i have conditional operator's statement and i have no idea how its works.
there are two questions:
Question 1 : what will the following statement do :
quotient=(b==0)?0:(a/b) \\ here a,b,quotient is integer
Question 2 : Can preceding statement be written as follow ?
quotient=(b)?(a/b):0;
NOW MY QUESTION IS :
Question:1 :: we do not know b's value then how can we check this condition(b==0)
Question 2:: what (b) indicate ?
The conditional check in the C ternary conditional operator is an implicit comparison to not-zero.
In other words
quotient = b ? a / b: 0;
is the same as
quotient = b != 0 ? a / b : 0;
or the absurd
quotient = (b != 0) != 0 ? a / b : 0;
This is consistent throughout C, e.g. in an if, a for stopping condition, a while, &&, ||, &c.
If you try
int b = 0;
if (b) {
printf("Hello World");
}
Does not print anything while :
int b = 1;
if (b) {
printf("Hello World");
}
Prints Hello World. Why ? Because 0 is false and 1 is true.
If you do quotient=(b)?(a/b):0; it is interpreted to is b true ? or in other words is b evaluated to 1 (while, again, 1 is true and 0 is false)
C did not originally have a Boolean type. Conditionals are simply int values in C. 0 is false, and any other value is truthy. If the type of b is int, or it can implicitly convert to int, then (b) ? foo : bar does the same thing as (b == 0) ? bar : foo. (However, b==0 will evaluate to 1 or 0, whereas b by itself might have other nonzero values that if or ? consider truthy.)
Related
Conditional operator in C is used like this:
condition ? value_if_true : value_if_false
What does 0 mean when it's used in the value_if_false?
I've seen some people using it like this, for example.
a == b ? i++ : 0
It seems like it does nothing. Does this work like return 0 in other functions?
In C language, ternary is shorter version of if statement and it requires both statements, if_true and if_false. It would be like this (in fact it can have multiple statements for one case, separated with comma):
Short:
condition ? if_true : if_false;
Long:
if (condition) {
if_true;
} else {
if_false;
}
You can also assign the value if you put something infront of condition.
Short:
result = condition ? if_true : if_false;
Long:
if (condition) {
result = if_true;
} else {
result = if_false;
}
Now here is the trick. In C language, writing 0; is a valid statement, so your ternary becomes in longer version same as code below:
if (a == b) {
i++;
} else {
0; /* This is valid C statement */
}
Or if you have assignment too, it would be:
if (a == b) {
result = i++;
} else {
result = 0;
}
You can also do this:
int a;
/* Code here ... */
condition ? a = 5: 0;
That is effectively the same as:
if (condition) {
a = 5;
} else {
/* DO not touch a */
}
The ?: operator is a ternary operator, but it is not called "ternary" as some answers and/or comments here suggest. It just is the arity of the operator, just as + is a binary operator or as & is unary. If it has a name at all, it is called "Conditional Expression"-operator
It is not quite equivalent to if/else, because it is a conditional value (with the consequence, that both expressions must have the same type) in the first place, not a conditional execution. Of course, both types can be cast to make them equal.
In the case of what the OP does, a better option (if if shall not be used) is in my opinion:
a == b && i++;
which resembles a bit more logical what happens. But of course it is a matter of style.
The reason why someone might want to write a == b ? i++ : 0; is that s/he probably wants to have an (Caution! You are now entering an opinion-based area) easier and faster alternative to if (a == b) i++; - although this is of course opinion-based and I personally not share the same opinion.
One thing I can think of as a "blocker" at the if statement is the requirement to write the parentheses () which can be omitted by using the conditional operator instead.
"But why the 0?"
The C syntax requires a third operand for the conditional operator. Else if you would want to compile for example:
a == b ? i++;
you will get an error from the compiler:
"error: expected ':' before ';' token"
Or respectively, doing so:
a == b ? i++ : ;
would raise:
"error: expected expression before ';' token"
So they use 0 as kind of "syntax satisfier" to be able to use the conditional operator as replacement for the if statement. You could use any other numeral value here as well, but 0 is the most readable value, which signifies that it has no use otherwise.
To showcase the use at an example:
#include <stdio.h>
int main (void)
{
int a, b, c = 4;
a = 2;
b = 2;
a == b ? c++ : 0;
printf("%d",c);
return 0;
}
The output for c will be 5, because a == b.
