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.
I am trying to search through an array, and only check specific values (the 4th,5th and so on)- ((0+n*4) and (3+n*4).
The first one found will be checked and if it has a value of 0 will it be changed to 1 and then the program should stop. If not it will try the next value and so on..
I have the following code, but it doesn't stop ..it makes all the values 1 at once..
Any suggestions?
{
for (i=0; i<(totalnumber); i++)
{ for (n=0; n<((totalnumber)/4); n++)
{ if (i==(0+(n*4)))
{ if (array[i]==0)
{
array[i]=1;
break;
}
}
else if ((i==(3+(n*4))))
{
if (array[i]==0)
{
array[i]=1;
break;
}
}
}
}
}
Using a single break statement only breaks out of the nearest loop. It doesnt break out of the outer i loop. So, change your code to break out of the outer loop too.
One other way is to use both counter variables i,n within the same for loop. That means, you only use break once to break out of the outer for loop.
I quote from MSDN
Within nested statements, the break statement terminates only the do, for, switch, or while statement that immediately encloses it. You can use a return or goto statement to transfer control elsewhere out of the nested structure.
This is related - Can I use break to exit multiple nested for loops?
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 having trouble understanding the control flow within a segment of my code. The code is written to take a startword (i.e cat) and an endword (i.e dog) change 1 letter of the original word at a time and reach the end word - checking against a dictionary of real words. If a non-real word is reached, i.e cat --> dat, it should break out of the loop and try changing a different letter.
while (strcmp (startword, endword) != FALSE)
{
change_letter(startword, endword, i++);
if ((check_dictionary(dictionary, startword)) == FALSE)
{
printf("%s --> ", startword);
printf("Bad route\n");
break;
}
if ((check_dictionary(dictionary, startword)) == TRUE)
{
printf("%s --> ", startword);
}
}
change_letter will just do startword[i] = endword[i], where i is initialized at 0 (for the first letter, and moves through the letters as i++ appears.
check dictionary compares the word against the dictionary, if the word is found a 1 is returned. (#define TRUE = 1, #define FALSE = 0).
Right now it will print (if using cat and dog as the words)
cat --> dat --> Bad route and thats all.
To my understanding it should never leave the top while loop whilst the two words are not equal.
while (strcmp (startword, endword))
{
change_letter(startword, endword, i++);
if (!check_dictionary(dictionary, startword))
{
printf("%s --> ", startword);
printf("Bad route\n");
}
else (check_dictionary(dictionary, startword))
{
printf("%s --> ", startword);
}
}
Modified your code a bit. Removed break, as it breaks control flow from loop and control is on the next statement after loop's }. That was your issue. Not sure if it works now, though, as it depends on your check_dictionary implementation.
Additionally, comparison == TRUE is useless. You can endlessly write check_dictionary(dictionary, startword) == TRUE == TRUE == TRUE == TRUE, the meaning will be all the same. Same as in while loop. It makes your code less understandable and is generally a bad practice.
After all, I removed second function call to make the function return value being evaluated only once.
Also, try to control your code readability by keeping same coding style (bracers!). It improves your code's readability.
Sorry for being boring.
I have a work to do in which I have to keep a loop inside the function expecting the following parameters:
-"i" to insert
-"s" to search
-"q" to quit
How do I keep this loop? I've looked up some options and it seems to be possible using a while or a switch, but I am not sure which is the best way to read those chars (with a fscanf perhaps?). I am also not sure how to read the things after the parameter "i" as the input would be "i word 9", so after detecting the i to insert I have to read a string and an int.
Anyone has any idea how to do this? I am sorry is this seems simple, but I am new to programming.
edit: Here is what I have so far
while (loop) {
fscanf(stdin,"%c",&par);
if (strcmp(&par,"i")){
scanf("%s %d",palavra,p);
raiz = insere(raiz,&palavra,p);
}
else if (strcmp(&par,"b")){
scanf("%s",palavra);
busca(raiz,&palavra);
}
else if (strcmp(&par,"q"))
loop = 0;
}
edit 2: This is what I have now, I am having problems reading the string and integer when the parameter is i, somehow it crashes the function
while (1) {
c = getchar();
if (c == 'f')
break;
else if (c == 'i'){
fscanf(stdin,"%s",&palavra);
scanf("%d",&p);
raiz = insere(raiz,palavra,p);
}
else if (c == 'b') {
scanf("%s",palavra);
busca(raiz,palavra);
}
}
Thanks in advance!
The code you have doesn't look too bad compared to what I believe you want. You can replace the "while (loop)" with "while (1)" and then your exist code "loop = 0;" with "break;" which is a bit more standard way of doing things. Also "fscanf(stdin..." is the same as "scanf(..." ... scanf will read from stdin by default. You might want to check the docs for strcmp because it returns 0 for an exact match and I don't think that will do what you want in your 'if' statements. You should be able to use scanf to read in the values you want, is it giving you an error?
You are using 3 separated scans. That means you can't input this "i word 9", but input one command or parameter at the time separated by EOL(pressing enter).. i, enter, word, enter, 9, enter ... Then the function should actually get further in those "if"s. With those scans you also should consider printing information about expected inputs ("Choose action q/i/f")
And I would recommend using something to test those inputs.
if (scanf("%d", &p) == 0) {
printf("Wrong input");
break;
}