Pass by name implementation in C

Question

How can I calculate the value of the arithmetic expression ^2 + 3i − 1 that is dependent on the index i by using pass-by-name mechanism in C language

9
∑ i^2 + 3i − 1
=0

through a call to a sum procedure with argument(s) passed by name

Pass by name examples written in C could also help me

This is not a good question! Please re-format it and ask specifics. Do not post links to pictures. Show us that you actually attempted the problem, then come back and ask! — Retro Gamer, May 12 '16 at 21:07
Everything @Developer said. You can find a guide on how to ask questions [here](http://stackoverflow.com/help/how-to-ask). — Steve Heim, May 13 '16 at 00:38
I know that C does not working with pass by name mehanism, but pass-by-name could be written in C. I did some research and there are two ways to do that 1) with macro's 2) by using thunk's But I couldn't understand working logic of thunk's in C — Kadir Erceylan, May 13 '16 at 11:20
"pass-by-name could be written in C" Macros use pass-by-name indeed. They are not functions but they are the closest thing you can get. There are no thunks in C. You might be able to write a thunk in assembly and call it from C, but that would be "using pass-by-name mechanism in the assembly language". — n. m. could be an AI, May 13 '16 at 14:06
Could you give me a small function as an example about pass-by-name mechanism in assembly. — Kadir Erceylan, May 16 '16 at 12:00

Bernd Elkemann · Answer 1 · 2016-05-13T14:07:20.973

2

There are two completely different topics here:

(1) You want to learn C. In C you always pass a value, but that value may be a pointer, which in effect works like pass-by-reference.
(2) you want to calculate that summation.

You could use (1) to solve (2) but it is not a good way to do it.

You could use (2) as an example to learn (1).

But it should be very clear that (1) and (2) are not the same thing.

This is how you pass a value to a function in C: void f(int i); ... f(123);
This is how you pass a pointer to a function in C: void f(int* i); ... int i=123; f(&i);

This is the typical way you would calculate the sum in C:

int sum = 0;
for(int i=0; i<=9; ++i)
    sum += 2 + 3*i - 1;
// now sum contains the sum

If for some reason (e.g. homework requirement?) you want to pass a pointer to calculate the sum then:

void f(int* psum, int i) {
    *psum += 2 + 3*i - 1;
}
...
int sum=0;
for(int i=0; i<=9; ++i)
    f(&sum, i);

edited May 13 '16 at 14:07

answered May 13 '16 at 10:31

Bernd Elkemann

23,242
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3

Pass by name and pass by reference are totally different. – n. m. could be an AI May 13 '16 at 10:41
OP is new to this, I assume he does not mean the p-by-name **you** are thinking of. It is almost certain that the confusion he experiences is exactly what i defuse in my answer. – Bernd Elkemann May 13 '16 at 10:47
u may confuse the OP with your C++ syntax ... `int& sum` – AndersK May 13 '16 at 13:47
1

As we can see from the comments, he means exactly what he says. – n. m. could be an AI May 13 '16 at 14:07
I am not sure. He sounds like someone who is just typing "pass by name" into a search engine. But if you are right, then you should write up an answer and if he tells me, then i will delete this answer. – Bernd Elkemann May 13 '16 at 16:18
1

The question is "how to calculate a sum using pass by name in C". There's no pass by name in C, so there's no answer I can write. – n. m. could be an AI May 14 '16 at 11:49

score 1 · Accepted Answer · answered May 16 '16 at 19:50

1

I have done such a solution as following it works but I am not sure whether it works with pass-by-name or not, Could you comment my solution?

#include <stdio.h>

int i;
typedef int* (*intThunk)(void);
int* vSubiThunk(void){ return &i; }

int sum(intThunk i){
    return (*i())* (*i()) + (*i() * 3) - 1 ;
}

int main(void){
    int total = 0;
    for(i=0;i<=9;i++)
        total += sum(vSubiThunk);

    printf("%d \n",total);
}

answered May 16 '16 at 19:50

Kadir Erceylan

41
4

It gives right calculation answer but ı am not sure about pass-by-name mechanism – Kadir Erceylan May 16 '16 at 19:51

Pass by name implementation in C

2 Answers2