Our organization has a required coding rule (without any explanation) that:
if … else if constructs should be terminated with an else clause
Example 1:
if ( x < 0 )
{
x = 0;
} /* else not needed */
Example 2:
if ( x < 0 )
{
x = 0;
}
else if ( y < 0 )
{
x = 3;
}
else /* this else clause is required, even if the */
{ /* programmer expects this will never be reached */
/* no change in value of x */
}
What edge case is this designed to handle?
What also concerns me about the reason is that Example 1 does not need an else but Example 2 does. If the reason is re-usability and extensibility, I think else should be used in both cases.
As mentioned in another answer, this is from the MISRA-C coding guidelines. The purpose is defensive programming, a concept which is often used in mission-critical programming.
That is, every if - else if must end with an else, and every switch must end with a default.
There are two reasons for this:
Self-documenting code. If you write an else but leave it empty it means: "I have definitely considered the scenario when neither if nor else if are true".
Not writing an else there means: "either I considered the scenario where neither if nor else if are true, or I completely forgot to consider it and there's potentially a fat bug right here in my code".
Stop runaway code. In mission-critical software, you need to write robust programs that account even for the highly unlikely. So you could see code like
if (mybool == TRUE)
{
}
else if (mybool == FALSE)
{
}
else
{
// handle error
}
This code will be completely alien to PC programmers and computer scientists, but it makes perfect sense in mission-critical software, because it catches the case where the "mybool" has gone corrupt, for whatever reason.
Historically, you would fear corruption of the RAM memory because of EMI/noise. This is not much of an issue today. Far more likely, memory corruption occurs because of bugs elsewhere in the code: pointers to wrong locations, array-out-of-bounds bugs, stack overflow, runaway code etc.
So most of the time, code like this comes back to slap yourself in the face when you have written bugs during the implementation stage. Meaning it could also be used as a debug technique: the program you are writing tells you when you have written bugs.
EDIT
Regarding why else is not needed after every single if:
An if-else or if-else if-else completely covers all possible values that a variable can have. But a plain if statement is not necessarily there to cover all possible values, it has a much broader usage. Most often you just wish to check a certain condition and if it is not met, then do nothing. Then it is simply not meaningful to write defensive programming to cover the else case.
Plus it would clutter up the code completely if you wrote an empty else after each and every if.
MISRA-C:2012 15.7 gives no rationale why else is not needed, it just states:
Note: a final else statement is not required for a simple if
statement.
Your company followed MISRA coding guidance. There are a few versions of these guidelines that contain this rule, but from MISRA-C:2004†:
Rule 14.10 (required): All if … else if constructs shall be terminated
with an else clause.
This rule applies whenever an if statement is followed by one or more
else if statements; the final else if shall be followed by an else
statement. In the case of a simple if statement then the else
statement need not be included. The requirement for a final else
statement is defensive programming. The else statement shall either
take appropriate action or contain a suitable comment as to why no
action is taken. This is consistent with the requirement to have a
final default clause in a switch statement. For example this code
is a simple if statement:
if ( x < 0 )
{
log_error(3);
x = 0;
} /* else not needed */
whereas the following code demonstrates an if, else if construct
if ( x < 0 )
{
log_error(3);
x = 0;
}
else if ( y < 0 )
{
x = 3;
}
else /* this else clause is required, even if the */
{ /* programmer expects this will never be reached */
/* no change in value of x */
}
In MISRA-C:2012, which supersedes the 2004 version and is the current recommendation for new projects, the same rule exists but is numbered 15.7.
Example 1:
in a single if statement programmer may need to check n number of conditions and performs single operation.
if(condition_1 || condition_2 || ... condition_n)
{
//operation_1
}
In a regular usage performing a operation is not needed all the time when if is used.
Example 2:
Here programmer checks n number of conditions and performing multiple operations. In regular usage if..else if is like switch you may need to perform a operation like default. So usage else is needed as per misra standard
if(condition_1 || condition_2 || ... condition_n)
{
//operation_1
}
else if(condition_1 || condition_2 || ... condition_n)
{
//operation_2
}
....
else
{
//default cause
}
† Current and past versions of these publications are available for purchase via the MISRA webstore (via).
