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>

#include<conio.h>

#include <string.h> //include the header file which contains definition of strlwr and strupr

functions.

int main()

{

clrscr();

char string[100] ;



printf("Enter a string : ");

fgets(string, 100, stdin);     //read string input from standard input screen.



printf("The string in lower case :\n", strlwr(string));

printf("The string in upper case :\n", strupr(string));

getch();



return  0;

}

Output:

Input a string: Hello! Howdy! HII

The string in lowercase is as follows: hello! howdy! hii


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);

int  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>

#include <ctype.h>

int main ()

{

int i=0, size = 100;

char *string;



printf("Input a string: ");

/* These 2 lines are very important. */

string = (char *) malloc (size);

getline (&string, &size, stdin);



while (string[i])                          //checking for null character

{

string[i] = tolower(string[i]);   //converting to lowercase

i++;

}



printf("\nThe string in lowercase is as follows: ");

puts(string);



i = 0;                                     //initializing index

while (string[i])                     //checking for null character

{

string[i] = toupper(string[i]);   //converting to lowercase

i++;

}



printf("\nThe string in uppercase is as follows: ");

puts(string);



return 0;

}

Output:

Input a string: Hello! Howdy! HII

The string in lowercase is as follows: hello! howdy! hii

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>

void lower_string(char []);

void upper_string(char []);



int main()

{

int size = 100;

char *string;



printf("Enter some text of your choice: ");



string = (char *) malloc (size);

getline (&string, &size, stdin);



lower_string(string);   //function call to convert to lowercase

upper_string(string);   //function call to convert to uppercase



return 0;

}



void lower_string(char str[])

{

int i = 0;



while (str[i] != '\0') {

if (str[i] >= 'A' && str[i] <= 'Z')    //checking for uppercase characters

{

str[i] = str[i] + 32;               //converting uppercase to lowercase

}

i++;

}



printf("The string in lower case: %s\n", str);

}



void upper_string(char str[])

{

int i = 0;



while (str[i] != '\0')

{

if (str[i] >= 'a' && str[i] <= 'z')   //checking for lowercase characters

{

str[i] = str[i] - 32;              //converting to uppercase from lowercase

}

i++;

}



printf("The string in upper case: %s\n", str);

}

Output:

Enter some text of your choice: Where have YOU been ?

The string in lower case: where have you been ?


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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