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C program to convert uppercase to lowercase and vice versa

In this program, you will learn how you can convert strings from uppercase to lowercase and lowercase to uppercase. In C language every character represents an integer value known as ASCII. ASCII value of small (a-z) are (97-122) and capital (A-Z) are (65-90)

small       capital
a   97      A   65
b   98      B   66
c   99      C   67
.           . 
.           .
.           .
z   122     Z   90

Convert string case program in c

#include<stdio.h>
void main()
{
 char str[20];
 int i;
 printf("\n enter a string:");
 gets(str);
 for(i=0;str[i]!='\0';i++)
 {
    //it will convert lowercase to uppercase
    if(str[i]>=97 && str[i]<=122)
    {
    	str[i]=str[i]-32;
    }
    //it will convert uppercase to lowercase
    else if(str[i]>=65 && str[i]<=90)
    {
    	str[i]=str[i]+32;
    }
 }
 printf(" %s",str);
}

output

enter a string:WELcome TO MY Website
welCOME to my wEBSITE

Convert string case using pointer

#include<stdio.h>
#include<conio.h>
int main()
{
	char a[20],*p;
	int c=0,i;
	p=a;
	printf("\nenter a string:");
	gets(a);
	for(i=0;*(p+i)!='\0';i++)
	{
	if(*(p+i)>='a' && *(p+i)<='z')   
	   *(p+i)=*(p+i)-32;
	else if(*(p+i)>='A' && *(p+i)<='Z')
	   *(p+i)=*(p+i)+32;
	}
	printf("string:%s",p);
	return 0;
}

output

enter a string:WELcome TO MY Website
welCOME to my wEBSITE

explanation of string case conversion

input string from the user, and store it some variable say str
run the loop from 0 till end of string
When you want to change from lowercase to uppercase, you have to substract the ASCII value of lowercase letters to 32.why subtracting it with 32 because diffrence of a-A=32
For example a-32=A i.e 97-32=65
When you want to change from uppercase to lowercase, you have to add the ASCII value of uppercase letters to 32.
For example A+32=a i.e 65+32=97
65 is ASCII value of capital A and 97 is ASCII value of small

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