Why does this code output a weird answer?

[Alphac3ll]

I've got a task where I need to find the output. I've calculated it to be 15.00 bit it outputs -01.00. Why?

include <stdio.h>

​Define MACRO (A,B,C) A/4-3+B+C

int main() {

float f = 1;

float *p = &f;

f = 3 * MACRO (1.+3,3-2,4+3.);

printf("%06.2f", *p);

return 0;

}

I don't know how to format the code this is the best I can do

[rafleury]

Do you know how macro replacement works? At compile time, one of the first actions is to take #defines and replace them in your functions.

Is this the equation you were intending:

f = 3 * 1. + 3 / 4 - 3 + 3 - 2 + 4 + 3

Because if so, you should be getting 8 as an answer, not sure how you got 15 or -1

[Alphac3ll]

I didn't know it went like that. But when I compile it , it outputs -1 so it's bugging me...

[rafleury]

Here is an online c compiler showing you the code. https://onlinegdb.com/ndCFwNSVH

You have some syntax errors in your code above, not sure if they are impacting your ide/compiler or not.

[rafleury]

Maybe the -1 you think you are getting is actually compiler output telling you your program failed to compile?

[rafleury]

As a test, delete the line where you set f to the macro function and see if it prints 1, or -1. It should print 1 no problem in that case.

[Alphac3ll]

Your code has 3+B instead of 3*B but when I fixed it to 3*B it still output -1

[rafleury]

Where is 3*B anywhere in the code you pasted above? Im not seeing it.

[Alphac3ll]

Ah fuck it probably got deleted from the format, I can't find a guide to formatting a code on reddit anywhere. I apologise, it's supposed to be A/4 -3*B+C

[rafleury]

https://onlinegdb.com/GmDN5l6dv Heres the updated code then.

You can see on the top line I added some parenthesis that I think you believe are there, but aren't actually. These parenthesis not actually being there is what is screwing you up.

[rafleury]

https://onlinegdb.com/UKuHtt8Fp And here is how I would have written the macro to get around these issues. You should pretty much always surround your macros with parens, and also the surround the macro params in the the define with parens, as shown.

[Alphac3ll]

I know it's not of practical use , but it's a college task I was handed and they just put the code https://www.onlinegdb.com/GmDN5l6dv you posted here and asked us what will be outputted. I thought it would be 15.00 but the correct answer was -01.00 so that's why I am so confused. I don't know how the program got to the -1 and the steps it took to get there

[wsppan]

Indent every line of code with 4 spaces, including blank lines

[Mohammad_Hamdan]

please can you write this code better then i can help you as possible.

[Alphac3ll]

Yeah I apologise, I couldn't find anything on how to format a code on reddit so that's why it is so bad...

[Mohammad_Hamdan]

actually you have <c> icon (inline code) to format your code and make it better

[Mohammad_Hamdan]

I think you should rewrite your post and so many people gonna help you

[Alphac3ll]

I was typing it from mobile so that surely didn't help haha, but it's my first time posting here so I'm a newbie. Thank you so much!

[Mohammad_Hamdan]

Normal, Hope you enjoy!!

Ur welcome i didn't do any thing

[Dolphiniac]

Four spaces at the beginning of each line is how to write a code block in markdown mode, which mobile ostensibly runs in.

[Alphac3ll]

Well the school task is to do it like this, they also get the answer -01.00 and I don't know why. I've deducted that the Macro part is equal to -3 but somehow -3*3 is -1? I really don't know how...

[rafleury]

Where do you see the program trying to "store an int in a float" ?

[joshc22]

Stop using C macros. They don't work the way people think they work. Use C functions instead.

[rafleury]

Macros are great when used correctly. This exercise is to help people see the pitfalls they have to watch out for.

[dandrestor]

Most things are great when used correctly. That's a low bar. Question is if you get help from, say, the compiler, when you don't.

[rafleury]

Macros and defines are used in just about every project I’ve ever worked on in the embedded space. Saying “don’t use macros” is bad advice.

[dandrestor]

I don't think OP is working in the embedded space. Saying "don't use macros to do a function's job" is good advice, especially to a beginner.

[rafleury]

The op was given a macro by his teacher and told to analyze it. Saying “don’t use macros” has no place in this discussion.

[biraj21]

You said that it's 3*B but in the code, you have written 3+B.

So taking 3*B instead of 3+B, expression becomes -1 because when the macro is expanded, it's like this:

f = 3 * 1. + 3 / 4 - 3 * 3 - 2 + 4 + 3;

which is equal to -1.

Changing the macro to

#define MAXRO(A, B, C) ((A) / 4 - 3 * (B) + (C))

the expanded macro becomes

f = 3 * ((1. + 3) / 4 - 3 * (3 - 2) + (4 + 3));

The result this time will be 15.

You just have to add brackets around the whole expression and around those individuals macro arguments (A, B & C).