This is the equivalent of requiring a default case in every switch.
This extra else will Decrease code coverage of your program.
In my experience with porting linux kernel , or android code to different platform many time we do something wrong and in logcat we see some error like
if ( x < 0 )
{
x = 0;
}
else if ( y < 0 )
{
x = 3;
}
else /* this else clause is required, even if the */
{ /* programmer expects this will never be reached */
/* no change in value of x */
printk(" \n [function or module name]: this should never happen \n");
/* It is always good to mention function/module name with the
logs. If you end up with "this should never happen" message
and the same message is used in many places in the software
it will be hard to track/debug.
*/
}
Only a brief explanation, since I did this all about 5 years ago.
There is (with most languages) no syntactic requirement to include "null" else statement (and unnecessary {..}), and in "simple little programs" there is no need. But real programmers don't write "simple little programs", and, just as importantly, they don't write programs that will be used once and then discarded.
When one write an if/else:
if(something)
doSomething;
else
doSomethingElse;
it all seems simple and one hardly sees even the point of adding {..}.
But some day, a few months from now, some other programmer (you would never make such a mistake!) will need to "enhance" the program and will add a statement.
if(something)
doSomething;
else
doSomethingIForgot;
doSomethingElse;
Suddenly doSomethingElse kinda forgets that it's supposed to be in the else leg.
So you're a good little programmer and you always use {..}. But you write:
if(something) {
if(anotherThing) {
doSomething;
}
}
All's well and good until that new kid makes a midnight modification:
if(something) {
if(!notMyThing) {
if(anotherThing) {
doSomething;
}
else {
dontDoAnything; // Because it's not my thing.
}}
}
Yes, it's improperly formatted, but so is half the code in the project, and the "auto formatter" gets bollixed up by all the #ifdef statements. And, of course, the real code is far more complicated than this toy example.
Unfortunately (or not), I've been out of this sort of thing for a few years now, so I don't have a fresh "real" example in mind -- the above is (obviously) contrived and a bit hokey.
This, is done to make the code more readable, for later references and to make it clear, to a later reviewer, that the remaining cases handled by the last else, are do nothing cases, so that they are not overlooked somehow at first sight.
This is a good programming practice, which makes code reusable and extend-able.
I would like to add to – and partly contradict – the previous answers. While it is certainly common to use if-else if in a switch-like manner that should cover the full range of thinkable values for an expression, it is by no means guaranteed that any range of possible conditions is fully covered. The same can be said about the switch construct itself, hence the requirement to use a default clause, which catches all remaining values and can, if not otherwise required anyway, be used as an assertion safeguard.
The question itself features a good counter-example: The second condition does not relate to x at all (which is the reason why I often prefer the more flexible if-based variant over the switch-based variant). From the example it is obvious that if condition A is met, x should be set to a certain value. Should A not be met, then condition B is tested. If it is met, then x should receive another value. If neither A nor B are met, then x should remain unchanged.
Here we can see that an empty else branch should be used to comment on the programmer's intention for the reader.
On the other hand, I cannot see why there must be an else clause especially for the latest and innermost if statement. In C, there is no such thing as an 'else if'. There is only if and else. Instead, the construct should formally be indented this way (and I should have put the opening curly braces on their own lines, but I don't like that):
if (A) {
// do something
}
else {
if (B) {
// do something else (no pun intended)
}
else {
// don't do anything here
}
}
Should any standard happen to require curly braces around every branch, then it would contradict itself if it mentioned "if ... else if constructs" at the same time.
Anyone can imagine the ugliness of deeply nested if else trees, see here on a side note. Now imagine that this construct can be arbitrarily extended anywhere. Then asking for an else clause in the end, but not anywhere else, becomes absurd.
if (A) {
if (B) {
// do something
}
// you could to something here
}
else {
// or here
if (B) { // or C?
// do something else (no pun intended)
}
else {
// don't do anything here, if you don't want to
}
// what if I wanted to do something here? I need brackets for that.
