I would like to convert a double into character string and I find gcvt() and _gcvt(). I just wondering what is the difference between them. Both return with char* and both need value, number of digits and buffer as a given parameters
As per the google search result
The _gcvt() function is identical to gcvt(). Use _gcvt() for ANSI/ISO naming conventions.
Related
int currD, currM, currY;
scanf("%d/%d/%d", &currD, &currM, &currY);
I saw this code receiving birth date in the format DD/MM/YYYY, but I wonder what's the meaning of putting '/', I know without this, it will lead to bad input because of the character '/'. So what does it actually mean?
When encountering code that you don’t understand, and which is calling a function from a library, your first order of business is to research the documentation for that function. For C standard functions it’s enough to google the function name.
A good reference in this case is cppreference (don’t be misled by the website name, this is the C reference, not the C++ reference). It gives the function’s definition as
int scanf( const char *format, ... );
Now look for the parameter description of the format parameter:
pointer to a null-terminated character string specifying how to read the input.
The subsequent text explains how to read the format string. In particular:
[…] character [except %] in the format string consumes exactly one identical character from the input stream, or causes the function to fail if the next character on the stream does not compare equal.
conversion specifications [in] the following format
introductory % character
conversion format specifier
d — matches a decimal integer.
In other words:
scanf parses a textual input based on the format string. Inside the format string, / matches a slash in the user input literally. %d matches a decimal integer.
Therefore, scanf("%d/%d/%d", …) will match a string consisting of three integers separated by slashes, and store the number values inside the pointed-to variables.
Is just the separator in the date format. The error must raise when some function searchs for those /.
The first parameter of scanf is a string specifying the format of the string you want to use to store the informations in the further arguments. You can see this format string as a pattern : %d means an integer, and without the '%' it means it just has to match exactly the characters.
Input is expected to provide like 04/07/2019.
If input is provided only 04072019. currD alone hold the value 04072019, currM and currY might garbage value as it is not initialised.
It expects the input to be in the format three integers separated by two slashes ("/"). For example: 10/11/1999.
Is it possible to define a string as char in C like this? I think C calls it multi character constant.
#define OK '_/'
I want C to treat '_/' as a char from now on, not a string, so this:
printf("%c", OK);
prints _/ and not /
While it is technically valid C to define OK as '_/', the value of a multi-character character constant is implementation defined, so this is probably not something you want to do.
There is no way you will be able to print more than one character without resorting to strings.
Multi character constants are of int type and their value is not strictly defined-- it's platform dependent stuff. So using them as normal letters is not best idea, even though you can use them in every context as normal char there is no guarantee that they will be compiled as you intend (as in your example you get only last char from ur string).
here you have explanation of the topic:
Multiple characters in a character constant
Why do we pass the format specifiers to scanf( ) in C as its parameters?
I mean, can't we just do
scanf( &var ); // Here, var is any pre-declared variable.
the type of var can be fetched from its declaration. Why is this not allowed ?
I see the below code as a waste of memory.
scanf( "%d" , &var );
The type cannot be "fetched from it's declaration". There is nothing magical about scanf(), it's just a function. A function in C cannot access meta information about variables.
This is why, in general, function arguments are declared including their type. For variable-argument functions such as scanf(), the first argument is not optional and it is used to describe the number and type of the other arguments in some fashion chosen by the function itself.
You clearly need to read some book on C programming to get better understanding of the core concepts. Unlike some other languages, C doesn't have I/O mechanism baked into the language. scanf() is just a library function and as such, this function has no way to automagically know the type of the variable it is supposed to fill.
Because %d will simply specify what the type of var is, there is no memory wastage. scanf(&var) would not work because the function is not designed to accept arguments that way.
You know that variables in C can be of different types:
int: Integer
char: Character
float: Floating point number.
...
Unlike other languages, variable types cannot be implicitly inferred at compilation time in C. That is why you always declare the type of your variables ( Example: int a or char c).
Because scanf is just a function in C, and because functions in C should take parameters of a specific type, people who coded C decided to use the following format:
scanf("%d", &var) ; // for integers
scanf("%c", &var); //for chars
scanf("%f", &var); //for double and floats.
using %d or %c does not waste memory or whatsoever. you can think about it as a flag that specifies the type of the input variable.
Could the developers of C do it without %d, %c...etc? Yes they could, but then, they have to handle all possible exceptions that might arise from sending the wrong type.
Suppose the developers of C used just the following format
scanf(&var);
That is surly very concise, but then you will have to use the same syntax to send chars/int/double...etc, and then the function scanf has to figure out a way to decide about the type of the variable that was sent. Remember what I told you before? variable types CANNOT be implicitly inferred at compilation time, and thus, this task will be almost impossible.
