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aade #1 / 10

help with K&R2
Hi all i am a newbie to c and have been reading 2ed the c language by K&R i am at 1.7 functions and i am looking at the raise an int to a power and have two questions heres the prog #include <stdio.h> int power(int m,int n); /* test power function */ main() { int i; for (i=0;i<10;++i) printf("%d %d %d\n",i power(2,i),power(3,i)); return 0; Quote: }
/* power raise base to nth power; n >=0 */ int power(int base, int n) { int i ,p; p =1; for (i=1;i<=n;++i) p = p * base; return p; Quote: }
my questions are 1: what does i have to do in the for loop p isnt being increased why a for loop?? question2: where does p come from (declared assigned ) and what has it to do with working out the power of some thing sorry if i have missed something most obvious but a iam a new learner thanks for takeing a moment of your time regards aade

Sun, 11 Jan 2004 18:55:06 GMT 


pete #2 / 10

help with K&R2
Quote:
> Hi all i am a newbie to c > and have been reading 2ed the c language by K&R > i am at 1.7 functions and i am looking at the raise an int to a power and > have two questions > heres the prog > #include <stdio.h> > int power(int m,int n); > /* test power function */ > main() > { > int i; > for (i=0;i<10;++i) > printf("%d %d %d\n",i power(2,i),power(3,i));
You're missing a comma. printf("%d %d %d\n", i, power(2, i), power(3, i)); Quote: > return 0; > } > /* power raise base to nth power; n >=0 */ > int power(int base, int n) > { > int i ,p; > p =1; > for (i=1;i<=n;++i) > p = p * base; > return p; > } > my questions are 1: > what does i have to do in the for loop > p isnt being increased why a for loop??
power() could have been written without i int power(int base, int n) { int p; p = 1; while(n) p *= base; return p; Quote: } > question2: > where does p come from (declared assigned ) > and what has it to do with working out the power of some thing
p is where the results of the calculations go. n is being used to count, and base is used as a constant, so the calculations have to go somewhere else.  pete

Sun, 11 Jan 2004 20:02:06 GMT 


john gal #3 / 10

help with K&R2
Quote: aade writes: > and have been reading 2ed the c language by K&R > i am at 1.7 functions and i am looking at the raise an int to a power and > have two questions > heres the prog > #include <stdio.h> > int power(int m,int n); > /* test power function */ > main() > { > int i; > for (i=0;i<10;++i) > printf("%d %d %d\n",i power(2,i),power(3,i)); > return 0; > } > /* power raise base to nth power; n >=0 */ > int power(int base, int n) > { > int i ,p; > p =1; > for (i=1;i<=n;++i) > p = p * base; > return p; > } > my questions are 1: > what does i have to do in the for loop p isnt being increased why a for > loop?? > question2: > where does p come from (declared assigned ) and what has it to do with > working out the power of some thing > sorry if i have missed something most obvious but a iam a new learner
Both i and p are 'inventions' of the person who wrote the power function. i is a counting variable, it provides a means to explicitly determine, when the code is written, how many times to execute the loop. A for loop is the simplest way to do a loop when you can satisfy the a priori requirement. i stands for 'index', a better choice might have been 'count'. The writers chose i because this variable often *does* serve as an index. But at this point in your learning process, you don't know about indexes yet, that has to do with arrays. p is acting as an accumulator. A better name might have been 'current_power'. Note that after one execution of the loop, p will be p^1. And the loop is always executed at least one time. When I say "a better choice" above it should in no way be construed as criticism of the book. It is a fantastic book! But it is written for people who already have some notion of programming and the common idioms, as above. It would only be a better choice if the book were intended for absolute beginners.

