(So far, 1850 people got it right out of 2786 for a success rate of 66%)
Predict the output of the following C program.
#include <stdio.h>
#define x 2+3
int main()
{
int y;
y = x*x*x;
printf("%d", y);
return 0;
}
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What do you think, do you agree with the solution/explanation?
I agree with the solution, but I think that maybe this quiz is only about C language. To make a quiz about embedded systems I would answer, for example, about different variables definitions (local, global, initalizated, malloc) and their localization (stack, heap, data, bss)...what do you think?
It is also interesting to consider aspects such as C/C++ incompatibilities, concurrency issues, bare metal vs RTOS vs Linux (and the underlying hardware requirements/differences), the use of other languages (e.g. Python).
If this were an interview question, what's more important is the follow-up: "Is this bad code? Why? What would you do to fix it, if anything?" Even experienced C programmers who would catch this "problem" immediately would have structured the code with parentheses, etc., to be more explicit on the page, even if the end resulting binary were the same.
Many people on Linkedin have complained as well about how bad this code is. If you were to re-write this code in a way that most Embedded Systems engineers would agree is following best practices, how would it look?
One of my favorite quotes goes something like:
"Which has higher precedence, || or & in C?"
The correct answer is "Who cares?!!!" If you have to ask you are writing bad code. Use parentheses liberally to make your intent clear. They are free!
While you have a point, you may also be missing the point. I don't have a problem with my own code, but I've spent a lot of time looking at code someone else wrote. It's important to understand the rules of the game, like order of precedence, if you want to understand what actually is happening.
The quick fix in this particular instance is to change the define to either: #define x (2+3) -or- #define x 5 even if that isn't the approach you would take starting from scratch.
Well, the answer is 0 depending on your semantics. The question asks for the "output". Now, the output of the function is 0, because you "return 0". If you are asking what is printed to the output stream, then the answer is what is stated. I wasn't sure if this was a trick question. For all those that chose "None of the above" because of this, I got your back. ;)
Not really.. here the "output" of the program is what it prints out to the standard output. Zero is the return/exit value.
However, as others are noting, and for new software developers, the key point is to remember that the preprocessor hits (effectively edits) the code before the compiler actually compiles it.
FYI: You can play with the answer here: https://godbolt.org/z/h6rqredPY
So:
x*x*x
is expanded (without any 'smarts') by the preprocessor to:
2+3*2+3*2+3
When the code executes, we get:
2+(6)+(6)+3
or
8+9 = 17
But again, please do not use #define macros in this manner. Use 'const' and 'constexpr' where possible. This will ensure type safety and also help avoid unexpected results due to accidental missing parans.
Hope that helps!
Best regards,
Matthew
https://covemountainsoftware.com/
Thanks!
A better solution would be to add a "++" after that "C", and make friends with constexpr.
We can add more architectural and OS related questions. These will help for block level understanding.
This is the reason I insist on parens to avoid the issue of precedence.
For a really bad example, look at the library code for time_t calculation - depends totally on precedence.
I wannt to know how many folks copied the code & compiled/ran it - I did. While the precedence in C is well-defined, it is error-prone to try to figure out how it applies.
Sensible.
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