Can someone explain what exactly is happening in these two statements listed below:
1) int enabled = val == 0; //val is an int
2) int sync = param || (foo->delay == NULL); //param and delay is both int
int enabled = val == 0;
read as
int enabled = (val == 0);
and
(val == 0)
will be either 0 or non zero if val is 0 or not. enabled will then be initialized with that value
Equivalent to:
int enabled;
if(val == 0)
{
enabled = 1;
}
else
{
enabled = 0;
}
now you do that same analysis on the second one
You will set the variable enabled to 1 if val is equal to 0 and 0 otherwise.
sync will be equal to 1 if param is non-zero and in case it is 0 then it will be 1 if foo->delay is NULL else it will be 0.
From standard §6.5.9p3 backing up what I said:
The == (equal to) and != (not equal to) operators are analogous to the relational operators except for their lower precedence.108) Each of the operators yields 1 if the specified relation is true and 0 if it is false. The result has type int. For any pair of operands, exactly one of the relations is true.
Also in case || there is standard saying the evaluation logic: from §6.5.14
Unlike the bitwise | operator, the || operator guarantees left-to-right evaluation; if the second operand is evaluated, there is a sequence point between the evaluations of the first and second operands. If the first operand compares unequal to 0, the second operand is not evaluated.
Also in the same section the rule which dictates what will be the result if param is non-zero.
The || operator shall yield 1 if either of its operands compare unequal to 0; otherwise, it yields 0. The result has type int.
C only adopted a true boolean type from C++ with C99, though it named it _Bool so it wouldn't conflict with pre-existing code and provided the header <stdbool.h> to make it nicer.
The prior convention that only 0 is considered falsey (a literal zero, which in a pointer-context is a null-pointer), and anything else truthy was not changed. (That's the reverse to how command shells do it by the way.)
All boolean operators and conversion from _Bool use canonical values of 0 and 1.
That should be enough history and details to understand the code.
Related
I have three variables: a, b, c. Let's say they are integers. I want to find the first non-zero value among them, in that particular order, without looping. The following seems to work, but I am not sure if that is because I am lucky, or because the language guarantees it:
int main(int argc, char *argv[]) {
int a = 0;
int b = 3;
int c = 5;
int test;
if ((test = a) != 0 || (test = b) != 0 || (test = c) != 0) {
printf("First non-zero: %d\n", test);
} else {
printf("All zero!\n");
}
return 0;
}
Is the repeated assignment with short-circuiting shown here guaranteed to work as intended, or am I missing something?
This might be one place where a three-letter answer would be acceptable, but a two-letter answer might require more explanation.
It would!
Because of the nature of the OR operator if any of the condition is
true then the test stops.
Thus i think what you did was basically equivalent to:
test = a != 0 ? a : b != 0 ? b : c != 0 ? c : 0;
printf("%d\n",test);
but heck yours looks good.
[update]
As per what chqrlie mentioned it can be further simplified to:
test = a ? a : b ? b : c;
Yes, your expression is fully defined because there is a sequence point at each || operator and the short circuit evaluation guarantees that the first non zero value assigned to test completes the expression.
Here is a crazy alternative without sequence points that may produce branchless code:
int test = a + !!a * (b + !!b * c);
printf("%d\n", test);
The code is very bad practice but it is guaranteed to work fine.
This is because the || and && operators have special characteristics - unlike most operators in C, they guarantee that the evaluation of the left operand is sequenced (executed) before the evaluation of the right operand. This is the reason that the code works. There's also a guarantee that the right operand will not be evaluated if it is sufficient to evaluate the left one ("short circuit"). Summarized in C17 6.5.14/4:
Unlike the bitwise | operator, the || operator guarantees left-to-right evaluation; if the
second operand is evaluated, there is a sequence point between the evaluations of the first
and second operands. If the first operand compares unequal to 0, the second operand is
not evaluated.
"Sequence point" being the key here, which is what gives the expression a deterministic outcome.
Had you used pretty much any other operator (like for example bitwise |), then the result would be undefined, because you have multiple side effects (assignments) on the same variable test in the same expression.
A more sound version of the same algorithm would involve storing the data in an array and loop through it.
I don't understand why it prints 1 at the end.
#include <stdio.h>
int main(){
printf("%d\n", -2&&2);
return 0；
}
&& is a Boolean operator, not an integer operator. In C any non-zero value is interpreted as true when applied to a Boolean operation, while zero is the only integer value regarded as false when applied to a Boolean operation.
