integer log base 10 function without using java.lang.Math?

Question

I am assigned to a task of creating a function int logBase10Estimate(int n) that returns the log of a number base 10. The answer is integer part only.(w/o using the Math class)

Is this okay?

static int logBase10Estimate(int n){
 int count = 0;
 for(int x = 10; ;x*=10){
 if(x < n){
 count+=1;
 System.out.println(x);
 }
 else if(x == n){
 count+=1;
 break;
 }
 else{
 break;
 }
 } 
 return count;
}

• greybeard: If you wanted a more symmetrical error, you could start at round(sqrt(10)).
• S.S. Anne: Why not just int count = 0; for(; n; n/=10) { ctr++; } return count;?
• slepic: @greybeard Error seems to be no concern here. And round(sqrt(10))=3 btw ;)
• greybeard: (@slepic 3 does not tell why/how it was chosen.)
• Mast: Please do not update the code in your question to incorporate feedback from answers, doing so goes against the Question + Answer style of Code Review. This is not a forum where you should keep the most updated version in your question. Please see what you may and may not do after receiving answers .

Answer 1

The printing should not be there :)

You don't need another variable x. You can modify the local copy n, because you won't need its original value for the entire algorithm and because it is passed by value, you won't change the value for the caller.

Also check for valid input.

static int logBase10Estimate(int n)
{
 if (n <= 0) {
 throw new IllegalArgumentException('Log of non-positive number is undefined');
 }
 int count = 0;
 for (; n>=10; n/=10) {
 ++count;
 }
 return count;
}

• zvz: Sorry i forgot to remove the print function...Can i ask of what's the difference of putting an increment before and after a variable in a loop?
• Mast: @ZiegfredZorrilla Postfix vs prefix

Answer 2

You have the possibility of an infinite loop.

Consider: logBase10Estimate(2147483647). Should be about 9. We need a value of \10ドル^i\$ which is greater than 1000000000. So, what values does x take on?

 10
 100
 1000
 10000
 100000
 1000000
 10000000
 100000000
 1000000000
 1410065408
 1215752192
 -727379968
 1316134912
 276447232
-1530494976
 1874919424
 1569325056
-1486618624
 : : :

Any value of num greater than 1000000000 is going to give incorrect results. And some values, like 2147483647 will never give an answer, because no int value is greater than it, and x will never be odd, so it will never be equal to it either.

Not only is slepic's solution cleaner, it is correct and it terminates for all values.