Note that a == b ? i++ : 0 is different when used f.e. inside of an assignment like f.e.:
int c = a == b ? i++ : 0;
Here c is either getting assigned by i or 0, dependent upon a == b is true or not. If a == b is true, c is assigned by i. If a == b is wrong, c is assigned by 0.
Side Notes:
To view it from a technical point, ?= is called the "conditional operator". The conditional operator is one of the group of ternary operators.
If you want to learn more about the conditional operator ?=, look at ISO:IEC 9899:2018 (C18), §6.5.15 - "Conditional operator" for more information.
There's nothing special about 0 one could write
a == b ? i++ : 1
And it would behave the same way.
Only difference is when you assign the expression to say another variable:
int c = a == b ? i++ : 1;
// a == b -> c will be i++
// a != b -> c will be 1
However it's much cleaner to write
if (a == b) i++;
It helps to think of the ternary operator as a shorthand way or writing an if-else statement.
If(a == b){
i++;
}else{
//if assigned, return 0. Else do nothing.
}
I am trying to understand the Pre-Processor syntax. Its really simple line of code that either returns "ON" or "OFF". However I am utterly confused as to what exactly the condition is?
I understand C's conditional statement as follows:
? x : y
If Condition ? return - replace? x : or y either way this line of code is as follows:
#define ONOFF(a) ((a) ? "ON" : "OFF")
I don't understand what condition must be met here? Is the condition that a has to be something other than null?
True and Flase can be more perfectly presented as 1 or 0 . As I can see you have declared
#define ONOFF(a) ((a) ? "ON" : "OFF")
Your condition here is (a), which istrueif the value ofa is non zero and false if ais 0
Which means in your program, if you write
int a=1;
char *str;
str=ONOFF(a);
The substitution which takes place is
int a=1;
char *str;
str=((a) ? "ON" : "OFF")// here a=1
Since here a is 1 and
1 is true and str gets the value ON. If a were 0, then str would get the value OFF
The condition is that a has to evaluate to true. In c, that means that a must be an expression that is non-zero.
If a is a pointer type, NULL is false and any other value is true.
If a is an integer type, 0 is false and any other value is true.
If a is a floating point type, 0 is false and any other value is true.
If a is a struct or a void type you will get a compile error.
To add a bit of context here, the first operand of the conditional operato has to be of scalar type. Now, from chapter §6.2.5, of C11,
Arithmetic types and pointer types are collectively called scalar types.
So, for the conditional expression,
any non-zero value gets evaluated as TRUE and zero (0) is evaluated as FALSE.
(In case of pointers) a NULL is FALSE, any non-NULL is TRUE.
Preprocessor macros do textual substitution, so a is not a variable -- it's just a placeholder for whatever text is in the parentheses when the macro is used.
You could use it for checking pointers are not null like this:
printf("%s\n", ONOFF(ptr));
printf("%s\n", ONOFF(ptr != null)); // This is the same
Or any other type of condition you like:
printf("%s\n", ONOFF(a > b));
printf("%s\n", ONOFF(a && b));
printf("%s\n", ONOFF(a == 1 || c == 4));
printf("%s\n", ONOFF(somefunction() != 0));
printf("%s\n", ONOFF((a == b && c == d) || (a == c && b == d));
printf("%s\n", ONOFF(my_bool_value));
#include<stdio.h>
main()
{
int big,x=3,y=2,z=1,q=4;
big=( x>y ? (x<z ? 20:10 && y>x ? 50:10 ) : (y>z ? 40:10 || x<q ? 30:10));
printf("big =%d",big);
return 0;
}
&& is a relational operator so it should return a true or false value i.e 0 or 1, but in this case its not. Please explain whats the logic behind its output?
Output: big =10
It's all about operator precedence (and a distressing lack of parentheses).
The output I get when I run your program is nbig =10 (with no newline; you should add a \n to your format string).
