# Thread: creating a "to the power" equivalent in c

1. ## creating a "to the power" equivalent in c

hi there, im using a while command to generate a number sequence and in the formula i am using, i need to raise -1 to the power of (k+1)

the whole equation is 2*k*(-1)to the power of (k+1) any help would be appreciated

2. What's wrong with pow? (Although (-1) to any power is either -1 or +1 so pow may be overkill.)

3. it is to generate 2, -4, 6, -8 but with just pow, it says syntax error, missing ")" before identifier "pow"

4. Post the smallest and simplest program that demonstrates the error.

6. Code:
```    k = 0;
while ( k < 20 )
{
printf("\n\t\t The numbers are %4d", 2*k*(-1)pow(k+1));
k = k + 1;
}```

7. pow requires two arguments...pow(base, exponent)...also, pow returns a double or some other kind of floating point number, not a decimal...instead of thinking in terms of pow, use the % operator...look at the (-1)^(k+1) sequence:

k | f(k)
0 | -1
1 | 1
2 | -1
3 | 1

8. Originally Posted by Epy
pow requires two arguments...pow(base, exponent)...also, pow returns a double or some other kind of floating point number, not a decimal...instead of thinking in terms of pow, use the % operator...look at the (-1)^(k+1) sequence:

k | f(k)
0 | -1
1 | 1
2 | -1
3 | 1
so would i do 2*k*pow(-1, k+1)

?

edit i tried this and generated 0, 2488, 4976, and so on with huge numbers

9. I don't think pow() accepts negative bases. Like I said, look at the modulo (%) operator...when k is even, 2k(-1)^(k+1) is negative...use that to your advantage.

10. that kind of just lost me.

we just learned while commands and he wants us to generate 2, -4, 6, -8, and so on for 20 terms. i have no idea what a modulo or % operator is. we havnt learned that yet.

11. Are you required to use pow()? It would be simpler if you did not use it, but instead relied on say, the fact that -x = -1 if x = 1, but -x = 1 if x = -1, and thus use the expression -x * n. (You can actually do without multiplication.)

12. no im not required to use pow, but i figured it woudl be easiest in 2*k*(-1)^k that generates 2, -4, 6, -8, and so on...

13. Ah, then you should recognise that (-1)^k is just mathematical notation to express alternating coefficients of 1 and -1.

14. so how would i set up that x and stuff in my code?

15. Originally Posted by tabstop
What's wrong with pow? (Although (-1) to any power is either -1 or +1 so pow may be overkill.)
-1 to the power of 2.5 is neither 1 nor -1.