As the function is computing a single value, this outcome is then returned, and it became the value of the expression. The Control is also passed back to the expression which called it in the first position, which then continues (example, in the first illustration the value would then be assigned to the variable lv, and in the second illustration the value was printed).