They could however use a different scanf function for every type. For example:
scanfInt(&var); //for integers.
scanfFloat(&var); //for floats.
...
...
That would work perfectly, but it makes less sense to replicate the same code of scanf and use different functions just because the type is different.
So what is the solution? ==> Use the same function name ( scanf ), and then add a parameter (%d, %f, %c..) that will be used internally as a flag by C to know the parameter type.
I hope now you have a better understanding of the use of %d, %f....
There are two major points you are missing here.
First, we humans sitting at the keyboard will write something like:
char var = '0';
And we know that the "type" of this variable is char and we probably intend to store a character there. Once the compiler gets a hold of this it removes these "human" elements all that is left is at some memory location there is 1 byte reserved, further references to places in the code where we wrote "var" will interact with this memory location. That is all the compiler knows, there is no understanding of the intended variable "type".
Second, the format specificers do so much more than just indicate a simple type. Look at any page explaining scanf() and you'll see a long list, take special note of things like scan sets, negated scan sets, and expected input lengths.
Let's say I want the user to enter just a single digit, 0-9, well I don't have to just assume they will do as I ask, I can help ensure they will by using the format specifiers in scanf():
int var = 0;
print("enter 1 digit (0-9):\n");
scanf("%1d", &var);
Now, no matter how many digits they enter, I'll only have stored the first one.
What if you have a string that you want to read from the user, and you want to read everything up until you hit a new line character (read over spaces). We'll there are a number of options for this, but scanf can do it too. The "standard" to read a string is:
scanf("%s",some_string);
But that will stop at any whitespace character, so you wouldn't want scanf() to make an assumption in this case, you'd want to be able to use a specific negated scanset:
scanf("%[^\n]",some_string);
I want to handle some char variables and would like to get a list of some functions that can do these tasks when it comes to handling chars.
Getting first characters of a char (var_name[1] doesnt seem to work)
Getting last characters of a char
Checking for char1 matches with char2 ( eg if "unicorn" matches words with "bicycle"
I am pretty sure some of these methods exist in libraries such as stdio.h or so but google isnt my friend.
EDIT:My 3rd question means not direct match with strcmp but single character match(eg if "hey" and "hello") have e as common letter.
Use var_name[0] to get first character (array indexes run from 0 to N - 1, where N is the number of elements in the array).
Use var_name[strlen(var_name) - 1] to get the last character.
Use strcmp() to compare two char strings.
EDIT:
To search for character in a string you can use strchr():
if (strchr("hello", 'e') && strchr("hey", 'e'))
{
}
There is also strpbrk() function that would indicate if two strings have any common characters:
if (strpbrk("hello", "hey"))
{
}
Assuming you mean a char[], and not a char which is a single character.
C uses 0-based indexing, var_name[0] gives you the first char.
strlen() gives you the length of the string, which together with my answer to 1. means
char lastchar = var_name[strlen(var_name)-1]; http://www.cplusplus.com/reference/clibrary/cstring/strlen/
strcmp(var_name1, var_name2) == 0. http://www.cplusplus.com/reference/clibrary/cstring/strcmp/
I am pretty sure some of these methods exist in libraries such as
stdio.h or so but google isnt my friend.
The string functions in the C standard library (libc) are described in the header file . If you're on a unix-ish machine, try typing man 3 string at a command line. You can then use the man program again to get more information about specific functions, e.g. man 3 strlen. (The '3' just tells man to look in "section 3", which describes the C standard library functions.)
What you're looking for is the string functions in the C runtime library. These are defined in string.h, not stdio.h.
But your list of problems is simple:
var_name[0] works perfectly well for accessing the first char in an array. var_name[ 1] doesn't work because arrays in C are zero-based.
The last char in an array is:
char c;
c = var_name[strlen(var_name)-1];
Testing for equality is simple:
if (var_name[0] == var_name[1])
; // they match
C and C++ strings are zero indexed. The memory you need to hold a particular length string has to be at least the string length and one character for the string terminator \0. So, the first character is array[0].
As #Carey Gregory said, the basic string handling functions are in string.h. But these are only primitives for handling strings. C is a low level enough language, that you have an opportunity to build up your own string handling library based on the functions in string.h.
On example might be that you want to pass a string pointer to a function and also the length of the buffer holding that sane string, not just the string length itself.
Is there a C function which can do the equivalent of find_first_not_of, receiving a string to search and a set of characters and returning the first character in the string that's not of the set?
The strspn function will get you most of the way there. (You just have to massage the return value a bit.)