C Video Tutorial 14

In the last part of my C Video Tutorial, I covered how to convert from base 10 to base 2, base 8 and base 16 with C.

At the end of that tutorial, I provided a homework assignment for you to write code that converts from any base back to base 10. In this tutorial, I will walk through exactly how to do that.

You can find all of the code below the video. Please leave any questions you may have.

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Code From the Video

CTutorial14.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h> // Used for pow()
#include <ctype.h> // Used for isalpha()

// To get the commented code for this function see
// http://www.newthinktank.com/2013/09/c-video-tutorial-13/

char * convertBase(unsigned int numberToConvert, int base)
{

	char buffer[33];
	char *pConvertedNumber;
	
	char allValues[] = "0123456789abcdef";

	if(base < 2 || base > 16){
		printf("Enter a Number Between 2 and 16\n");
		exit(1);
	}

	pConvertedNumber = &buffer[sizeof(buffer)-1];
	
	*pConvertedNumber = '\0';

	do {
	
		int value = numberToConvert % base;
		
		pConvertedNumber = pConvertedNumber - 1;
		
		*pConvertedNumber = allValues[value];
		
		numberToConvert /= base;
		
	} while(numberToConvert != 0);
	
	printf("%s", pConvertedNumber);

	return pConvertedNumber;
}

/* WORKS UNTIL LETTERS ARE USED

int baseToDecimal(char* number, int baseFrom, int sizeOfNumber){

    int result = 0;
    int toThePowerOf = 0;
    
    // Cycle through all letters in the array starting with the last one
    // We subtract 2 because indexing starts at 0 and to avoid the '\0'
    
    for ( int i = (sizeOfNumber - 2); i >= 0; --i ){
    
    	// Copying this style
    	// ... +  ( 4 * 8^1 )  +  ( 5 * 8^0 )
    
        result += (number[i] - '0') * pow(baseFrom, toThePowerOf);
        
        toThePowerOf++;
        
    }
        
    printf("%s in base %d equals %d in base 10\n", number, baseFrom, result);

    return result;
}
*/

// WORKS WITH HEXADECIMALS

int baseToDecimal(char* number, int baseFrom, int sizeOfNumber){

    int result = 0;
    int toThePowerOf = 0;
    
    for ( int i = (sizeOfNumber - 2); i >= 0; --i ){
    
    	// Check if character in the array is a letter
    
    	if(isalpha(number[i])){
    	
    		// Convert the letter into the right character code
    		// and subtract 87 to make 'a' = 10 along with other
    		// letters
    	
    		int charCode = ((int)tolower(number[i])) - 87;
    		
    		// Copying this style
    		// ... +  ( 4 * 8^1 )  +  ( 5 * 8^0 )
    		
    		result += charCode * pow(baseFrom, toThePowerOf);
    	
    	} else {
    
        	result += (number[i] - '0') * pow(baseFrom, toThePowerOf);
        	
        }
        
        toThePowerOf++;
        
    }
        
    printf("%s in base %d equals %d in base 10\n", number, baseFrom, result);

    return result;
}

int main(void){

	unsigned int numberOne = 60; // 111100
	
	printf("%d in Base 2 = ", numberOne);
	convertBase(numberOne, 2);
	
	printf("\n%d in Base 8 = ", numberOne);
	convertBase(numberOne, 8);
	
	printf("\n%d in Base 16 = ", numberOne);
	convertBase(numberOne, 16);
	
	printf("\n");
	
	// Work Out the Key Codes for Hexadecimals
	/*
	printf("a : %d\n", 'A');
	printf("a : %d\n", 'b');
	printf("a : %d\n", 'c');
	printf("a : %d\n", 'd');
	printf("a : %d\n", 'e');
	printf("a : %d\n", 'f');
	*/
	
	char numberToConvert[] = "3C";
	
	baseToDecimal(numberToConvert, 16, sizeof(numberToConvert));
	
	return 0;
	
}