{"post":"

Converting a string into either from lowercase to uppercase or from uppercase to lowercase can be done in two ways i.e with pre-defined C functions and without them.

First let us see the simpler approach of using pre-defined C functions.

Approach 1: Using the functions strlwr() and strupr() from string.h:

strlwr() converts the input string into lowercase and strupr() converts the string to uppercase. Both are a part of the string.h library.
This is again a not recommended approach as strlwr and strupr are from Microsoft C library and does not work in standard C library. If you are using a Turbo C++ IDE, only then will the below approach work. The GNU library has no such function defined and will throw you an error.
Here to scan the string we are using fgets method in order to understand how it is used, although it is not recommended.

Code:

#include <stdio.h>\n#include<conio.h>
\n#include <string.h> //include the header file which contains definition of strlwr and strupr
\nfunctions.
\nint main()
\n{
\nclrscr();
\nchar string[100] ;
\nprintf(\"Enter a string : \");
\nfgets(string, 100, stdin);     //read string input from standard input screen.
\nprintf(\"The string in lower case :\\n\", strlwr(string));
\nprintf(\"The string in upper case :\\n\", strupr(string));
\ngetch();
\nreturn  0;
\n}

Output:

Input a string: Hello! Howdy! HII\nThe string in lowercase is as follows: hello! howdy! hii \n

The string in uppercase is as follows: HELLO! HOWDY! HII

Approach 2: Using the functions tolower() and toupper():

Since, the above code is not compatible with standard C library; we have an alternate approach.
The ctype.h library includes function tolower() – to convert string to lower case and toupper() – to convert string to uppercase.
The ctype.h header file of the C Standard Library declares several functions that are useful for testing and mapping characters.
The difference here is that functions included under ctype.h , work on integer values.
Hence, while case conversion the function will consider the ASCII value of the character in order to convert it to the desired case.

Declaration:Following is the declaration for tolower() and toupper() functions respectively.

int  tolower(int c);\nint  toupper(int c);

Here c is the letter to be converted to lowercase /uppercase; This function returns lowercase / uppercase equivalent to c, if such value exists, else c remains unchanged. The value is returned as an int value that can be implicitly casted to char.

Code: We keep converting character at each index into lower / uppercase, continuously checking for string end in every iteration.

#include <stdio.h>\n#include <ctype.h>
\nint main ()
\n{
\nint i=0, size = 100;
\nchar *string;
\nprintf(\"Input a string: \");
\n/* These 2 lines are very important. */
\nstring = (char *) malloc (size);
\ngetline (&string, &size, stdin);
\nwhile (string[i])                          //checking for null character
\n{
\nstring[i] = tolower(string[i]);   //converting to lowercase
\ni++;
\n}
\nprintf(\"\\nThe string in lowercase is as follows: \");
\nputs(string);
\ni = 0;                                     //initializing index
\nwhile (string[i])                     //checking for null character
\n{
\nstring[i] = toupper(string[i]);   //converting to lowercase
\ni++;
\n}
\nprintf(\"\\nThe string in uppercase is as follows: \");
\nputs(string);
\nreturn 0;
\n}

Output:

Input a string: Hello! Howdy! HII\nThe string in lowercase is as follows: hello! howdy! hii \n

The string in uppercase is as follows: HELLO! HOWDY! HII

Approach 3: Without using pre-defined functions of C:

In this approach we will create two user defined functions upper and lower to convert case of characters in the string.
Remember that strings are nothing but character arrays and have the same properties for function call and return as previously discussed while learning arrays.
We know that inn computer memory the ASCII codes of the characters are actually stored and understood.
The ASCII code for A-Z varies from 65 to 91 and for a-z varies from 97 to 123.
So in order to convert a character to lowercase we would have to add 32 i.e 97-65 = 32; the difference between both cases to the character value.
In order to convert string's character to upper case we subtract 32 from character.

Code:

#include <stdio.h>\nvoid lower_string(char []);
\nvoid upper_string(char []);
\nint main()
\n{
\nint size = 100;
\nchar *string;
\nprintf(\"Enter some text of your choice: \");
\nstring = (char *) malloc (size);
\ngetline (&string, &size, stdin);
\nlower_string(string);   //function call to convert to lowercase
\nupper_string(string);   //function call to convert to uppercase
\nreturn 0;
\n}
\nvoid lower_string(char str[])
\n{
\nint i = 0;
\nwhile (str[i] != '\\0') {
\nif (str[i] >= 'A' && str[i] <= 'Z')    //checking for uppercase characters
\n{
\nstr[i] = str[i] + 32;               //converting uppercase to lowercase
\n}
\ni++;
\n}
\nprintf(\"The string in lower case: %s\\n\", str);
\n}
\nvoid upper_string(char str[])
\n{
\nint i = 0;
\nwhile (str[i] != '\\0')
\n{
\nif (str[i] >= 'a' && str[i] <= 'z')   //checking for lowercase characters
\n{
\nstr[i] = str[i] - 32;              //converting to uppercase from lowercase
\n}
\ni++;
\n}
\nprintf(\"The string in upper case: %s\\n\", str);
\n}

Output:

Enter some text of your choice: Where have YOU been ?\nThe string in lower case: where have you been ? \n

The string in upper case: WHERE HAVE YOU BEEN ?

Thus, we have seen various methods to perform case change operation. We also saw use of fgets and getline. Henceforth getline / getdelim will be used in all string programs. Also, key takeaway is always write a code that is compatible on all platforms. Approach 2 and 3 are examples for the same. Approach 1 is restricted to Microsoft C library.

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