Thread: Raising an Integer to a power

Hello, I'm currently writing a program that is supposed to raise an integer to a power. However, the output keeps printing as going to infinity rather than the actual number when using the test results. I tried changing the double type to a long double, but it still is not large enough. Earlier when I was working on the program though, it did not seem to have this problem.


Any help at all would be greatly appreciated.

Thank you.

EDIT: The iterative function on the bottom does not work. I have not been able to finish it due to trying to stop the result from printing as 1.#NF00.

Code:

#include "stdafx.h"//standard library inclusion
#include "stdio.h"
#include "conio.h"
#include "time.h"

clock_t start,stop;//built in type for processor time (ticks)
double duration;//Records the run time, in seconds, of a function 

long double Recur(double base, int pow);//Declaration of function for recursion.
long double Expon(double base, int pow);//Declaration of function for N-1 loop.
long double Iter(double base, int pow);//Declaration of function using iteration.

int  main(void)
{
    //This program raises a given integer (base) to a power (pow)

    int  pow=0, algo, test=0;//algo will be used to initiate the chosen algorithm. Test for choosing whether or not to run the test cases.
    double base;
    long double result;//Stored as a long double as in the test files the number is very large

    printf("If you want to run Algorithm 1: Enter 1\nIf you want to run Algorithm 2(recursive): Enter 2\nIf you want to run Algorithm 3(iterative): Enter 3\n");
    scanf("%d", &algo);//Choose which algorithm to run.

    printf("If you want to run the test cases: Enter 1: ");
    scanf("%d", &test);//Choose whether or not to run the test cases.
    
    if(test!=1){//If Not running test cases, user inputs own base and power.
        printf("Please enter your base: ");
        scanf("%lf", &base);
        printf("Please enter your power: ");
        scanf("%d",&pow);//take in input for functions.

        if(algo=1){//Will run algorithm one and clock time
            start=clock();
            result=Expon(base,pow);
            stop=clock();
        }
        else if(algo=2){//Will run algorithm two and clock time
            start=clock();
            result=Recur(base,pow);
            stop=clock();
        }
        else{//Will run algorithm three and clock time
            start=clock();
            result=Iter(base,pow);
            stop=clock();
        }

        printf("%llf to the power of %d = %llf.\n",base, pow, result);//Print out results of chosen algorithm

        duration = ((double)(stop - start))/CLK_TCK;  //Print out duration of algorithm time
        printf("Duration = %llf",duration);

    }


    else{//Run Test cases
        base=1.0001;
        int arraypow[8]= {1000,5000,10000, 20000, 40000, 60000, 80000, 100000};//Go through the array of powers in the test cases
        int i=0;

        for(i=0;i<8;i++){//Use loop to go through the test cases
            if(algo=1){//Test cases for algorithm 1
                pow=arraypow[i];
                start=clock();
                result=Expon(base,pow);//Function call for algorithm 1
                stop=clock();
                printf("%lf to the power of %d = %Lf.\n",base, pow, result);//Print out results as each power is used.

                duration = ((double)(stop - start))/CLK_TCK;   //Calculate duration of algorithm
                printf("Duration = %lf\n",duration);
            }
            else if(algo=2){//Test cases for algorithm 2 (recursive)
                pow=arraypow[i];
                start=clock();
                result=Recur(base,pow);//Function call for algorithm 2 Recursive
                stop=clock();
                printf("%lf to the power of %d = %Lf.\n",base, pow, result);//Print out results as each power is used.

                duration = ((double)(stop - start))/CLK_TCK;  //Calculate duration of algorithm
                printf("Duration = %lf",duration);
            }
            else{//Test cases for algorithm 2 (iteration)
                pow=arraypow[i];
                start=clock();
                result=Iter(base,pow);//Function call for algorithm 2 Iterartive
                stop=clock();
                printf("%lf to the power of %d = %Lf.\n",base, pow, result);//Print out results as each power is used.

                duration = ((double)(stop - start))/CLK_TCK;  //Calculate duration of algorithm
                printf("Duration = %lf",duration);
            }
        }
    }

    getch();//Stop prgram from exiting after executing
    getch();
    return 0;
}


long double Expon(double base, int pow)//Function for algorithm 1
{
    double result;
    if(pow==0)
        result = 1;
    else if(pow==1)
        result = base;
    for(pow;pow>2;pow--){//Loop for N-1 
        base=base*base;
        result=base;//Update result as power increases
    }

    return result;
}


long double Recur(double base, int pow)//Function for algorithm 2 (Recursive)
{
    double result;
    if(pow==0)
        return result = 1;//Case for when power is equal to 0.
    else if(pow==1)
        return result = base;//Case for when power is equal to 1
    else if(pow%2==0){
        return Recur(base*base,pow/2);//Case for if pow is even
    }
    else
        return Recur(base*base,pow/2)*base;//Case for if pow is odd
}

long double Iter(double base, int pow)//Function for algorithm 2 (Iterative)
{
    double result;

    while(pow>1){

        if (pow%2==0){
            base=(base*base);
            pow=pow/2;
            result=base;
        }
        else{
            base=base*base;
            pow= pow/2 +1;
            result=base;
        }
    }
    return result;
}

Typo in expon()

Code:

base=base base*;

What is the function supposed to do?

Code:

if(algo=1)

Should this be

Code:

if(algo==1)

The function is supposed to raise a an integer ( or a decimal) (base )to a power (pow) using recursion, iteration, and N-1.


I have included stdio.h in the function, but if I don't include stdafx.h as well, it won't run. I tried saving the program as a .c instead as well, but when I did that it said that my array had terms with too many characters and wouldn't run.

And, thank you for the data type advice. I will try it out and see how it works.


Oh, I can't believe I missed that I didn't use two '='. I'll fix that now.

EDIT:And, now my recursive function works. Thank you so much.

Originally Posted by Adak

you should be using include of "stdio.h", instead of stdafx.h.

I think you were confused by his inaccurate comment. stdafx.h has nothing to do with "standard library inclusion". It's related to pre-compiled headers in MSVC.

Doubles are floats and all floats involve CLOSE approximations of numbers, BUT NOT QUITE. So they won't match up with int's, on many cases.

Actually, doubles will represent ints perfectly unless the magnitude is too great.

now my recursive function works. Thank you so much

That's great to hear.

What appears to have been happening is that the =/== mistake was forcing your code to do the expon() function every time - With that typo in it, it would not work the way you wanted to.

Yes, It seems to be working properly now and I can focus on finishing it.

It might be running successfully now, but the result will be horribly wrong.
You have gotten the implementation of Expon wrong, and are actually calculating "base to the power of (base to the power of pow)" i.e. base^(base^pow) where ^ means "to the power of".
Or as an example, 2 to the power of 8 should be 256, but instead you'd get a result that is 77 decimal digits long!

First work out which algorithm you are intending to use. The slow one where you multiply the answer by base each time, or the binary exponentation method where you multiply either by base or by a binary power of base.

This is why you need to test your code with inputs and corresponding expected outputs, in order to test if it is working correctly.