I'm using codeblocks and it is giving a different output to other compilers and I can't find a solution to it.What's the undefined behaviour in this program and is there any solution to avoid it?
This is the code to print the nth number in a number system with only 3 & 4.
#include<stdio.h>
#include<math.h>
int main(void)
{
int n,i,value;
scanf("%d",&n);
value=i=0;
while(n>0)
{
if((n%2)==0)
{
value+=4*pow(10,i);
}
else
{
value+=3*pow(10,i);
}
n=(n-1)/2;
i=i+1;
}
printf("\nThe number is : %d",value);
}
It works fine for numbers upto 6..And the output for numbers greater than 6 is one less than what it actually should be. E.g. if n=7,output=332 where it should be 333.
EDIT : Provided the full code with braces.
you're using the function pow(), which has the signature
double pow(double x, double y);
and calculations as int. Rounding/truncation errors ?
There is no undefined behavior in this code. i=i+1; is well-defined behavior, not to be confused with i=i++; which gives undefined behavior.
The only thing that could cause different outputs here would be floating point inaccuracy.
Try value += 4 * (int)nearbyint(pow(10,i)); and see if it makes any difference.
Seems that floating point gets truncated.
It sounds like a compiler bug.
You are computing the result as value+=3*pow(10,i); but what this actually translates to is value+= (int)(3*pow(10,i));
One of two things might be wrong here:
pow(10,0)!=1.0
cast to int is truncating the result incorrectly.
To debug it easily just try printing the partial results and see there the problem is.
The problem here is most likely that the pow function on this particular platform performs its computations by taking the log of the argument (perhaps natural log; perhaps log base 2), multiplying by the exponent, and then raising the base of the first logarithm to the power of that product. Performing such an operation with infinite-precision numbers would yield a mathematically-correct result, as would performing the operation on extended-precision numbers and returning a double result. My guess would be that the pow function used on this implementation may have been written for a platform which could perform the intermediate computations using extended-precision numbers, and which would consequently return correct double-precision values, but it is being run on a platform which lacks an extended-precision type. As a consequence of this, pow(10,3) may be returning something like 999.9999999997, and coercing that to int yields 999 rather than 1000.
If you're trying to get an integer-type result, there's really no reason to compute the power as a floating-point value. Rather than computing 10^i within the loop, it would be better to have a variable that's initialized to 1 and gets multiplied by 10 each time through the loop.
Related
While running the following lines of code:
int i,a;
for(i=0;i<=4;i++)
{
a=pow(10,i);
printf("%d\t",a);
}
I was surprised to see the output, it comes out to be 1 10 99 1000 9999 instead of 1 10 100 1000 10000.
What could be the possible reason?
Note
If you think it's a floating point inaccuracy that in the above for loop when i = 2, the values stored in variable a is 99.
But if you write instead
a=pow(10,2);
now the value of a comes out to be 100. How is that possible?
You have set a to be an int. pow() generates a floating point number, that in SOME cases may be just a hair less than 100 or 10000 (as we see here.)
Then you stuff that into the integer, which TRUNCATES to an integer. So you lose that fractional part. Oops. If you really needed an integer result, round may be a better way to do that operation.
Be careful even there, as for large enough powers, the error may actually be large enough to still cause a failure, giving you something you don't expect. Remember that floating point numbers only carry so much precision.
The function pow() returns a double. You're assigning it to variable a, of type int. Doing that doesn't "round off" the floating point value, it truncates it. So pow() is returning something like 99.99999... for 10^2, and then you're just throwing away the .9999... part. Better to say a = round(pow(10, i)).
This is to do with floating point inaccuracy. Although you are passing in ints they are being implicitly converted to a floating point type since the pow function is only defined for floating point parameters.
Mathematically, the integer power of an integer is an integer.
In a good quality pow() routine this specific calculation should NOT produce any round-off errors. I ran your code on Eclipse/Microsoft C and got the following output:
1 10 100 1000 10000
This test does NOT indicate if Microsoft is using floats and rounding or if they are detecting the type of your numbers and choosing the appropriate method.
So, I ran the following code:
#include <stdio.h>
#include <math.h>
main ()
{
double i,a;
for(i=0.0; i <= 4.0 ;i++)
{
a=pow(10,i);
printf("%lf\t",a);
}
}
And got the following output:
1.000000 10.000000 100.000000 1000.000000 10000.000000
No one spelt out how to actually do it correctly - instead of pow function, just have a variable that tracks the current power:
int i, a, power;
for (i = 0, a = 1; i <= 4; i++, a *= 10) {
printf("%d\t",a);
}
This continuing multiplication by ten is guaranteed to give you the correct answer, and quite OK (and much better than pow, even if it were giving the correct results) for tasks like converting decimal strings into integers.
