output of a c program and the reason behind it

[pannu]

tell me the output of the following c program and the reason behind it

main()
{
int a=400;
int b;
b=a*a/a;
printf("%d",b);
}

[horace1]

tell me the output of the following c program and the reason behind it

main()
{
int a=400;
int b;
b=a*a/a;
printf("%d",b);
}

the output is 400
a equals 400, the result of the expression a*a/a equals a which is assigned to b, b then equals 400

[thefarmer]

tell me the output of the following c program and the reason behind it

main()
{
int a=400;
int b;
b=a*a/a;
printf("%d",b);
}

hi there,

in precedence of operators when writing complex expressions with several operands, we may have some doubts about which operand is evaluated first and which later. in your expression
b = a*a/a
we may doubt if it really means:

a = (400*400)/400 // with a result of 400, or a = 400 *(400/400) // with a result of 400 , although you can get the same result this is very critical, what if the values are changed?

The correct answer is the first of the two expressions, with a result of 400. There is an established order with the priority of each operator, and not only the arithmetic ones (those whose preference come from mathematics) but for all the operators which can appear in C++. From greatest to lowest priority,in reading data it always finds the which always comes first.


ok

regards,
mike

[Banfa]

because * and / have left to right associativity

b=a*a/a;

is evaluated as

b=(a*a)/a;

However I not that a and b are ints so the result of this is dependent on the size of int, typically int is either 2 or 4 bytes. If int is 4 bytes then the result is

b = (400*400) / 400
b = (160000) / 400
b = 400

However if int is 2 bytes then the multiplicaltion 400 * 400 = 160000 is behond the bounds of an int. What you actually get is undefined behaviour, i.e. anything could happen. However probably the most likely result is that the extra bits are droped therefore 400 * 400 = 28928 so

b = (400*400) / 400
b = (28928) / 400 (note this is not a whole number but fractions are dropped)
b = 72



This is a feature of integer arithmatic, when doing sums with big numbers you always need to check that you will not run into wrapping errors.