}
In the end, it comes down for them to defining precisely what is meant with an "if ... else if construct"
The basic reason is probably code coverage and the implicit else: how will the code behave if the condition is not true? For genuine testing, you need some way to see that you have tested with the condition false. If every test case you have goes through the if clause, your code could have problems in the real world because of a condition that you did not test.
However, some conditions may properly be like Example 1, like on a tax return: "If the result is less than 0, enter 0." You still need to have a test where the condition is false.
Logically any test implies two branches. What do you do if it is true, and what do you do if it is false.
For those cases where either branch has no functionality, it is reasonable to add a comment about why it doesn't need to have functionality.
This may be of benefit for the next maintenance programmer to come along. They should not have to search too far to decide if the code is correct. You can kind of Prehunt the Elephant.
Personally, it helps me as it forces me to look at the else case, and evaluate it. It may be an impossible condition, in which case i may throw an exception as the contract is violated. It may be benign, in which case a comment may be enough.
Your mileage may vary.
Most the time when you just have a single if statement, it's probably one of reasons such as:
Function guard checks
Initialization option
Optional processing branch
Example
void print (char * text)
{
if (text == null) return; // guard check
printf(text);
}
But when you do if .. else if, it's probably one of reasons such as:
Dynamic switch-case
Processing fork
Handling a processing parameter
And in case your if .. else if covers all possibilities, in that case your last if (...) is not needed, you can just remove it, because at that point the only possible values are the ones covered by that condition.
Example
int absolute_value (int n)
{
if (n == 0)
{
return 0;
}
else if (n > 0)
{
return n;
}
else /* if (n < 0) */ // redundant check
{
return (n * (-1));
}
}
And in most of these reasons, it's possible something doesn't fit into any of the categories in your if .. else if, thus the need to handle them in a final else clause, handling can be done through business-level procedure, user notification, internal error mechanism, ..etc.
Example
#DEFINE SQRT_TWO 1.41421356237309504880
#DEFINE SQRT_THREE 1.73205080756887729352
#DEFINE SQRT_FIVE 2.23606797749978969641
double square_root (int n)
{
if (n > 5) return sqrt((double)n);
else if (n == 5) return SQRT_FIVE;
else if (n == 4) return 2.0;
else if (n == 3) return SQRT_THREE;
else if (n == 2) return SQRT_TWO;
else if (n == 1) return 1.0;
else if (n == 0) return 0.0;
else return sqrt(-1); // error handling
}
This final else clause is quite similar to few other things in languages such as Java and C++, such as:
default case in a switch statement
catch(...) that comes after all specific catch blocks
finally in a try-catch clause
Our software was not mission critical, yet we also decided to use this rule because of defensive programming.
We added a throw exception to the theoretically unreachable code (switch + if-else). And it saved us many times as the software failed fast e.g. when a new type has been added and we forgot to change one-or-two if-else or switch. As a bonus it made super easy to find the issue.
Well, my example involves undefined behavior, but sometimes some people try to be fancy and fails hard, take a look:
int a = 0;
bool b = true;
uint8_t* bPtr = (uint8_t*)&b;
*bPtr = 0xCC;
if(b == true)
{
a += 3;
}
else if(b == false)
{
a += 5;
}
else
{
exit(3);
}
You probably would never expect to have bool which is not true nor false, however it may happen. Personally I believe this is problem caused by person who decides to do something fancy, but additional else statement can prevent any further issues.
I'm currently working with PHP. Creating a registration form and a login form. I am just purely using if and else. No else if or anything that is unnecessary.
If user clicks submits button -> it goes to the next if statement... if username is less than than 'X' amount of characters then alert. If successful then check password length and so on.
No need for extra code such as an else if that could dismiss reliability for server load time to check all the extra code.
As this question on boolean if/else if was closed as a duplicate. As well, there are many bad answers here as it relates to safety-critical.
For a boolean, there are only two cases. In the boolean instance, following the MISRA recommendation blindly maybe bad. The code,
if ( x == FALSE ) {
// Normal action
} else if (x == TRUE ) {
// Fail safe
}
Should just be refactored to,
if ( x == FALSE ) {
// Normal action
} else {
// Fail safe
}
Adding another else increases cyclometric complexity and makes it far harder to test all branches. Some code maybe 'safety related'; Ie, not a direct control function that can cause an unsafe event. In this code, it is often better to have full testability without instrumentation.
