GATE CSE 1988 | Question: 8i

Question

Consider the procedure declaration:

Procedure
P (k: integer)

where the parameter passing mechanism is call-by-value-result. Is it correct if the call, P (A[i]), where A is an array and i an integer, is implemented as below.

1. create a new local variable, say z;
2. assign to z, the value of A [i];
3. execute the body of P using z for k;
4. set A [i] to z;

Explain your answer. If this is incorrect implementation, suggest a correct one.

Comments

is it out of syllabus?

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Nopes. Parameter passing is in Compilers.

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call by value: copy going into the procedure

call by result: copy going out of the procedure

call by value result: copy going in, and again going out

call by reference: pass a pointer to the actual parameter, and indirect through the pointer.

Good Read: CSE 341 -- Parameter Passing

Answer 1

The given implementation is correct.

In call-by-value-result parameter passing mechanism, formal parameter gets its initial value from the actual parameter being passed. During execution of the function, there is no effect on the actual argument but once the function is returned, the final value of the parameter is copied back to the actual parameter. And this is exactly what the given implementation does.

This is different from call-by-reference in the sense that there whatever modification happens to the parameter in a function, it must reflect in the actual argument too. These two can produce different outputs if a global variable is being used as a parameter.

Comments

@Arjun

What would be the difference if Global variable is used

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Excerpt from Ullman