Programming Forums > Application Development > C++

Jared · Feb 17th, 2005, 4:43 PM

First of all, this is standard C.

I have a statement in my code like this:

int x = log(8)/log(2)

After much confusion I found that x was assigned 2. This is not what I want though. I know that log(8)/log(2) equals 3.000000... in double and I want it to convert that to the integer 3 and save it into x. For some reason it will not do this though. I have tested by just putting

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double x = 3;
int a = x;
printf("a: %d", a);

and it prints out a 3 so I do know that it works in that case. When I use the log() function for some reason it floors the 3.000000... and makes it a 2. Is there any way to fix this so int x equals 3?
I also tried this:

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double x = log(8)/log(2);
int a = ceil(x);
printf("a: %d", a);

which gave me the right number 3. I don't know how the ceiling of 3.000000... would be 3 and the floor of 3.000000... would be 2.

The actual use of the statement in my code is in a function:

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arrayInsert(int n, int *array){
int a = log(n)/log(2);
int b = a - 1;
int j = n + pow(2, b);
.
.
.
}

a is assigned the right value everytime except when n = 8. log(2)/log(2) works, log(4)/log(2) works, log(16)/log(2) works, but log(8)/log(2) never works. I don't know how one value of it would cause problems. I'm using Visual Studio, could it have a bug in it?

Jared · Feb 17th, 2005, 5:50 PM

For now, I'm just using this little check.

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if(n == 8) a = 3;
else a = log(n)/log(2);

I still don't know why it does not return the right value when n = 8. I'm going to try it in Linux and see if it works right there.

Dietrich · Feb 18th, 2005, 1:53 AM

Can't figure out your problem. Must be your compiler, this works just fine with Pelles C under Windows XP.

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// PellesC

#include <stdio.h>
#include <math.h>

int main()
{
  int x = log(8)/log(2);
  
  printf("x = %d\n", x);  // result is x = 3
  
  getchar();  // wait
  return 0;
}

Feb 17th, 2005, 4:43 PM	#1
Jared Newbie Join Date: Feb 2005 Posts: 2 Rep Power: 0	Problem converting double to int First of all, this is standard C. I have a statement in my code like this: (Toggle Plain Text) int x = log(8)/log(2) int x = log(8)/log(2) After much confusion I found that x was assigned 2. This is not what I want though. I know that log(8)/log(2) equals 3.000000... in double and I want it to convert that to the integer 3 and save it into x. For some reason it will not do this though. I have tested by just putting (Toggle Plain Text) double x = 3; int a = x; printf("a: %d", a); double x = 3; int a = x; printf("a: %d", a); and it prints out a 3 so I do know that it works in that case. When I use the log() function for some reason it floors the 3.000000... and makes it a 2. Is there any way to fix this so int x equals 3? I also tried this: (Toggle Plain Text) double x = log(8)/log(2); int a = ceil(x); printf("a: %d", a); double x = log(8)/log(2); int a = ceil(x); printf("a: %d", a); which gave me the right number 3. I don't know how the ceiling of 3.000000... would be 3 and the floor of 3.000000... would be 2. The actual use of the statement in my code is in a function: (Toggle Plain Text) arrayInsert(int n, int array){ int a = log(n)/log(2); int b = a - 1; int j = n + pow(2, b); . . . } arrayInsert(int n, int array){ int a = log(n)/log(2); int b = a - 1; int j = n + pow(2, b); . . . } a is assigned the right value everytime except when n = 8. log(2)/log(2) works, log(4)/log(2) works, log(16)/log(2) works, but log(8)/log(2) never works. I don't know how one value of it would cause problems. I'm using Visual Studio, could it have a bug in it?

Feb 17th, 2005, 5:50 PM	#2
Jared Newbie Join Date: Feb 2005 Posts: 2 Rep Power: 0	For now, I'm just using this little check. (Toggle Plain Text) if(n == 8) a = 3; else a = log(n)/log(2); if(n == 8) a = 3; else a = log(n)/log(2); I still don't know why it does not return the right value when n = 8. I'm going to try it in Linux and see if it works right there.

Feb 18th, 2005, 1:53 AM	#3
Dietrich Professional Programmer Join Date: Feb 2005 Posts: 434 Rep Power: 4	Can't figure out your problem. Must be your compiler, this works just fine with Pelles C under Windows XP. (Toggle Plain Text) // PellesC #include <stdio.h> #include <math.h> int main() { int x = log(8)/log(2); printf("x = %d\n", x); // result is x = 3 getchar(); // wait return 0; } // PellesC #include <stdio.h> #include <math.h> int main() { int x = log(8)/log(2); printf("x = %d\n", x); // result is x = 3 getchar(); // wait return 0; }

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Programming Forums > Application Development > C++

Problem converting double to int