Thread: Recursion function to change the base of a number and return the result

I am thinking the problem is too big for you to solve.

I would try to break the problem down into two separate recursive functions.
One that converts the input value of input base into base 10.
Two converts the base 10 value into a char array.

Then, the third function calls the two recursive functions.

NOTE: I think Grumpy was hinting that this is not the best method; but, it is easier for me to understand so, likely easier for you to understand.
I think he implied you did NOT need to convert to base 10 when going from input base to output base value.

Tim S.

Please explain your logic on the recursive step?

Code:

return num + baseConversion( num%newBase, num/base, newBase );

Why do you think the 2rd parameter value should be changing?
Why do you think the 1st parameter should be num%newBase ?

Tim S.

My quick 15 minute test program.
Note: My program only does base 2 to 10; but, I think this just means the code in fromBaseTen into a function that returns a char array. This would likely take another 15 minutes to half for me to do. But, I have no idea if you know malloc or static char arrays.
I used this code in my baseConversion function; but, it will likely need moved to the new fromBaseTen function.

Code:

char* baseConversion(int num, int base, int newBase)
{
    static char result_string[MAX_RESULT_LENGTH+1] = {0};
    
    snprintf(result_string, MAX_RESULT_LENGTH, "%d", fromBaseTen( toBaseTen( num, base ), newBase ));
    
    return result_string;
}

Code:

#include <stdio.h>
#include <stdlib.h>

/* Max number of charators in the result of baseConversion */
#define MAX_RESULT_LENGTH 32

char* baseConversion(int num, int base, int newBase);
int toBaseTen( int num, int base );
int fromBaseTen( int num, int base );

int main()
{
    printf("Testing!\n");
    
    printf("toBaseTen(0,8) := %d\n", toBaseTen(0,8));
    
    printf("fromBaseTen(0,8) := %d\n", fromBaseTen(0,8));
    
    printf("toBaseTen(10,8) := %d\n", toBaseTen(10,8));
    
    printf("fromBaseTen(8,8) := %d\n", fromBaseTen(8,8));
    
    printf("toBaseTen(17,8) := %d\n", toBaseTen(17,8));
    
    printf("fromBaseTen(15,8) := %d\n", fromBaseTen(15,8));
    
    printf("toBaseTen(127,8) := %d\n", toBaseTen(127,8));
    
    printf("fromBaseTen(87,8) := %d\n", fromBaseTen(87,8));
    
    printf("baseConversion(127, 8, 10) := %s\n", baseConversion(127, 8, 10));
    
    printf("baseConversion(127, 8, 2) := %s\n", baseConversion(127, 8, 2));
    
    return 0;
}

Code:

Testing!
toBaseTen(0,8) := 0
fromBaseTen(0,8) := 0
toBaseTen(10,8) := 8
fromBaseTen(8,8) := 10
toBaseTen(17,8) := 15
fromBaseTen(15,8) := 17
toBaseTen(127,8) := 87
fromBaseTen(87,8) := 127
baseConversion(127, 8, 10) := 87
baseConversion(127, 8, 2) := 1010111

Tim S.

FYI: I am NOT posting the code that takes real thought because the only way to learn it is to figure it out yourself.

4 chars

I made it!!! Sorry for the double post. Thank you for your help too!, can you help me understand the converting numbers to strings like you do in your baseConversion function?
I literally wrote down on paper what was happening and what my number literally must equate to in a recursive call.

Code:

int baseTen( int num, int base )
{
    if( num < 10 )
        {
        return num;
        }
    return num + baseTen( num/10%10*base , base*base );
}

The method I used will not apply to you; I would use a char array set equal to "0123456789ABCDEF" and then use a index of it to set the value in a allocated char array.

NOTE: your "return num + baseTen( num/10%10*base , base*base );" has little resemblance to my code.

Note: Your method might work; but, mine is much simpler than yours.

Tim S.

toBaseTen(127,8) := 87

((((1)*8) + (2)) * 8) + (7) = 87

Maybe re-looking at the math will give you a hint.

Edit: added extra unneeded () to maybe give better hint.

Tim S.

Originally Posted by jcconnell

Code:

if ( input != 0 ) {

Code:

baseConversion(input/outBase, base, outBase);

I think these two steps are closer than the OP; but, still NOT right, yet.

Your recursion step likely only works for input 0 thru 9.

Tim S.

Originally Posted by jcconnell

Are you guys suggesting we look into use the ASCII tables?

No, I am NOT.

Tim S.

Hint: Many (I think likely all) recursive function has one or more recursive steps.

One form of recursive step is

return <Value1> <OP1> <Value2> <OP2> recursive_function_name(Parameter1<OP3> <Value3>, ...);

Where <OP1>, <OP2>, and <OP3> are operations like +,-,/,*,%, and etc.
Where Value1, Value2, or Value3 are constants or input parameters.
Where Parameter1 is the first parameter passed to the recursive function.

Tim S.