I am applying printf and/or other functions to a certain string of characters, read from a file. I want to skip the first 5 characters under certain conditions. Now I thought to be clever by, if the conditions apply, increasing the string pointer by 5:
if (strlen(nav_code) == 10 ) {nav_code = 5+nav_code;}
but the compiler refuses this:
error: assignment to expression with array type
What have I misunderstood? How to make my idea work - or is it a bad idea anyway?
It's probably becuase nav_code is not a pointer but a character array like char nav_code[50]. Try the following:
char nav_code[50];
char *nav_code_ptr = nav_code;
if (strlen(nav_code_ptr) == 10 ) {nav_code_ptr += 5;}
// forth on, use nav_code_ptr instead of nav_code
I am applying printf and/or other functions to a certain string of characters, read from a file. I want to skip the first 5 characters under certain conditions.
If printf is all what you need, then sure you can skip the first 5 characters.
Given nav_code is string (either char array or char pointer), then:
printf( "%s", nav_code + 5 ); // skip the first 5 characters
Of course you need to make sure your string has more than 5 characters, otherwise it's flat out illegal as out-of-bound access.
In your code, nav_code is an array and arrays cannot be assigned.
Instead, use a pointer, initialize that with the address of the first element of the array, make pointer arithmetic on that pointer and store the updated result back to the pointer.
Related
I actually have a question regarding the concept of a char array, especially the one which is declared and initialized like below.
char aString[10] = "";
What i was taught was that this array can store up to 10 characters (index 0-9) and that at index 10 there is an automatically placed null terminating character (i know that accessing it would not be right) such that if we use string handling functions (printf, scanf, strcmp, etc.) they would know when the string stops.
However when I tried making a struct data type like below,
typedef struct customer{
char accountNum[10];
char name[100];
char idNum[15];
char address[200];
char dateOfBirth[10];
unsigned long long int balance;
char dateOpening[10];
}CUSTOMER;
inserted 10 characters into accountNum (any method, e.g. scanf), and printf it, what is printed out will be accountNum and values in the first word of name (i know that printf will stop at a space or a '\0'). This indicates that a char array does not have a terminating null at the end of the array.
Does this mean that if we have a char array of size 10 (char aString[10]), its maximum number of char it can store is 9 characters? or does things work differently in a struct? It would be nice if someone can help me the concept because it seems like i may have been working with undefined behaviour this whole time.
char aString[10] = "";
What i was taught was that this array can store up to 10 characters (index 0-9)
Yes.
and that at index 10 there is an automatically placed null terminating character
That is wrong. For one thing, index 10 would be out of bounds of the array. The compiler will certainly not initialize data outside of the memory it has reserved for the array.
What actually happens is that the compiler will copy the entire string literal including the null-terminator into the array, and if there are any remaining elements then they will be set to zeros. If the string literal is longer than the array can hold, the compile will simply fail.
In your example, the string literal has a length of 1 char (the null terminator), so the entire array ends up initialized with zeros.
i know that accessing it would not be right
There is no problem with accessing the null terminator, as long as it is inside the bounds of the array.
such that if we use string handling functions (printf, scanf, strcmp, etc.) they would know when the string stops.
Yes, they expect C-style strings and so will look for a null terminator - unless they are explicitly told the actual string length, ie by using a precision modifier for %s, or using strncmp(), etc.
However when I tried making a struct data type like below,
<snip>
inserted 10 characters into accountNum (any method, e.g. scanf), and printf it, what is printed out will be accountNum and values in the first word of name
That means you either forgot to null-terminate accountNum, or you likely overflowed it by writing too many characters into it. For instance, that is very easy to do when misusing scanf(), strcpy(), etc.
i know that printf will stop at a space or a '\0'
printf() does not stop on a space, only on a null terminator. Unless you tell it the max length explicitly, eg:
CUSTOMER c;
strncpy(c.accountNum, "1234567890", 10); // <-- will not be null terminated!
printf("%.10s", c.accountNum); // <-- stops after printing 10 chars!
If it has not encountered a null terminator by the time it reaches the 10th character, it will stop itself.
This indicates that a char array does not have a terminating null at the end of the array.
An array is just an array, there is no terminator, only a size. If you want to treat a character array as a C-style string, then you are responsible for making sure the array contains a nul character in it. But that is just semantics of the character data, the compiler will not do anything to ensure that behavior for you (except for in the one case of initializing a character array with a string literal).