Sun, 11 Jan 2004 20:21:26 GMT 


aade #4 / 10

help with K&R2
thanks people for the reply but i think iknow enought to know that p=p *base will never work out the nth rasied to b power espeacially as p is initialised to =1 and never changes ??? i am truly stumped regards aade
Quote: > aade writes: > > and have been reading 2ed the c language by K&R > > i am at 1.7 functions and i am looking at the raise an int to a power and > > have two questions > > heres the prog > > #include <stdio.h> > > int power(int m,int n); > > /* test power function */ > > main() > > { > > int i; > > for (i=0;i<10;++i) > > printf("%d %d %d\n",i power(2,i),power(3,i)); > > return 0; > > } > > /* power raise base to nth power; n >=0 */ > > int power(int base, int n) > > { > > int i ,p; > > p =1; > > for (i=1;i<=n;++i) > > p = p * base; > > return p; > > } > > my questions are 1: > > what does i have to do in the for loop p isnt being increased why a for > > loop?? > > question2: > > where does p come from (declared assigned ) and what has it to do with > > working out the power of some thing > > sorry if i have missed something most obvious but a iam a new learner > Both i and p are 'inventions' of the person who wrote the power function. i > is a counting variable, it provides a means to explicitly determine, when > the code is written, how many times to execute the loop. A for loop is the > simplest way to do a loop when you can satisfy the a priori requirement. i > stands for 'index', a better choice might have been 'count'. The writers > chose i because this variable often *does* serve as an index. But at this > point in your learning process, you don't know about indexes yet, that has > to do with arrays. > p is acting as an accumulator. A better name might have been > 'current_power'. Note that after one execution of the loop, p will be p^1. > And the loop is always executed at least one time. > When I say "a better choice" above it should in no way be construed as > criticism of the book. It is a fantastic book! But it is written for > people who already have some notion of programming and the common idioms, as > above. It would only be a better choice if the book were intended for > absolute beginners.

Sun, 11 Jan 2004 21:19:15 GMT 


pete #5 / 10

help with K&R2
Quote:
> > Hi all i am a newbie to c > > and have been reading 2ed the c language by K&R > > i am at 1.7 functions and i am looking at the raise an int to a power and > > have two questions > > heres the prog > > #include <stdio.h> > > int power(int m,int n); > > /* test power function */ > > main() > > { > > int i; > > for (i=0;i<10;++i) > > printf("%d %d %d\n",i power(2,i),power(3,i)); > You're missing a comma. > printf("%d %d %d\n", i, power(2, i), power(3, i)); > > return 0; > > } > > /* power raise base to nth power; n >=0 */ > > int power(int base, int n) > > { > > int i ,p; > > p =1; > > for (i=1;i<=n;++i) > > p = p * base; > > return p; > > } > > my questions are 1: > > what does i have to do in the for loop > > p isnt being increased why a for loop?? > power() could have been written without i > int power(int base, int n) > { > int p; > p = 1; > while(n) > p *= base; > return p; > }
Change that to: int power(int base, int n) { int p; p = 1; if (p > 0) while(n) p *= base; return p; Quote: }
 pete

Sun, 11 Jan 2004 21:30:40 GMT 


Richard B #6 / 10

help with K&R2
Quote:
> /* power raise base to nth power; n >=0 */ > int power(int base, int n) > { > int i ,p; > p =1; > for (i=1;i<=n;++i) > p = p * base; > return p; > } > what does i have to do in the for loop p isnt being increased
Yerwhat? What do you mean, isn't being increased? What do you think this line does: Quote: > p = p * base;
It multiplies p by base, thereby increasing its magnitude (provided, of course, abs(base)>1...). Richard

Sun, 11 Jan 2004 21:28:54 GMT 


KBu #7 / 10

help with K&R2
Quote:
> Hi all i am a newbie to c > and have been reading 2ed the c language by K&R > i am at 1.7 functions and i am looking at the raise an int to a power and > have two questions > heres the prog > #include <stdio.h> > int power(int m,int n); > /* test power function */ > main() > { > int i; > for (i=0;i<10;++i) > printf("%d %d %d\n",i power(2,i),power(3,i));
^^ Oops! Missed a comma ;) ! Quote: > return 0; > } > /* power raise base to nth power; n >=0 */ > int power(int base, int n) > { > int i ,p; > p =1; > for (i=1;i<=n;++i) > p = p * base; > return p; > } > my questions are 1: > what does i have to do in the for loop p isnt being increased why a for > loop??
Everything! You may consider i as some sort of counter that tells you where you are in the process of repeated multiplication (to obtain the power). As for p, it /is/ being increased in every iteration of the loop  in fact, it increases by a factor of base, every iteration. Quote: > question2: > where does p come from (declared assigned ) and what has it to do with > working out the power of some thing
The first statement in the function power() declares p, and the second assigns the value 1 to it. By repeatedly multiplying p by the base (n times), you get base raised to the power n (from the definition of exponentiation). Quote: > sorry if i have missed something most obvious but a iam a new learner > thanks for takeing a moment of your time > regards > aade
I don't know if I've understood your question properly, but anyway, I hope this helps. Perhaps you may want to look at K&R2, Section 1.3 again. Cheers! Regards, M.Q.

