I'm having problem using int values in C that starts with zero's (like 00111001).
I know that C compiler understand zero's in the beginning of a number as an octal number.
My question is how to disable it? I want to turn an 8 digit int into a char array[8].
e.g. 01010001={'0','1','0','1','0','0','0','1'}
If you're using GCC, you can use binary literals. Set the variable to 0b00111001.
https://gcc.gnu.org/onlinedocs/gcc/Binary-constants.html
Surround your number with quotes:
char array[] = "01010001";
The above will make each digit a character in array that you can read as well as placing a '\0' character at the end so that it can be used as a C string - as per your last sentence. Beware, the length of this string (in memory) will be 9 characters though because of this added NUL character.
Related
I have the following C code:
#include <stdio.h>
#include <strings.h>
int main(void){
char * str = "\012\0345";
char testArr[8] = {'\0','1','2','\0','3','4','5','\0'};
printf("%s\n",str);
printf("**%s**",testArr);
return 0;
}
See live code here
I'm having trouble understanding the results and I have googled but am unsure that I understand why a null character at the start of a string and why one in the middle would cause only the string "5" to display. Also, when I assign each string character to array testArr and then attempt to display that array of characters the result is different despite the string and the array having the same characters. So, I'm struck by the confounding results, especially their disparity. With the string str, does the code display "5" because the null characters overwrite what is in memory?
Also, with the array I created using the same characters, nothing displays of the data contained in array testArr. Is it that once the first null is encountered for some reason everything else is ignored? If so, why doesn't the same behavior occur with string str which contains the same characters?
An octal escape sequence is \ followed by one to three octal digits, per C 2018 6.4.4.4 1. Per 6.4.4.4 7: “Each octal or hexadecimal escape sequence is the longest sequence of characters that can constitute the escape sequence.” So, when the compiler sees "\012\0345", it interprets it as the sequence \012 (which is ten), the sequence \034 (which is twenty-eight), and the character 5.
To represent the string you intended, you could use "\00012\000345". Since an octal escape sequence stops at three digits, this is interpreted as the sequence \000, the characters 1 and 2, the sequence \000, and the characters 3, 4, and 5. (A null terminating character will also be appended automatically.)
When you printed "\012\0345", the characters with codes ten and twenty-eight were printed but had no visible effect. (Your C implementation likely uses ASCII, in which case they are control characters. \012 is new-line, so it should have caused a line advance, but you probably did not notice that. \034 is a file-separator control character, which likely has no effect when printed to a regular terminal display.)
When you printed testArr, the null character in the first position ended the string.
When I came across this C language implementation of Porters Stemming algorithm I found a C-ism I was confused about.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void test( char *s )
{
int len = s[0];
printf("len= %i\n", len );
printf("s[len] = %c\n", s[len] );
}
int main()
{
test("\07" "abcdefg");
return 0;
}
and output:
len = 7
s[len] = g
However, when I input
test("\08" "abcdefgh");
or any string constant that is longer than 7 with the corresponding length in the first pair of parenthesis ( i.e. test("\09" "abcdefghi"); the output is
len = 0
s[len] =
But any input like test("\01" "abcdefgh"); prints out the character in that position ( if we call the first character position 1 and not 0 for the moment )
It appears if test( char *s ) reads the number in the first pair of parenthesis ( how it does this I am not sure since I thought s[0] would be able to only read a single char, i.e. the '\' ) and prints the last character at that index + 1 of the string constant in the second pair of parenthesis.
My question is this: It seems as if we are passing two string constants into test( char *s ). What exactly is happening here, meaning, how does the compiler seem to "split" up the string over two pairs of parenthesis? Another question one might have is, is a string of the form "blah" "abcdefg" one consecutive block of memory? It may be the case that I have overlooked something elementary, but even so I would like to know what I overlooked. I know this is a basic concept but I could not find a clear example or situation on the web that explains this and in all honesty I don't follow the output. Any helpful comments are welcomed.
There are at least three things going on here:
Literal strings juxtaposed against one another are concatenated by the compiler. "a" "b" is exactly the same as "ab".
The backslash is an escape character, which means it is not copied literally into the resulting string. The notation \01 means "the character with ASCII value 1".
The notation \0... means an octal character constant. Octal numbers are base 8, made up from digits that range from 0 through 7 inclusive. 8 is not a valid octal constant, so "\08" does not follow "\07".
The problem is not in the length of the string, but in the \o syntax for specifying non-printable values in string literals. \o, \oo, and \ooo denote octal constants, i.e. a single character whose value is written in base 8. Since 08 in \08 doesn't represent a valid base 8 number, it is interpreted as \0 followed by the ASCII character 8.
To fix the problem, represent 8 as \10 or \010:
test("\007" "abcdefg");
test("\010" "abcdefgh");
...or switch to hexadecimal, where the \x prefix makes the base more explicit to the casual reader:
test("\x07" "abcdefg");
test("\x08" "abcdefgh");
test("\x09" "abcdefghi");
test("\x0a" "abcdefghij");
...
\number in a character or string literal is means the character whose code is the value number. number is interpreted in octal, so the first non-octal digit terminates the number. So "\07" is a one-character string containing the character with code 7, but \08 is a two-character string containing the character with code 0 followed by the digit 8.
Additionally, code 0 the null terminator that's used in C to indicate the end of the string. So that second string ends at the beginning, because its first byte is the terminator. This why the length of the string in your second example is 0.
When two or more string literals are adjacent (separated only by white-space), the compiler will join them into a single string. Therefore "\07" "abcdefg" is equivalent to "\07abcdefg".
