CODEWITHSUNDEEP
cparametersreferencesemantics
Bobby S
Bobby S

Reputation: 4116

Specific Parameter-Passing Styles: Call-By-Value, Call-By-Name, etc

Studying for my final exam, and came across this past exam question:

Consider the following program written in a C-like notation:

int i = 1;
A[] = {4, 0, 1, 2};

void mystery05(int from, int to)
{
    int temp;
    temp = A[from];
    A[from] = A[to];
    A[to] = temp;
    i = i + 2;
    to = -1;
}

int main(void)
{
    mystery05(A[i+2], A[i]);
}

In the table below, fill in the boxes with the appropriate variable values after the call to mystery05 in main. Each row corresponds to a specific parameter-passing style (ie. use the style listed instead of the default C-language semantics). Assume arrays are indexed from 0.

style               |___i___|__A[0]__|__A[1]__|__A[2]__|__A[3]__| 
call-by-value       |_______|________|________|________|________|
call-by-name        |_______|________|________|________|________|
call-by-reference   |_______|________|________|________|________|
call-by-value-result|_______|________|________|________|________|

I'm not sure on how to go about this, but if it was regular C semantics, I supposed the answers would be

i = 3; A[0] = 4; A[1] = 2; A[2] = 1; A[3] = 0

Upvotes: 3

Views: 729

Answers (2)

S.Lott
S.Lott

Reputation: 391962

call-by-value is -- I think -- what you mean by "regular C semantics"

call-by-name is something C doesn't have. Look it up. That's not "regular C semantics"

call-by-reference assumes that all the arguments have "&" and the parameters have "*". That's not "regular C semantics" that's a different semantics, but easily built in C.

call-by-value-result is something C doesn't have. Look it up.

Each is different. Don't assume C. Don't read the code as if it was C. You have to read the code in different ways assuming different things.

Upvotes: 0

JerryK
JerryK

Reputation: 82

@S.Lott : I thought 'pointer's to strings and arrays are call by reference. Am I wrong?

I agree: don't want to do all the question. If he has an exam he ought to be more clued up. I would like to answer the first line though just to see if i have understood correctly. So I could be wrong!

Call by value: doesn't change the values unless the variables are global and in this case they have to be; for how otherwise can the proc make use of i.

Both i and the A array are global.

What happens in the proc changes the values.

i begins with value 1 so values of A[3] and A[1] swapped.

A[3] now 0 , A[1] now 2 . A[0] and A[2] unchanged.

finally i value changed to 3

I think the exam q missed a trick by not asking about the value of 'to' after the proc call.

Upvotes: 1

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