So:
(<non-zero> && <non-zero>) == true
then conversely when a true result is treated as an integer (as the %d format specifier dictates in this case), true is represented by 1, while false is zero. So in this case true becomes 1.
If you want to print a Boolean result, then:
printf( "%s\n", (-2 && 2) == 0 ? "false" : "true" ) ;
Strictly the expression:
-2 && 2
is equivalent to:
(-2 != 0) && (2 != 0)
Which has strict type agreement since != has a Boolean result from integer operands, and so && is presented with Boolean operands only with no implicit conversion.
Because -2&&2 is evaluated to true = 1 (when you AND two numbers together you will get true unless one or both of the numbers is 0).
Try changing -2&&2 to true && false; it will print 0.
The expression -2&&2 resumes to 1 or true (true is always != 0, while false == 0) based on the boolean arithmetic opration.
And here is how it resumes to 1:
&& represents the logical AND, not bitwise AND, which is &, in C. So you are logical ANDing the values of -2 and 2.
Let´s consider the following sentence as one of the statements for the Logical AND (&&)-operation:
If both the operands are non-zero, then the condition becomes true.
-2 is non-zero, equals 1.
2is also non-zero, equals 1.
So the result is also 1 (1AND 1 = 1) or true represents 1.
The && operator is for logical and, not bitwise and.
The expression in your program evaluates as
printf("%d\n", (-2 != 0) && (2 != 0));
Which is 1 because boolean expressions have value 0 for false and 1 for true in C.
Conversely, if you had written -2 & 2, you program would have implementation defined behavior, depending on the representation of negative integers used for the target system. On modern systems with two's complement representation, this bitwise and expression evaluates to 2. Same value for sign-magnitude representation, but on rare systems with ones' complement representation, the value would be 0.
Please consider these piece of C code:
if ((value & 1) == 1)
{
}
Assuming value equals 1, will (value & 1) return 1 or any unspecified non zero number?
§6.5.8 Relational operators
Each of the operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to) shall yield 1 if the specified relation is true and 0 if it is false.) The result has type int.
§6.5.9 Equality operators
The == (equal to) and != (not equal to) operators are analogous to the relational operators except for their lower precedence.) Each of the operators yields 1 if the specified relation is true and 0 if it is false. The result has type int. For any pair of operands, exactly one of the relations is true.
§6.5.13 Logical AND operator
The && operator shall yield 1 if both of its operands compare unequal to 0; otherwise, it yields 0. The result has type int.
§6.5.14 Logical OR operator
The || operator shall yield 1 if either of its operands compare unequal to 0; otherwise, it yields 0. The result has type int.
Does “true” in C always mean 1?
No. Any expression that evaluates to a non-zero value is "true" in C. For example,
if (-1) {
}
if (5-10) {
}
if( "string literal") {
}
are all "true" and pass the condition.
In your specific example, the relational operator == yields 1 which is also "true"(the same holds for all other relational operators as noted by Govind).
If you are really asking about whether the bit-wise AND (&) yields 1 when value is 1, then yes, value & 1 yields 1 (assuming value is an integer type -- & operator requires its operands to be integers).
In fact, you can probably try to understand the individual parts and (generally how the & and == operators behave) by using a simple program:
#include <stdio.h>
int main(void)
{
int value = 1;
printf(" value & 1 = %d\n", value & 1);
printf(" 2 & 1 = %d\n", 2 & 1);
printf("((value & 1) == 1) = %d", (value & 1) == 1);
}
I've a C university exam coming up next week and i was looking at old exam papers a one of the questions gives this fragmented bit of code.
int a=2, b=-1, c=0;
if (a-2||b&&c||a){
printf("True\n");
} else {
printf("False\n");
}
We have to determine what the output of this code will be but the if statement makes no sense to me any if statement I've come across has been very specific like saying
if( x == 0)
I don't know what this is looking for my only assumption is that its going to be always true. Am I right or is there more to it then that?
This assignment has two goals:
to show what booleans are in C: Essentially they evaluate to ints with false mapping to 0 and true mapping to 1. In turn, any numeric or pointer value can be used in an integer context, with the respective zero value (0, 0.0, NULL (pointer), 0.0f, 0L etc.) evaluating as false and all others as true.
to show the precedence of operators
&& has a higher precedence than ||, so this statement is equivalent to
a-2 || (b&&c) || a
which will evaluate to true if any of the values is true.
As a==2, a-2 is 0. c is 0, so b && c is 0 as well.
So we have 0 || 0 || a, which is true as a is 2.