While running the following lines of code:
int i,a;
for(i=0;i<=4;i++)
{
a=pow(10,i);
printf("%d\t",a);
}
I was surprised to see the output, it comes out to be 1 10 99 1000 9999 instead of 1 10 100 1000 10000.
What could be the possible reason?
Note
If you think it's a floating point inaccuracy that in the above for loop when i = 2, the values stored in variable a is 99.
But if you write instead
a=pow(10,2);
now the value of a comes out to be 100. How is that possible?
You have set a to be an int. pow() generates a floating point number, that in SOME cases may be just a hair less than 100 or 10000 (as we see here.)
Then you stuff that into the integer, which TRUNCATES to an integer. So you lose that fractional part. Oops. If you really needed an integer result, round may be a better way to do that operation.
Be careful even there, as for large enough powers, the error may actually be large enough to still cause a failure, giving you something you don't expect. Remember that floating point numbers only carry so much precision.
The function pow() returns a double. You're assigning it to variable a, of type int. Doing that doesn't "round off" the floating point value, it truncates it. So pow() is returning something like 99.99999... for 10^2, and then you're just throwing away the .9999... part. Better to say a = round(pow(10, i)).
This is to do with floating point inaccuracy. Although you are passing in ints they are being implicitly converted to a floating point type since the pow function is only defined for floating point parameters.
Mathematically, the integer power of an integer is an integer.
In a good quality pow() routine this specific calculation should NOT produce any round-off errors. I ran your code on Eclipse/Microsoft C and got the following output:
1 10 100 1000 10000
This test does NOT indicate if Microsoft is using floats and rounding or if they are detecting the type of your numbers and choosing the appropriate method.
So, I ran the following code:
#include <stdio.h>
#include <math.h>
main ()
{
double i,a;
for(i=0.0; i <= 4.0 ;i++)
{
a=pow(10,i);
printf("%lf\t",a);
}
}
And got the following output:
1.000000 10.000000 100.000000 1000.000000 10000.000000
No one spelt out how to actually do it correctly - instead of pow function, just have a variable that tracks the current power:
int i, a, power;
for (i = 0, a = 1; i <= 4; i++, a *= 10) {
printf("%d\t",a);
}
This continuing multiplication by ten is guaranteed to give you the correct answer, and quite OK (and much better than pow, even if it were giving the correct results) for tasks like converting decimal strings into integers.
While running the following lines of code:
int i,a;
for(i=0;i<=4;i++)
{
a=pow(10,i);
printf("%d\t",a);
}
I was surprised to see the output, it comes out to be 1 10 99 1000 9999 instead of 1 10 100 1000 10000.
What could be the possible reason?
Note
If you think it's a floating point inaccuracy that in the above for loop when i = 2, the values stored in variable a is 99.
But if you write instead
a=pow(10,2);
now the value of a comes out to be 100. How is that possible?
You have set a to be an int. pow() generates a floating point number, that in SOME cases may be just a hair less than 100 or 10000 (as we see here.)
Then you stuff that into the integer, which TRUNCATES to an integer. So you lose that fractional part. Oops. If you really needed an integer result, round may be a better way to do that operation.
Be careful even there, as for large enough powers, the error may actually be large enough to still cause a failure, giving you something you don't expect. Remember that floating point numbers only carry so much precision.
The function pow() returns a double. You're assigning it to variable a, of type int. Doing that doesn't "round off" the floating point value, it truncates it. So pow() is returning something like 99.99999... for 10^2, and then you're just throwing away the .9999... part. Better to say a = round(pow(10, i)).
This is to do with floating point inaccuracy. Although you are passing in ints they are being implicitly converted to a floating point type since the pow function is only defined for floating point parameters.
Mathematically, the integer power of an integer is an integer.
In a good quality pow() routine this specific calculation should NOT produce any round-off errors. I ran your code on Eclipse/Microsoft C and got the following output:
1 10 100 1000 10000
This test does NOT indicate if Microsoft is using floats and rounding or if they are detecting the type of your numbers and choosing the appropriate method.
So, I ran the following code:
#include <stdio.h>
#include <math.h>
main ()
{
double i,a;
for(i=0.0; i <= 4.0 ;i++)
{
a=pow(10,i);
printf("%lf\t",a);
}
}
And got the following output:
1.000000 10.000000 100.000000 1000.000000 10000.000000
No one spelt out how to actually do it correctly - instead of pow function, just have a variable that tracks the current power:
int i, a, power;
for (i = 0, a = 1; i <= 4; i++, a *= 10) {
printf("%d\t",a);
}
This continuing multiplication by ten is guaranteed to give you the correct answer, and quite OK (and much better than pow, even if it were giving the correct results) for tasks like converting decimal strings into integers.