For truly safety functional code, it might make sense to separate the cases to detect a fault in this code and have it reported. Although I think logging 'x' on the failure would handle both. For the other cases, it will make the system harder to test and could result in lower availability depending on what the second 'error handling' action is (see other answers where exit() is called).
For non-booleans, there may be ranges that are nonsensical. Ie, they maybe some analog variable going to a DAC. In these cases, the if(x > 2) a; else if(x < -2) b; else c; makes sense for cases where deadband should not have been sent, etc. However, these type of cases do not exist for a boolean.
This is for a school project I'm working on, it's just a small part of the code but for some reason the program doesn't seem to go inside the if() no matter what the input is. I've tried anything and everything I know of (also used the isalpha() function) but it just won't run the commands inside the if().
do
{
flag=1;
gets(input.number);
printf("\n%s\n",input.number);
for(i=0;i<strlen(input.number);i++)
{
printf("yolo1\n"); //debug
if(input.number[i]<48 || input.number[i]>57) //PROBLEM HERE
{
printf("\nyolo2\n"); //debug
flag=-1;
break;
}
}
if(strlen(input.number)<1000 || strlen(input.number)>9999 || flag==-1) printf("\nINVALID INPUT\n\nARI8MOS: ");
}while(strlen(input.number)<1000 || strlen(input.number)>9999 || flag==-1);
Can you guys help me out here??? I've been staring and the code for the better part of 3 days now....
I presume you declared char input.number[].
Your condition in if says that you only want to get into its body if the character is NOT a digit. This is somehow contradictory to the name input.number of the variable (but perhaps you are just checking for incorrect characters here...)
To better see what is happening with the condition, you can choose to print the values of its components, like this:
printf("%c[%d]", input.number[i], input.number[i]);
printf("%d, %d, %d\n", input.number[i]<48 , input.number[i]>57, input.number[i]<48 || input.number[i]>57);
(you will se a 0 for false and 1 for true)
BTW: You could write the same condition in a more readable manner, using char constants like this:
input.number[i]<'0' || input.number[i]>'9')
I'm working on a program and I keep getting "undefined reference to 'dosell' " and I can't quite figure out what is going on. Here's the declaration of the function:
void dosell(int *cash, int *numchips);
The use of the function:
choice = menu();
// Execute the appropriate choice.
if (choice == 1) {
dobuy(&cash, &numchips);
}
else if (choice == 2) {
dosell(&cash, &numchips);
}
And the function itself:
void dosell(int *cash, int *numchips) {
int numsell;
// Determine the number of chips to be sold.
printf("How many chips do you want to sell?\n");
scanf("%d", &numsell);
// Print out the error message if this is too much.
if (numsell > *numchips)
printf("Sorry, you do not have that many chips. No chips sold.\n");
// Execute the transaction.
else {
(*cash) += sellchips(numsell);
(*numchips) -= numsell;
}}}
Put a declaration of the function at the beginning of the file i.e. before the main function and after the includes and defines:
void dosell(int *cash, int *numchips) ;
Transferring key comments into an answer
Is dosell() in the same file as the call to it? If not, are you linking both (all) the files to create the program?
What's with the }}} at the end of dosell(); it looks like a syntax error, unless you've accidentally managed to use a GCC extension — nested functions.
It actually takes quite a bit of effort to make GCC give you a warning about a nested function. You can do it by specifying a standard such as -std=c11 and -pedantic. However, you should not plan on using nested functions, especially not by accident.
I took a look at the }}} and what turns out is one of those was misplaced. One of brackets should have been at the end of the dobuy() function, which is immediately before the dosell() function. Because of this, it included dosell() within dobuy(), so it was as if I hadn't even written the dosell() function.
I observe that if your code had been indented by an automatic indenter, you would have seen that the start line for dosell() was indented, which would perhaps have tipped you off that there was something amiss. The symptoms you describe are exactly consistent with a nested function.
