How to convert string representation of octal number to decimal? - c

Question

I want to make a program that converts binary number to decimal, octal and hexadecimal.
While taking input I am considering their standard representation. For example: octal is "o10"
So I convert 10 into decimal, which gives 8.
Here's my code for inputting string and converting it to decimal:
int main()
{
int ch=0;
char str[1000];
printf("Enter the number\n");
scanf(" %s",str);
l=strlen(str);
for(int j=0;j<l;j++)
{
if(str[j]>=48 && str[j]<=57)
n=n*10+(str[j]-48);
}
}
But the problem is that it doesn't work for octal. How to take the input correctly?

Answer 1

On the post you are taking about creating Binary/Octal/Hexadecimal to Decimal converter, but you code only address Octal, My answer will try to be generic so you could implement the other cases with it, but will focus on Octal.
First of all, your code has some minor issues not related to the algorithm itself:
You call printf and strlen function, which does not come built in with C and need to be included with their respective libraries:
#include <stdio.h>
#include <string.h>
You assign l to the length of the string, but you never declare it type, so this is needed:
l = strlen(str);
Same problem for n, which is never declared. Not only that, you say n=n*10 ... so noy only not declaring it, you use it value.. what do you think n*10 will return? My guess is garbage. Since you use it in the loop, you should declare it outside the loop itself, so I would suggest:
int l = strlen(str), n = 0;
Okay not that we got this out of the way, your Octal converted code is actually almost done.
Only problem with the algorithm, is the n*10 ... part. If you multiply n by 10, you acually get decimal converted.. so Decimal to Decimal converted, if you input for example 48 you will get 48 as output. What do we need to fix it? change it to 2 for Binary converted, 8 for Octal, and 16 for Hexa. Your code only speak about octal, so if you change this line to:
n = n * 8 + (str[j] - 48);
You will get a working Octal converted. For example, if run the program and input 10, you will get 8 which is what we want.
Now as I mentioned earlier, if you want to make it more generic you should have a variable called int base = 10; which you can set according to the first character, for example:
int base = 10; // default value
if (str[0] == 'o')
{
base = 8;
}
Of course you can do the same for other bases, and it will pretty much work the same. Finally, you never did actually with the value of n after you calculated it, So someting like that would work to check the answer we got:
printf("In Decimal => %d\n", n);
This kinda works, but we still need to check digit boundaries. For example, octal number can't have a digit larger then 7, and Hexa number can have A as a digit. In the for loop, we do not check for anything like that. For example, if you input o48 in your code it will print an answer, 40, which is not correct since 48 is not an octal number.
So you in order to complete the program, you need to check if boundaries, and implement the rest of the bases check (h for hexa, b for binary), This I will leave to you.
GOOD LUCK~!