Sun, 11 Jan 2004 21:35:55 GMT 


pete #8 / 10

help with K&R2
Quote:
> thanks people for the reply but i think iknow enought to know > that p=p *base will never work out the nth rasied to b power > espeacially as p is initialised to =1 and never changes ??? > i am truly stumped
Should be "base raised to the nth power". If you had compiled and run the program, you would know whether or not it works. If you can't compile and run C programs, then there's not much you can do here.  pete

Sun, 11 Jan 2004 21:39:57 GMT 


pete #9 / 10

help with K&R2
Quote:
> > > Hi all i am a newbie to c > > > and have been reading 2ed the c language by K&R > > > i am at 1.7 functions and i am looking at the raise an int to a power and > > > have two questions > > > heres the prog > > > #include <stdio.h> > > > int power(int m,int n); > > > /* test power function */ > > > main() > > > { > > > int i; > > > for (i=0;i<10;++i) > > > printf("%d %d %d\n",i power(2,i),power(3,i)); > > You're missing a comma. > > printf("%d %d %d\n", i, power(2, i), power(3, i)); > > > return 0; > > > } > > > /* power raise base to nth power; n >=0 */ > > > int power(int base, int n) > > > { > > > int i ,p; > > > p =1; > > > for (i=1;i<=n;++i) > > > p = p * base; > > > return p; > > > } > > > my questions are 1: > > > what does i have to do in the for loop > > > p isnt being increased why a for loop?? > > power() could have been written without i > > int power(int base, int n) > > { > > int p; > > p = 1; > > while(n) > > p *= base; > > return p; > > } > Change that to: > int power(int base, int n) > { > int p; > p = 1; > if (p > 0) > while(n) > p *= base; > return p; > }
int power(int base, int n) { int p; p = 1; if (n > 0) while(n) p *= base; return p; Quote: }
Good night.  pete

Sun, 11 Jan 2004 21:42:25 GMT 


aade #10 / 10

help with K&R2
thanks guys i can now see that p=p*base does infact increase p i am sorry i must have had a matrix moment regards aade
Quote: > > Hi all i am a newbie to c > > and have been reading 2ed the c language by K&R > > i am at 1.7 functions and i am looking at the raise an int to a power and > > have two questions > > heres the prog > > #include <stdio.h> > > int power(int m,int n); > > /* test power function */ > > main() > > { > > int i; > > for (i=0;i<10;++i) > > printf("%d %d %d\n",i power(2,i),power(3,i)); > ^^ > Oops! Missed a comma ;) ! > > return 0; > > } > > /* power raise base to nth power; n >=0 */ > > int power(int base, int n) > > { > > int i ,p; > > p =1; > > for (i=1;i<=n;++i) > > p = p * base; > > return p; > > } > > my questions are 1: > > what does i have to do in the for loop p isnt being increased why a for > > loop?? > Everything! You may consider i as some sort of counter that tells you where > you are in the process of repeated multiplication (to obtain the power). As > for p, it /is/ being increased in every iteration of the loop  in fact, it > increases by a factor of base, every iteration. > > question2: > > where does p come from (declared assigned ) and what has it to do with > > working out the power of some thing > The first statement in the function power() declares p, and the second assigns > the value 1 to it. By repeatedly multiplying p by the base (n times), you get > base raised to the power n (from the definition of exponentiation). > > sorry if i have missed something most obvious but a iam a new learner > > thanks for takeing a moment of your time > > regards > > aade > I don't know if I've understood your question properly, but anyway, I hope > this helps. Perhaps you may want to look at K&R2, Section 1.3 again. > Cheers! > Regards, > M.Q.

Sun, 11 Jan 2004 22:13:05 GMT 


