C++ Convert Ascii Int To Char To Int - c++

Im able to convert most things without a problem, a google search if needed. I cannot figure this one out, though.
I have a char array like:
char map[40] = {0,0,0,0,0,1,1,0,0,0,1,0,1... etc
I am trying to convert the char to the correct integer, but no matter what I try, I get the ascii value: 48/ 49.
I've tried quite a few different combinations of conversions and casts, but I cannot end up with a 0 or a 1, even as a char.
Can anyone help me out with this?
Thanks.

The ascii range of the characters representing integers is 48 to 57 (for '0' to '9'). You should subtract the base value 48 from the character to get its integer value.
char map[40] = {'0','0','0','0','0','1','1','0','0','0','1','0','1'...};
int integerMap[40];
for ( int i = 0 ;i < 40; i++)
{
integerMap[i] = map[i] - 48 ;
// OR
//integerMap[i] = map[i] - '0';
}

If the char is a literal, e.g. '0' (note the quotes), to convert to an int you'd have to do:
int i = map[0] - '0';
And of course a similar operation across your map array. It would also be prudent to error-check so you know the resulting int is in the range 0-9.
The reason you're getting 48/49 is because, as you noted, direct conversion of a literal like int i = (int)map[0]; gives the ASCII value of the char.

Related

Is it possible to add two indexes of a string to a character to then convert that character to an int in c++?

For example, I have a string st="1234567" and I want to add st[0] and st[1] to char ch so that ch = 12, and then to convert that to an int x so that x = 12.
Is there a way to do this?
You can achieve it like this:
int x = 10*(st[0]-'0') + (st[1]-'0');
This works because subtracting '0' from a char representing a number results in that actual number. So for instance '5' - '0' == 5.
char ch so that ch = 12
That's not possible though. You can do char ch = 12, but that's not going to represent a 12, that will most likely just result in something that might not be printable. In ASCII that would be a "form feed" and if you tried to print that, it could perhaps put a newline (or skip to the next page if your editor supports that). If you want a string to hold "12", you can do it this way:
std::string twelve = st.substr(0, 2);
And from there a general way of converting to int would be
int x = std::stoi(twelve);

How to convert single char to double

I try to create a program that can evaluate simple math expression like "4+4". The expression is given from the user.
The program saves it in a char* and then searches for binary operation (+,-,*,:) and does the operation.
The problem is that I can't figure out how to convert the single char into a double value.
I know there is the atof function but I want to convert single char.
There is a way to do that without creating a char*?
A char usually represents a character. However, a single char is simply an integer in range of at least [-127,+127] (signed version) or at least [0,255] (unsigned version).
If you obtained a character looking as a digit, the value stored in it is an ASCII number representing it. Digits start at code 48 (for zero) and go up incrementally till code 57 (for nine). Thus, if you take the code and subtract 48, you get the integer value. From there, converting it to double is a matter of casting.
Thus:
char digit = ...
double value = double(digit - 48);
or even better, for convenience:
char digit = ...
double value = double(digit - '0'); //'0' has a built-in value 48
There is a way to do that without creating a char* ???
Sure. You can extract the digit number from a single char as follows:
char c = '4';
double d = c - '0';
// ^^^^^^^ this expression results in a numeric value that can be converted
// to double
This uses the circumstance that certain character tables like ASCII or EBCDIC encode the digits in a continuous set of values starting at '0'.

How to subtract integers from characters in C?

