change ascii to string after adding number to it

Question

I am trying to reform a string from ascii characters. I noticed when I break down ascii it matches the number with the ascii char, this changes when I tried to do (c+=c+0) on it as seen below, I tried to revert back to the original string but it doesn't seems to work. I was wondering does the ascii number change when you append it to a string?

StringBuilder b = new StringBuilder();
char c = "e".charAt(0);
System.out.println(c);   //'e'
System.out.println(c+0); //101 -ascii
b.append(c+=c+0);
char d = b.charAt(0);
System.out.println(d-=d+0);   //blank

What are you trying to accomplish? Your code has lots of strange things going on, like b.append(c += c+0)... it will be helpful if you tell us what you're trying to do on each line, what happens, and what you expected to happen. — k_ssb
– k_ssb, Commented Apr 19, 2018 at 15:06
well is just (c = c+c+0) and yes you can write it as c*=2 but currently I have cut the method into small pieces for debugging so c*=2 will not work for the full form. I am trying to get String b back to the original form. 'e' — logger
– logger, Commented Apr 19, 2018 at 15:07

xingbin · Accepted Answer · 2018-04-19 16:24:54Z

1

When you use + to sum a char with and int, the result will be promoted to int.
When you use compound assignment operators +=, the result will be converted to left operand data type(in this case, char)

This is what happens with your code:

StringBuilder b = new StringBuilder();
char c = "e".charAt(0);
System.out.println(c);   //'e'
System.out.println(c+0); //101 -ascii
b.append(c+=c+0); // result of c+c+0 is int 202, it is converted to char Ê
char d = b.charAt(0); // char d = Ê
System.out.println(d-=d+0); // result of d-(d+0) is int 0, it will be converted to null

edited Apr 19, 2018 at 16:24

answered Apr 19, 2018 at 15:22

xingbin

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7 Comments

logger Over a year ago

actually you are right, I missed that into a zero instead. System.out.println(String.valueOf(Character.toChars(d/2))); works

k_ssb Over a year ago

@logger were you responding to the right answer? Also, System.out.println((char) (d/2)); is much simpler. All you need to do is cast the int expression d/2 into a char.

xingbin Over a year ago

@pkpnd Are you downvoting this answer? Is it not useful?

k_ssb Over a year ago

@user6690200 Your answer doesn't show the OP what the best code practices are. Converting a char to an int should be done with a cast; printing out d-=d+0 is just asking for trouble.

xingbin Over a year ago

@pkpnd It does. If OP dose not know what is happening with his code, how can he found out how to solve it? I did not provide a solution, but it does not mean it is not useful.

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k_ssb · Accepted Answer · 2018-04-19 15:15:40Z

0

It's not clear what you're trying to accomplish, but hopefully this example helps you figure out what's going on:

public static void main(String[] args) {
    StringBuilder b = new StringBuilder();
    char c = "e".charAt(0);
    System.out.println(c);   // e
    System.out.println((int) c); // 101
    char doubleC = (char) (c + c);
    b.append(doubleC);
    char d = b.charAt(0);
    System.out.println((int) d); // 202
    System.out.println(d - c); // 101
    System.out.println((char) (d - c)); // e
}

Whether a character is printed as a letter or its ASCII value depends on its type. You can see that (int) c casts the character to its int ASCII value. Your approach of c+0 also does this but in a less intuitive way.

You also apparently want to "undo" your modification to c, but subtracting d-d always gives you 0. You need to store your value of c and subtract that from d.

answered Apr 19, 2018 at 15:15

k_ssb

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5 Comments

logger Over a year ago

assume you dont have access to var c, just the string b to work with.

k_ssb Over a year ago

Well.. what are you trying to do? I can't help you solve your problem if I don't know what your problem is.

DodgyCodeException Over a year ago

You can do System.out.println(d / 2);.

logger Over a year ago

so I have a string, the encoding sequence is like this....it breaks string down into characters then it encrypt by doubles then add a integer sequence number. Then it pack back into a string. Now I am trying to recover that original string.

k_ssb Over a year ago

Then you need to reverse the "encryption" scheme. Suppose your encryption changes every character c to 2*c + n. To decrypt, you need to reverse those operations in the reverse order: (d - n) / 2, where d is your encrypted character.

Camile · Accepted Answer · 2018-04-19 15:15:48Z

+0s don't do anything because the character is underneath a number (so it's essentially like saying 101 + 0).

d-=d+0 evaluates to d = d - d + 0, so (because we're manipulating chars) d - d = 0

If you wanted to get c from d, you'd have to do d = d - c, because 'c' is the difference between the initial and the final value.

So no, the ASCII number of a character doesn't change, you're just not doing the reverse calculation as you should be.

It's like saying: x + y = z, and then wondering why z - z != y.

You should be doing: z - x = y

@Edit To add to your comment under pkpnd's answer - if you have only String b to work with, you'd still need to know the difference (i.e. by how much you increased/decreased the number) to reverse to the original chars.

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