While running the following lines of code:
int i,a;
for(i=0;i<=4;i++)
{
a=pow(10,i);
printf("%d\t",a);
}
I was surprised to see the output, it comes out to be 1 10 99 1000 9999 instead of 1 10 100 1000 10000.
What could be the possible reason?
Note
If you think it's a floating point inaccuracy that in the above for loop when i = 2, the values stored in variable a is 99.
But if you write instead
a=pow(10,2);
now the value of a comes out to be 100. How is that possible?
You have set a to be an int. pow() generates a floating point number, that in SOME cases may be just a hair less than 100 or 10000 (as we see here.)
Then you stuff that into the integer, which TRUNCATES to an integer. So you lose that fractional part. Oops. If you really needed an integer result, round may be a better way to do that operation.
Be careful even there, as for large enough powers, the error may actually be large enough to still cause a failure, giving you something you don't expect. Remember that floating point numbers only carry so much precision.
The function pow() returns a double. You're assigning it to variable a, of type int. Doing that doesn't "round off" the floating point value, it truncates it. So pow() is returning something like 99.99999... for 10^2, and then you're just throwing away the .9999... part. Better to say a = round(pow(10, i)).
This is to do with floating point inaccuracy. Although you are passing in ints they are being implicitly converted to a floating point type since the pow function is only defined for floating point parameters.
Mathematically, the integer power of an integer is an integer.
In a good quality pow() routine this specific calculation should NOT produce any round-off errors. I ran your code on Eclipse/Microsoft C and got the following output:
1 10 100 1000 10000
This test does NOT indicate if Microsoft is using floats and rounding or if they are detecting the type of your numbers and choosing the appropriate method.
So, I ran the following code:
#include <stdio.h>
#include <math.h>
main ()
{
double i,a;
for(i=0.0; i <= 4.0 ;i++)
{
a=pow(10,i);
printf("%lf\t",a);
}
}
And got the following output:
1.000000 10.000000 100.000000 1000.000000 10000.000000
No one spelt out how to actually do it correctly - instead of pow function, just have a variable that tracks the current power:
int i, a, power;
for (i = 0, a = 1; i <= 4; i++, a *= 10) {
printf("%d\t",a);
}
This continuing multiplication by ten is guaranteed to give you the correct answer, and quite OK (and much better than pow, even if it were giving the correct results) for tasks like converting decimal strings into integers.
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main()
{
int n,i,ele;
n=5;
ele=pow(n,2);
printf("%d",ele);
return 0;
}
The output is 24.
I'm using GNU/GCC in Code::Blocks.
What is happening?
I know the pow function returns a double , but 25 fits an int type so why does this code print a 24 instead of a 25? If n=4; n=6; n=3; n=2; the code works, but with the five it doesn't.
Here is what may be happening here. You should be able to confirm this by looking at your compiler's implementation of the pow function:
Assuming you have the correct #include's, (all the previous answers and comments about this are correct -- don't take the #include files for granted), the prototype for the standard pow function is this:
double pow(double, double);
and you're calling pow like this:
pow(5,2);
The pow function goes through an algorithm (probably using logarithms), thus uses floating point functions and values to compute the power value.
The pow function does not go through a naive "multiply the value of x a total of n times", since it has to also compute pow using fractional exponents, and you can't compute fractional powers that way.
So more than likely, the computation of pow using the parameters 5 and 2 resulted in a slight rounding error. When you assigned to an int, you truncated the fractional value, thus yielding 24.
If you are using integers, you might as well write your own "intpow" or similar function that simply multiplies the value the requisite number of times. The benefits of this are:
You won't get into the situation where you may get subtle rounding errors using pow.
Your intpow function will more than likely run faster than an equivalent call to pow.
You want int result from a function meant for doubles.
You should perhaps use
ele=(int)(0.5 + pow(n,2));
/* ^ ^ */
/* casting and rounding */
Floating-point arithmetic is not exact.
Although small values can be added and subtracted exactly, the pow() function normally works by multiplying logarithms, so even if the inputs are both exact, the result is not. Assigning to int always truncates, so if the inexactness is negative, you'll get 24 rather than 25.
The moral of this story is to use integer operations on integers, and be suspicious of <math.h> functions when the actual arguments are to be promoted or truncated. It's unfortunate that GCC doesn't warn unless you add -Wfloat-conversion (it's not in -Wall -Wextra, probably because there are many cases where such conversion is anticipated and wanted).
For integer powers, it's always safer and faster to use multiplication (division if negative) rather than pow() - reserve the latter for where it's needed! Do be aware of the risk of overflow, though.
When you use pow with variables, its result is double. Assigning to an int truncates it.
So you can avoid this error by assigning result of pow to double or float variable.
So basically
It translates to exp(log(x) * y) which will produce a result that isn't precisely the same as x^y - just a near approximation as a floating point value,. So for example 5^2 will become 24.9999996 or 25.00002