The value assigned to big isn't the result of an && or || operator. Let's reduce that over-complicated expression, one step at a time. (I've confirmed at each step that the result is unchanged.)
big=(x>y?(x<z?20:10 && y>x?50:10) : (y>z?40:10 || x<q?30:10));
We know that x>y is true, so we can drop the test and the third operand of the corresponding ?: operator:
big=(x<z?20:10 && y>x?50:10);
Let's remove the extraneous outer parentheses, add some new around the third operand of the outer ?: operator, and change the spacing a bit:
big = x<z ? 20 : (10 && y>x?50:10);
We know that x<z is false, so we can drop that and the second operand of the outer ?::
big = (10 && y>x?50:10);
Obviously 10 is true, so:
big = (y > x ? 50 : 10);
And y > x is false, so the result is 10 -- which is what I get when I run your program.
You probably assumed that this:
a ? b : c && d ? e : f
is equivalent to:
(a ? b : c) && (d ? e : f)
but in fact it's equivalent to:
a ? b : ((c && d) ? e : f)
because the && operator binds more tightly than the ?: operator.
In any case, if this is real code, you should definitely add enough parentheses so that a reasonably knowledgeable reader can understand the code without having to consult an operator precedence table. Mixing &&, ||, and ?: can be particularly confusing. Breaking down the expression into subexpressions, and assigning each one to a temporary variable (so it has a meaningful name) can also be helpful.
The above applies if you're trying to write a complex expression. If you're trying to understand something that someone else has written, you pretty much have to parse it yourself. Try doing what I did: incrementally simplify the expression (by removing parts or adding parentheses) in ways that don't change the meaning, confirming at each step that you get the same result. And if it's production code (rather than a quiz, which this appears to be), consider complaining bitterly encouraging the author to write clearer code.
Let's make your expression a bit more explicit by adding parethesis to show the precedence:
((x>y) ?
((x<z) ?
20 : ((10 && (y>x)) ? 50 : 10)
) : (
(y>z)?40:((10 || (x<q))?30:10)
)
)
x is greater than y, so let's consider
((x<z) ?
20 : ((10 && (y>x)) ? 50 : 10)
)
and x is not less than z, so
((10 && (y>x)) ? 50 : 10)
y is not greater than x, so
10
You can see that the results are not actually of your logical operators. Because of the complexity involved, you should almost certainly express such an evaluation using if statements to break up the logic in a clean way.
I was given this example in class and I am unsure what it does. I understand the colon adds a bit-field, but I am still uncertain about this question:
a = b > 0 ? 3:1;
?: operator is called the conditional operator.
If b value is > 0, the value 3 is assigned to a else the value 1 is assigned to a.
Take your Kernighan & Ritchie book 2nd edition, chapter 2.11 Conditional expressions, the behavior of the operator is explained.
? is a conditional operator:
a = b > 0 ? 3:1;
is equivalent to:
if(b > 0)
a = 3;
else
a = 1;
This is the conditional operator. It's equivalent to:
if (b > 0)
a = 3;
else
a = 1;
Read it as "a = if b > 0 then 3 else 1".
It's the conditional operator (generally called the ternary operator), which is used as a short way of writing if statements.
In general, it can be read:
condition ? value_if_true : value_if_false
So, in your case:
a = b > 0 ? 3:1;
Can be rewritten as:
if(b > 0) a = 3;
else a = 1;
The colon in this example doesn't mean anything related to bit fields - it's just the second part of the conditional.
This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
How do I use the conditional operator?
I’m new to C language, and in one of the sample codes I was reviewing I faced the statement:
A = A ? B: C[0]
I was just wondering what the task of the previous statement is and what will be the result after execution of the mentioned statement.
It's called a ternary operator. expr ? a : b returns a if expr is true, b if false. expr can be a boolean expression (e.g. x > 3), a boolean literal/variable or anything castable to a boolean (e.g. an int).
int ret = expr ? a : b is equivalent to the following:
int ret;
if (expr) ret = a;
else ret = b;
The nice thing about the ternary operator is that it's an expression, whereas the above are statements, and you can nest expressions but not statements. So you can do things like ret = (expr ? a : b) > 0;
As an extra tidbit, Python >=2.6 has a slightly different syntax for an equivalent operation: a if expr else b.
It assigns to A the value of B if A is true, otherwise C[0].
?:
result = a > b ? x : y; is identical to this block:
if (a > b) {
result = x;
}
else
{
result = y;
}
It's the same as an if else statement.
It could be rewritten as:
if ( A != 0 )
{
A = B;
}
else
{
A = C[ 0 ];
}
A gets assigned to B if A exists (not NULL), otherwise C[0].
if A equal 0 then A = C[0] else A = B