Why does pow(n,2) return 24 when n=5, with my compiler and OS?

by

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:

  1. You won’t get into the situation where you may get subtle rounding errors using pow.

  2. Your intpow function will more than likely run faster than an equivalent call to pow.

More Related Contents:

  1. Why do you have to link the math library in C?
  2. Using pow() function throws undefined reference error in C
  3. What is the difference between getch() and getchar()?
  4. Linking with gcc and -lm doesn’t define ceil() on Ubuntu
  5. sqrt() function not working with variable arguments [duplicate]
  6. Why do you have to link the math library in C?
  7. c math linker problems on Ubuntu 11.10 [duplicate]
  8. Does stdlib’s rand() always give the same sequence?
  9. log(10.0) can compile but log(0.0) cannot with undefined reference?
  10. Divide one list into two. C
  11. expected ‘void (**)(void *, const char *)’ but argument is of type ‘void (*)(void *, const char *)
  12. What can I use for input conversion instead of scanf?
  13. Valid use of goto for error management in C?
  14. What is the simplest way of implementing bigint in C?
  15. In C how do you redirect stdin/stdout/stderr to files when making an execvp() or similar call?
  16. Compiling without libc
  17. C – allocating a matrix in a function
  18. how to print memory bits in c
  19. Finding the length of an integer in C
  20. using printf to print out floating values
  21. What is the difference between exit and return?
  22. “&&” and “and” operator in C
  23. What’s the difference between a file descriptor and a file pointer?
  24. Why is int rather than unsigned int used for C and C++ for loops?
  25. How to use SO_KEEPALIVE option properly to detect that the client at the other end is down?
  26. strdup() function
  27. Timing CUDA operations
  28. Why does static initialization of flexible array member work?
  29. gdb prints wrong values when modifying arguments
  30. Multiple assignment in one line

Leave a Comment