"\07" is an octal escape. An octal escape ends after three digits or with first non-octal character. So, when you enter "\08", 8 is a non octal character therefore escape ends and 0 is stored at s[0].
Now, len is 0 and printing s[len] will try to print the character at s[0] which has a non printable ASCII code (Only character above ASCII value above 32 are printable).
I'm still new to the forum so I apologize in advance for forum - etiquette issues.
I'm having trouble understanding the differences between int arrays and char arrays.
I recently wrote a program for a Project Euler problem that originally used a char array to store a string of numbers, and later called specific characters and tried to use int operations on them to find a product. When I used a char string I got a ridiculously large product, clearly incorrect. Even if I converted what I thought would be compiled as a character (str[n]) to an integer in-line ((int)str[n]) it did the exact same thing. Only when I actually used an integer array did it work.
Code is as follows
for the char string
char str[21] = "73167176531330624919";
This did not work. I got an answer of about 1.5 trillion for an answer that should have been about 40k.
for the int array
int str[] = {7,3,1,6,7,1,7,6,5,3,1,3,3,0,6,2,4,9,1,9};
This is what did work. I took off the in-line type casting too.
Any explanation as to why these things worked/did not work and anything that can lead to a better understanding of these ideas will be appreciated. Links to helpful stuff are as well. I have researched strings and arrays and pointers plenty on my own (I'm self taught as I'm in high school) but the concepts are still confusing.
Side question, are strings in C automatically stored as arrays or is it just possible to do so?
To elaborate on WhozCraig's answer, the trouble you are having does not have to do with strings, but with the individual characters.
Strings in C are stored by and large as arrays of characters (with the caveat that there exists a null terminator at the end).
The characters themselves are encoded in a system called ascii which assigns codes between 0 - 127 for characters used in the english language (only). Thus "7" is not stored as 7 but as the ascii encoding of 7 which is 55.
I think now you can see why your product got so large.
One elegant way to fix would be to convert
int num = (int) str[n];
to
int num = str[n] - '0';
//thanks for fixing, ' ' is used for characters, " " is used for strings
This solution subtracts the ascii code for 0 from the ascii code for your character, say "7". Since the numbers are encoded linearly, this will work (for single digit numbers). For larger numbers, you should use atoi or strtol from stdlib.h
Strings are just character arrays with a null terminating byte.
There is no separate string data type in c.
When using a char as an integer, the numeric ascii value is used. For example, saying something like printf("%d\n", (int)'a'); will result in 97 (the ascii value of 'a') being printed.
You cannot use a string of numbers to do numeric calculations unless you convert it to an integer array. To convert a digit as a character into its integer form, you can do something like this:
char a = '2';
int a_num = a - '0';
//a_num now stores integer 2
This causes the ascii value of '0' (48) to be subtracted from ascii value '2' (50), finally leaving 2.
char str[21] = "73167176531330624919"
this code is equivalent to
char str[21] = {'7','3','1','6','7','1','7','6','5',/
'3','1','3','3','0','6','2','4','9','1','9'}
so whatever stored in str[21] is not numbers, but the char(their ASCII equivalent representation is different).
side question answer - yes/no, the strings are automatically stored as char arrays, but the string does has a extra character('\0') as the last element(where a char array need not have such a one).
How can I extract numbers from a char array, separated with spaces, convert them to integers and sum them? For example:
"34 54 3 23"
I'd start at the beginning of the array, check each character in turn with isdigit() and keep a current value and a current total.
When reaching the terminating NUL char (or last element of the array), the current total is already calculated.
You need to parse the string.
If you know how many integers are in there, you could use just a sscanf.
Otherwise find out where blanks are (with something similar to strtok, for example) and then read integers using atoi
char label[8] = "abcdefgh";
char arr[7] = "abcdefg";
printf("%s\n",label);
printf("%s",arr);
====output==========
abcdefgh
abcdefgÅ
Why Å is appended at the end of the string arr?
I am running C code in Turbo C ++.
printf expects NUL-terminated strings. Increase the size of your char arrays by one to make space for the terminating NUL character (it is added automatically by the = "..." initializer).
If you don't NUL-terminate your strings, printf will keep reading until it finds a NUL character, so you will get a more or less random result.
Your variables label and arr are not strings. They are arrays of characters.
To be strings (and for you to be able to pass them to functions declared in <string.h>) they need a NUL terminator in the space reserved for them.
Definition of "string" from the Standard
7.1.1 Definitions of terms
1 A string is a contiguous sequence of characters terminated by and including
the first null character. The term multibyte string is sometimes used
instead to emphasize special processing given to multibyte characters
contained in the string or to avoid confusion with a wide string. A pointer
to a string is a pointer to its initial (lowest addressed) character. The
length of a string is the number of bytes preceding the null character and
the value of a string is the sequence of the values of the contained
characters, in order.
Your string is not null terminated, so printf is running into junk data. You need to use the '\0' at the end of the string.
Using GCC (on Linux), it prints more garbage:
abcdefgh°ÃÕÄÕ¿UTÞÄÕ¿UTÞ·
abcdefgabcdefgh°ÃÕÄÕ¿UTÞÄÕ¿UTÞ·
This is because, you are printing two character arrays as strings (using %s).
This works fine:
char label[9] = "abcdefgh\0"; char arr[8] = "abcdefg\0";
printf("%s\n",label); printf("%s",arr);
However, you need not mention the "\0" explicitly. Just make sure the array size is large enough, i.e 1 more than the number of characters in your strings.