Brian Kernighnan in his book Programming with C says
By definition, chars are just small integers, so char variables and
constants are identical to ints in arithmetic expressions.
Does this mean we can subtract char variable from int ??
I wrote a small piece of code:
#include <stdio.h>
main()
{
int a ;
int c;
a = 1;
c = 1 - '0' ;
printf("%d", c);
}
However it gives me output = -47...
What is that I'm doing wrong ?? Are the variables I assigned have the right type??
The output is to be expected. '0' is a char value that, since your compiler presumably uses the ASCII encoding, has value 48. This is converted to int and subtracted from 1. Which gives the value -47.
So the program does what it is expected to do, just not what you might hope it would do. As for what you are doing wrong, it is hard to say. I'm not sure what you expect the program to do, or what problem you are trying to solve.
The characters from '0'-'9'' have values 48-57 when converted to integer ('0' = 48, '1' = 49 etc). Read more about ASCII Values. When used in numerical calculation, first they are converted to int, so 1- '0' = 1-48 =-47.
You're mixing here the actual operation with the form of representation. printf outputs the data according to the specified format - integer in your case. If you want to print it as a character, switch %d with %c.
What you are doing is treating with the ASCII code of the chars, each char has an ASCII value assigned.
Now, playing a little with the ASCII of each char you can do things like:
int a;
a = 'a' - 'A' ;
printf("%d", a);
And get 32 as output, due to the ASCII value to 'a' = 97 and for 'A' = 65, then you have 97-65 = 32
I think this gives you more clear understanding...
#include <stdio.h>
main()
{
int a ;
a = 1;
printf("%d", char(a+48));
//or printf("%d", char(a+'0'));
}

How to convert a "char number" into an int WITHOUT use of magic numbers? (in C++)

In other words, based on the ASCII table, from the range of '0' to '9',
how may I convert them into integers 0 to 9?
A solution such as:
char a = '6';
int b = a-48;
has already been floating around these parts, but I was wondering if there are other ways to go about this without the use of magic numbers?
Since '0' is not guaranteed to be 48, but the numbers are guaranteed to be consecutive, you can use a-'0'.
If you really want to, you could use a stringstream like this:
#include <string>
#include <sstream>
int charToInt(char c) {
// initialize a buffered stream with a 1-character string
std::stringstream ss(std::string(1,c));
// read an int from the stream
int v;
ss >> v;
return v;
}
Not the simplest way to do the conversion, but this way you don't see any of the implementation details involving "magic" number or character. You also get error handling (an exception is thrown) if the caracter was not a number.
On the other hand, if you're absolutely certain that the character c is in the '0'..'9' range, I don't see why not use c - '0'.
Another solution is to replace c - 48 with c & 0xf, but that still involves magic numbers and is less readable than c - '0'
The ascii table is ordered in an hexadecimal way, so it's very easy to change numbers characters to real number value, or another things like to Uppercase to Lower...
As the numbers begin in the 0x30, then 0x30 =0 , 0x31 = 1, 0x32 =2, etc, you must just remove the 0x30 to get the real value.
char number='2';
int numberValue = (int)number - 0x30; /* you can rest the '0' value too */
As it, to convert an int to char is the same, just add it the 0x30.
int numberValue=5;
char number = (int)numberValue +0x30; /* or add '0' to your var */
Subtract ASCII zero from the number:
char a = '2';
int b = a-'0';
If you can't use '0', how about that kind of cheating?
(int)(a + 2) % 10;
If it's a char, not a char pointer, you can do this:
int convert (char x)
{
return (int(x) - int('0'));
}

convert char[] of hexadecimal numbers to char[] of letters corresponding to the hexadecimal numbers in ascii table and reversing it

