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Wednesday, September 14, 2011

Some details about pointers and strings in C...

There doesn’t exist anything in C like real strings. A string is an array. It can be defined as an array of characters. A string is terminated by a nul character “\0”. A string can be initialized normally as follows:

Char string[10];
String[0] = ‘c’;
String[1] = ‘a’;
String[2] = ‘t’;
String[3] = ‘s’;

Another way of initializing and declaring a string is as follows:
Char string[10] = “cats”;

The following statement asks compiler to allocate 10 bytes of memory for the string entered by the user. The string can store only 9 characters and the tenth one being used for “nul” character.

We can easily access strings and their constituent characters using character pointers. The string arrays are mostly used for storing char data type elements in the memory. The array of string pointers is preferred over two dimensional arrays of characters mainly for two reasons. The first reason is by implementing array of pointers, you can make more efficient use of available memory since it consumes lesser number of bytes for storing arrays of strings. The second main reason is that an array of string pointers makes the manipulation of strings easier. You can easily exchange the positions of strings in the array using pointers without touching their memory locations. This makes managing strings very convenient. Another way of initializing a string is by using pointers like

for example:
Char string[ ] = “cats”;
Char *a;

Here is a pointer pointing to the string “string”. Pointers are another suitable way to access string arrays and modify them as we want. Since strings are composed of characters, we should use pointers to characters. The types of the variable and the pointers must match otherwise an error will be generated. Be careful that you don’t use “&” (address of) operator to assign the address of a variable to the desired pointer. If you used it, the name of the array will be used as the address of that particular array. For this reason only we do not use “&” (address of) while passing a string to the function scanf(). For example see the following code:

Int a;
Char name[10];
Scanf(“%d%s”, &a, name);

The pointer allows us to access the required character in the string because we made it to point to that string. Functions can be passed to print labels and to call the function. Below is an example:

Void printlbl(char lbl[ ])
Printf (“the label: %s\n”, lbl);
int main()
char lbl2[] = "abc";



Now if we want to pass a pointer to the string array lbl2, we will use the following code:
Char *lblptr = lbl2;

The above declared pointer can be used in the function also like shown below:
Printlbl (lblptr) ;

After execution you will observe that both the statements give the same output. Now you will ask me why so? The answer for this is that when we pass an array as a parameter or an argument to a function, a pointer is automatically created without we knowing it and the arrays are passed by reference method automatically the way a pointer is passed. So the above written code can also be written the other way as follows:

void Printlbl(char *lbl)
printf("the Label: %s\n",lbl);
a code for dynamically allocated string is given below:
string = (char *)malloc(sizeof(char) * (len+1));

Some stings can be allocated dynamically. This is done when we don’t know how much long a string will go until the program is executed or when we don’t what size of string the user will enter.

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