Does this mean that if we have a char array of size 10 (char aString[10]), its maximum number of char it can store is 9 characters?
Its maximum storage will always be 10 chars, period. But if you want to treat the array as a C-style string, then one of those chars must be a nul.
or does things work differently in a struct?
No. Where an array is used does not matter. The compiler treats all array the same, regardless of context (except for the one special case of initializing a character array with a string literal).
What i was taught was that this array can store up to 10 characters (index 0-9) and that at index 10 there is an automatically placed null terminating character (i know that accessing it would not be right) such that if we use string handling functions (printf, scanf, strcmp, etc.) they would know when the string stops.
Yes, but accessing the null terminating character is absolutely safe.
inserted 10 characters into accountNum (any method, e.g. scanf), and printf it, what is printed out will be accountNum and values in the first word of name (i know that printf will stop at a space or a '\0'). This indicates that a char array does not have a terminating null at the end of the array.
printf does not stop for a space, only for a null terminating character. In this case, printf will print all characters until it sees '\0'.
Does this mean that if we have a char array of size 10 (char aString[10]), its maximum number of char it can store is 9 characters?
Yes.
or does things work differently in a struct?
There is no difference.
I'm working with char arrays in C. I'm setting the size in a previous step. When I print it out it clearly shows the num_digits as 1.
But then when I put it in to set the size of a char array to make it a char array of size num_digits, its setting the size of the array as 6.
In the next step when I print strlen(number_array), it prints 6. Printing it out I get something with a lot of question marks. Does anyone know why this is happening?
int num_digits = get_num_digits(number);
printf("Num digits are %d\n", num_digits);
char number_array[num_digits];
printf("String len of array: %d\n", strlen(number_array));
You need to null terminate your array.
char number_array[num_digits + 1];
number_array[num_digits] = '\0';
Without this null terminator, C has no way of know when you've reached the end of the array.
just use 'sizeof' instead of 'strlen'
printf("String len of array: %d\n", sizeof(number_array));
There are a couple possible issues I see here:
As noted in Michael Bianconi's answer, C character arrays (often called strings) require null terminators. You would explicitly set this this with something like:
number_array[number + 1] = '\0'; /* See below for why number + 1 */
Rather than just setting the last element to null, pre-initializing the entire character array to nulls might be helpful. Some compilers may do this for you, but if not you'll need to do this explicitly with something like:
for (int i = 0; i < num_digits + 1; i ++) number_array[i] = '\0';
Note that with gcc I had to use C99 mode using -std=c99 to get this to compile, as the compiler didn't like the initialization within the for statement.
Also, the code presented sets the length of the character array to be the same length as number's length. We don't know what get_num_digits returns, but if it returns the actual number of significant digits in an integer, this will come up one short (see above and other answer), as you need an extra character for the null terminator. An example: if the number is 123456 and get_number_digits returns 6, you would would need to set the length of number_array to 7, instead of 6 (i.e. number + 1).
char number_array[num_digits]; allocates some space for a string. It's an array of num_digits characters. Strings in C are represented as an array of characters, with a null byte at the end. (A null byte has the value zero, not to be confused with the digit character '0'.) So this array has room for a string of up to num_digits - 1 characters.
sizeof(number_array) gives you the array storage size. That's the total amount of space you have for a string plus its null terminator. At any given time, the array can contain a string of any length up to number_array - 1, or it might not contain a string at all if the array doesn't contain a null terminator.
strlen(number_array) gives you the length of the string contained in the array. If the array doesn't contain a null terminator, this call may return a garbage value or crash your program (or make demons fly out of your nose, but most computers fortunately lack the requisite hardware).
Since you haven't initialized number_array, it contains whatever happened to be there in memory before. Depending on how your system works, this may or may not vary from one execution of the program to the next, and this certainly does vary depending on what the program has been doing and on the compiler and operating system.
What you need to do is:
Give the array enough room for the null terminator.
Initialize the array to an empty string by making setting the first character to zero.
Optionally, initialize the whole array to zero. This is not necessary, but it may simplify further work with the array.