I have a char a[] of hexadecimal characters like this:
"315c4eeaa8b5f8aaf9174145bf43e1784b8fa00dc71d885a804e5ee9fa40b16349c146fb778cdf2d3aff021dfff5b403b510d0d0455468aeb98622b137dae857553ccd8883a7bc37520e06e515d22c954eba5025b8cc57ee59418ce7dc6bc41556bdb36bbca3e8774301fbcaa3b83b220809560987815f65286764703de0f3d524400a19b159610b11ef3e"
I want to convert it to letters corresponding to each hexadecimal number like this:
68656c6c6f = hello
and store it in char b[] and then do the reverse
I don't want a block of code please, I want explanation and what libraries was used and how to use it.
Thanks
Assuming you are talking about ASCII codes. Well, first step is to find the size of b. Assuming you have all characters by 2 hexadecimal digits (for example, a tab would be 09), then size of b is simply strlen(a) / 2 + 1.
That done, you need to go through letters of a, 2 by 2, convert them to their integer value and store it as a string. Written as a formula you have:
b[i] = (to_digit(a[2*i]) << 4) + to_digit(a[2*i+1]))
where to_digit(x) converts '0'-'9' to 0-9 and 'a'-'z' or 'A'-'Z' to 10-15.
Note that if characters below 0x10 are shown with only one character (the only one I can think of is tab, then instead of using 2*i as index to a, you should keep a next_index in your loop which is either added by 2, if a[next_index] < '8' or added by 1 otherwise. In the later case, b[i] = to_digit(a[next_index]).
The reverse of this operation is very similar. Each character b[i] is written as:
a[2*i] = to_char(b[i] >> 4)
a[2*i+1] = to_char(b[i] & 0xf)
where to_char is the opposite of to_digit.
Converting the hexadecimal string to a character string can be done by using std::substr to get the next two characters of the hex string, then using std::stoi to convert the substring to an integer. This can be casted to a character that is added to a std::string. The std::stoi function is C++11 only, and if you don't have it you can use e.g. std::strtol.
To do the opposite you loop over each character in the input string, cast it to an integer and put it in an std::ostringstream preceded by manipulators to have it presented as a two-digit, zero-prefixed hexadecimal number. Append to the output string.
Use std::string::c_str to get an old-style C char pointer if needed.
No external library, only using the C++ standard library.
Forward:
Read two hex chars from input.
Convert to int (0..255). (hint: sscanf is one way)
Append int to output char array
Repeat 1-3 until out of chars.
Null terminate the array
Reverse:
Read single char from array
Convert to 2 hexidecimal chars (hint: sprintf is one way).
Concat buffer from (2) to final output string buffer.
Repeat 1-3 until out of chars.
Almost forgot to mention. stdio.h and the regular C-runtime required only-assuming you're using sscanf and sprintf. You could alternatively create a a pair of conversion tables that would radically speed up the conversions.
Here's a simple piece of code to do the trick:
unsigned int hex_digit_value(char c)
{
if ('0' <= c && c <= '9') { return c - '0'; }
if ('a' <= c && c <= 'f') { return c + 10 - 'a'; }
if ('A' <= c && c <= 'F') { return c + 10 - 'A'; }
return -1;
}
std::string dehexify(std::string const & s)
{
std::string result(s.size() / 2);
for (std::size_t i = 0; i != s.size(); ++i)
{
result[i] = hex_digit_value(s[2 * i]) * 16
+ hex_digit_value(s[2 * i + 1]);
}
return result;
}
Usage:
char const a[] = "12AB";
std::string s = dehexify(a);
Notes:
A proper implementation would add checks that the input string length is even and that each digit is in fact a valid hex numeral.
Dehexifying has nothing to do with ASCII. It just turns any hexified sequence of nibbles into a sequence of bytes. I just use std::string as a convenient "container of bytes", which is exactly what it is.
There are dozens of answers on SO showing you how to go the other way; just search for "hexify".
Each hexadecimal digit corresponds to 4 bits, because 4 bits has 16 possible bit patterns (and there are 16 possible hex digits, each standing for a unique 4-bit pattern).
So, two hexadecimal digits correspond to 8 bits.
And on most computers nowadays (some Texas Instruments digital signal processors are an exception) a C++ char is 8 bits.
This means that each C++ char is represented by 2 hex digits.
So, simply read two hex digits at a time, convert to int using e.g. an istringstream, convert that to char, and append each char value to a std::string.
The other direction is just opposite, but with a twist.
Because char is signed on most systems, you need to convert to unsigned char before converting that value again to hex digits.
Conversion to and from hexadecimal can be done using hex, like e.g.
cout << hex << x;
cin >> hex >> x;
for a suitable definition of x, e.g. int x
This should work for string streams as well.