Most languages interprets non-zero integers as true and zero as false, so here you would have to calculate each one of the terms. Without any parenthesis, I would suggest that the && statement is taken in account first. So we have:
if (2-2 // gives zero
|| // OR
-1 && 0 // -1 AND 0 gives false
|| // OR
a) // Which is 2, which is true
So you're right, this statement is always true. This exercice was about showing predecence orders, and the fact that everything is numerical, even in boolean logic.
This is really important for you to understand.
If the predecence was the other way around (|| > &&), you must understand that it would have been false instead. I think this example's whole point is here.
(a-2 || b) && (c || a)
false && true
--> false
You need to understand that truth and falsity in C is always numerical.
https://www.le.ac.uk/users/rjm1/cotter/page_37.htm
Namely, anything that evaluates to numerical zero is false, and anything that evaluates to numerical non-zero is true.
In c language integers 0 is treated as false and any non-zero integer value is true but it should be noted that it is language specific and the sme statement will show compilation error in java as java is more strict and integers are not converted to booleans.
Talking about the above assignment problem the expression inside if statement will evaluate to true as
(a-2||b&&c||a) is same as
(2-2||-1&&0||2) which is same as
(0||0||2) which is evaluated as
(false||false||true) and hence the entire expression evaluates to
true.
hope it helps.
int a=2, b=-1, c=0;
int first=a-2; //0 -> false
bool second= b&& c; // nonZero&&zero -> true&&false -> false
int third = 2; // nonZero -> true
// false|| false|| true -> true
if (first || second || third ){
printf("True\n");
} else {
printf("False\n");
}
you need to understand two things before solving this problem that is
operator precedence and
associativity of operators
operator precedence tells c compiler that which operation to perform first.
and if two operators have same precedence than associativity tells evaluate left to right or right to left in you expression
int a=2, b=-1, c=0;
if (a-2||b&&c||a){
you can think it as
if((a-2)||(b&&c)||a){}
means - has top precedence so it will solved first
reduced to if(0||(b&&c)||a){}
then && has higher precedence so
reduced to if(0||false||a)
then the associativity is left to right so
reduced to if(false||a)
that is(false||2)
return true
In almost every programming language as far as I know 0 means false and 1 means true.
So coming up to your question: you have used && and || operators. Both of these are called Logical operators.
Now first block of yours is a-2||b :-
2-2||-1 so 0||-1. Now since the right expression of || is -1 the or operator will return 1 i.e. True because one of the values of 0 and -1 is non-zero 0 i.e. -1.
Therefore the expression resolves to 1&&c||a :-
Now c=0, therefore 1&&0 returns a 0 because && will only return 1 if both the expressions right and left of it are non zero.
So expression becomes 0||2 :-
Now since || (or operator) requires only one of operands either on right or left side to be non zero hence 0||2 returns 1.
Now your if (a-2||b&&c||a) statement resolves to
if (1)
{
printf("True\n"); }
else......
Therefore since 1 means TRUE the if statement will execute and you will get output as True.
I know that in C, for if statements and comparisons FALSE = 0 and anything else equals true.
Hence,
int j = 40
int k = !j
k == 0 // this is true
My question handles the opposite. What does !0 become? 1?
int l = 0
int m = !l
m == ? // what is m?
Boolean/logical operators in C are required to yield either 0 or 1.
From section 6.5.3.3/5 of the ISO C99 standard:
The result of the logical negation operator ! is 0 if the value of its operand compares unequal to 0, 1 if the value of its operand compares equal to 0.
In fact, !!x is a common idiom for forcing a value to be either 0 or 1 (I personally prefer x != 0, though).
Also see Q9.2 from the comp.lang.c FAQ.
§6.5.3.3/5: "The result of the logical negation operator ! is 0 if the value of its operand compares unequal to 0, 1 if the value of its operand compares equal to 0. The result has type int."
The other logical operators (e.g., &&, ||) always produce either 0 or 1 as well.
Generally, yes, it'll become 1. That said even if that is guaranteed behavior (which I'm not sure of) I'd consider code that relied on that to be pretty awful.
You can assume that it's a true value. I wouldn't assume anything more.
The Bang operator (!) is the logical not operator found commonly in C, C++ and C#, so
!0 == 1
!1 == 0
This is based on the language characteristic of what is interpreted to be either true or false... in more modern languages it would be like this
!false == true
!true == false
See DeMorgan Law concerning truth tables...
!x will be expand to (x==0) so:
if x=0 -> !x take value from (0==0) = TRUE (value 1)
if x!=0 -> !x take value from (x==0) = FALSE (value 0)