Use %zu rather than %d to print a size. %d is for an int, but sizeof and strlen return a size_t, which depending on your system may or may not be the same size of integers.
char number_array[num_digits + 1];
number_array[0] = 0; // or memset(number_array, 0, sizeof(number_array));
printf("Storage size of array: %zu\n", sizeof(number_array));
printf("The array contains an empty string: length=%zu\n", strlen(number_array));
I'm making a simple program in C, which checks the length of some char array and if it's less than 8, I want to fill a new array with zeroes and add it to the former array. Here comes the problem. I don't know why the last values are some signs(see the photo).
char* hexadecimalno = decToHex(decimal,hexadecimal);
printf("Hexadecimal: %s\n", hexadecimalno);
char zeroes [8 - strlen(hexadecimalno)];
if(strlen(hexadecimalno) < 8){
for(i = 0; i < (8-strlen(hexadecimalno)); i++){
zeroes[i]='0';
}
}
printf("zeroes: %s\n",zeroes);
strcat(zeroes,hexadecimalno);
printf("zeroes: %s\n",zeroes);
result
In C, strings (which are, as you are aware, arrays of characters) do not have any special metadata that tells you their length. Instead, the convention is that the string stops at the first character whose char value is 0. This is called "null-termination". The way your code is initializing zeroes does not put any null character at the end of the array. (Do not confuse the '0' characters you are putting in with NUL characters -- they have char value 48, not 0.)
All of the string manipulation functions assume this convention, so when you call strcat, it is looking for that 0 character to decide the point at which to start adding the hexadecimal values.
C also does not automatically allocate memory for you. It assumes you know exactly what you are doing. So, your code is using a C99 feature to dynamically allocate an array zeroes that has exactly the number of elements as you need '0' characters appended. You aren't allocating an extra byte for a terminating NUL character, and strcat is also going to assume that you have allocated space for the contents of hexadecimalno, which you have not. In C, this does not trigger a bounds check error. It just writes over memory that you shouldn't actually write over. So, you need to be very careful that you do allocate enough memory, and that you only write to memory you have actually allocated.
In this case, you want hexadecimalno to always be 8 digits long, left-padding it with zeroes. That means you need an array with 8 char values, plus one for the NUL terminator. So, zeroes needs to be a char[9].
After your loop that sets zeroes[i] = '0' for the correct number of zeroes, you need to set the next element to char value 0. The fact that you are zero-padding confuses things, but again, remember that '0' and 0 are two different things.
Provided you allocate enough space (at least 9 characters, assuming that hexadecimalno will never be longer than 8 characters), and then that you null terminate the array when putting the zeroes into it for padding, you should get the expected result.
I construct an array with:
char *state[] = {"California", "Oregon", "Texas"};
I want to get the length of California which should be 10 but when I do sizeof(state[0]), it just gives me 8 ( I think this means 8 bytes since the size of a char is 1 byte). But why 8 though instead of 10? I'm still able to print out each chars of California by looping through state[0][i].
I'm new to C, can someone please explain this to me?
The simplest explanation is that sizeof is a compile-time evaluated expression. Therefore it knows nothing about the length of a string which is essentially something that needs to be evaluated at run-time.
To get the length of a string, use strlen. That returns the length of a string not including the implicit null-terminator that tells the C runtime where the end of the string is.
One other thing, it's a good habit to get into using const char* [] when setting up a string array. This reinforces the fact that it's undefined behaviour to modify any of the array contents.
So I want to know how to store an actual character into an array.
char arr[4] = "sup!";
char backwards[4];
backwards[0] = *(arr + 3);
I guess my second question is if I do this, will, if i prompt a printf of backwards[0] using %c, will the actual character appear?
First off, let's fix your array size problem:
Character arrays in C are null-terminated: This means that you need to add space for a trailing \0 to terminate your string. You could use char arr[5] on the first line. (Note that if you are ABSOLUTELY CERTAIN you are NEVER going to use this array of characters with any of the C string handling functions, AND you assigned your characters individual as chars instead of as a string, this is not technically required. But save yourself some debugging time and use up the extra byte.) This is probably the source of your "weird error."
It seems like you know, but the other thing to bear in mind is in C, arrays are zero-based. This means that when you declare and array like char arr[4], you really get
arr[0]
arr[1]
arr[2]
arr[3]
C has no qualms about letting you walk off the end of your array and stomp on data or read in bad values. Here be dragons. Be careful.
Now, on to your actual questions:
1) You assign actual characters using single quotes arr[2]='x'; If you use double quotes, you are assigning a C string, which is null-terminated, as discussed.
2) Yes, printf with %c should do the trick.