C Program to convert lowercase char to uppercase of string

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In this tutorial we are going to learn how to write a program in C to convert a lowercase string to uppercase.

There are multiple ways to write the program in C for the above problem. Here we will see 3 ways to do this.

In this tutorial we will learn writing the C program to convert all lowercase character to uppercase character of String. To perform this task first we will identify the lowercase character. If we find the lowercase then convert this lowercase to uppercase.

1) Without using string manipulation library functions

Here we will use the ASCII value of the character to convert lower case value to upper case value. The ASCII value of a is 97 and z is 122.

As we all know that ASCII value of a is 97. Now when we subtract 32 it will become 65 which the ASCII value of A.
Using above formula we can find upper character ASCII value.

Below is the example

ASCII value of b is 98.

Now to calculate ASCII value of B we will do

98 – 32 = 66 (It is a ASCII value of B)

Similarly for
c = 99 – 32 = 67 = C
d = 100 – 32 = 68 = D etc.

#include<stdio.h>
int main()
{
    char str[100];
    int i = 0;
    printf("Convert lower case into upper case Char using C \n");
    printf("Please enter a string: ");
    fgets(str, 100, stdin);
    while( str[i] != '\0' )
    {
        if( str[i] >= 'a' && str[i] <= 'z' )
        {
            str[i] = str[i] - 32;
        }
        i++;
    }
    printf("String after converting into upper case : \n%s", str);
    return 0;
}

Output

Convert lower case into upper case Char using C 
Please enter a string: Quescol Website
String after converting into upper case : 
QUESCOL WEBSITE

2) Using strupr() function

The strupr() is the library function available at string.h header file which converts lower case string into upper case string.

#include<stdio.h>
#include<string.h>
int main()
{
    char str[100];
    printf("Convert lower case into upper case Char using C \n");
    printf("Please enter a string: ");
    fgets(str, 100, stdin);
    strupr(str);
    printf("String after converting into uppercase = %s",str);
    return 0;

}

Output

Convert lower case into upper case Char using C 
Please enter a string: Quescol Website
String after converting into upper case : 
QUESCOL WEBSITE

3) Using the pre-defined function toupper()

toupper() function is predefined function which is available in ctype.h library. We can convert uppercase character of any string to lowercase character using this function. toupper() function only converts lowercase characters to uppercase characters without other characters.

#include<stdio.h>
#include<ctype.h>
int main()
{
   char str[100];
   printf("Convert lower case into upper case Char using C \n");
   printf("Please enter a String: ");
   fgets(str, 100, stdin);
   for(int i=0; str[i]!='\0'; i++)
   {
      str[i] = toupper(str[i]);
   }
   printf("String after converting into uppercase: %s \n", str);
   return 0;
}

Output

Convert lower case into upper case Char using C 
Please enter a String: Quescol Education
String after converting into uppercase: QUESCOL EDUCATION

Difference between strupr() and toupper()

  • strupr() function converts all lowercase characters of a string to the uppercase character at a time.
  • But toupper() function converts character by character. It works on one character at a time and convert it into uppercase if it is in lowercase.

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