Hello I am studying for a test for an intro to C programming class and yesterday I was trying to write this program to print out the even prime numbers between 2 and whatever number the user enters and I spent about 2 hours trying to write it properly and eventually I did it. I have 2 pictures I uploaded below. One of which displays the correct code and the correct output. The other shows one of my first attempts at the problem which didn't work correctly, I went back and made it as similar to the working code as I could without directly copying and pasting everything.
unfortunately new users aren't allowed to post pictures hopefully these links below will work.
This fails, it doesn't print all numbers in range with natural square root:
for (i = 2; i <= x; i++)
{
//non relevant line
a = sqrt(i);
aa = a * a;
if (aa == i);
printf("%d ",i);
}
source: http://i.imgur.com/WGG6n.jpg
While this succeeds, and prints even numbers with natural sqaure root
for (i = 2; i <= x; i++)
{
a = sqrt(i);
aa = a * a;
if (aa == i && ((i/2) *2) == i)
printf("%d ", i);
}
source: http://i.imgur.com/Kpvpq.jpg
Hopefully you can see and read the screen shots I have here. I know that the 'incorrect code' picture does not have the (i/2)*2 == i part but I figured that it would still print just the odd and even numbers, it also has the code to calculate "sqrd" but that shouldn't affect the output. Please correct me if I'm wrong on that last part though.
And Yes I am using Dev-C++ which I've read is kinda crappy of a program but I initally did this on code::blocks and it did the same thing...
Please I would very much appreciate any advice or suggestions as to what I did wrong 2 hours prior to actually getting the darn code to work for me.
Thank you,
Adam
your code in 'that' includes:
if (aa == i);
// ^
printf(...);
[note the ; at the end of the if condition]
Thus, if aa == i - an empty statement happens, and the print always occures, because it is out of the scope of the if statement.
To avoid this issue in the future, you might want to use explicit scoping1 [using {, } after control flow statements] - at least during your first steps of programming the language.
1: spartan programmers will probably hate this statement
Such errors are common. I use "step Over", "Step Into", "Break Points" and "watch window" to debug my program. Using these options, you can execute your program line by line and keep track of the variables used in each line. This way, u'll know which line is not getting executed in the desired way.
I have a function with an absurd number of return points, and I don't want to caveman each one, and I don't want to next through the function. Is there any way I can do something like finish, except have it stop on the return statement?
You can try reverse debugging to find out where function actually returns. Finish executing current frame, do reverse-step and then you should stop at just returned statement.
(gdb) fin
(gdb) reverse-step
There is already similar question
I think you're stuck setting breakpoints. I'd write a script to generate the list of breakpoint commands to run and paste them into gdb.
Sample script (in Python):
lines = open(filename, 'r').readlines()
break_lines = [line_num for line_num, line in enumerate(lines) if 'return' in line and
line_num > first and line_num <= last]
break_cmds = ['b %s:%d' % (filename, line_num) for line_num in break_lines]
print '\n'.join(break_cmds)
Set filename to the name of the file with the absurd function, first to the first line of the function (this is a quick script, not a C parser) and last to the number of the last line of the function. The output ought to be suitable for pasting into gdb.
Kind of a stretch, but the catch command can stop on many kinds of things (like forking, exiting, receiving a signal). You may be able to use catch catch (which breaks for exceptions) to do what you want in C++ if you wrap the function in try/finally. For that matter, if you break on a line inside the finally you can probably single-step through the return after that (although how much that will tell you about where it came from is highly dependent on optimization: common return cases are often folded by gcc).
How about taking this opportunity to break up what seems to be clearly a too-large function?
This question's come up before on SO. My answer from there:
Obviously you ought to refactor this function, but in C++ you can use this simple expedient to deal with this in five minutes:
class ReturnMarker
{
public:
ReturnMarker() {};
~ReturnMarker()
{
dummy += 1; //<-- put your breakpoint here
}
static int dummy;
}
int ReturnMarker::dummy = 0;
and then instance a single ReturnMarker at the top of your function. When it returns, that instance will go out of scope, and you'll hit the destructor.
void LongFunction()
{
ReturnMarker foo;
// ...
}