C program to Convert String to Uppercase

How to write a C Program to Convert String to Uppercase without using strupr function. We can achieve this in multiple way, but we will discuss four different approaches: using For Loop, While Loop, Functions, and ASCII Values

C program to Convert String to Uppercase without using strupr()

This program allows the user to enter any string or character array. Next, it will use For Loop to iterate each and every character in that string, and convert them to Uppercase.

/* C program to Convert String to Uppercase without using strupr() */

#include <stdio.h>
#include <string.h>
 
int main()
{
  	char Str1[100];
  	int i;
 
  	printf("\n Please Enter a String that you want to Convert into Uppercase :  ");
  	gets(Str1);
  	
  	for (i = 0; Str1[i]!='\0'; i++)
  	{
  		if(Str1[i] >= 'a' && Str1[i] <= 'z')
  		{
  			Str1[i] = Str1[i] -32;
		}
  	}

  	printf("\n The given String in Upper Case = %s", Str1);
  	
  	return 0;
}

OUTPUT

ANALYSIS
We declared a character arrays of size 100. Below statements asks the user to enter any word, and assign the user entered string to Str variable.

printf("\n Please Enter any String :  ");
gets(Str);

In the next line we have For loop to iterate and convert string to uppercase,

for (i = 0; Str1[i]!='\0'; i++)
{
	if(Str1[i] >= 'a' && Str1[i] <= 'z')
	{
		Str1[i] = Str1[i] -32;
	}
}

User Entered the String “WorLd”

First Iteration: for (i = 0; Str[0]!=’\0′; 0++)
(Str[0] != ‘\0’) condition is TRUE because Str[0] = W
if(Str1[i] >= ‘a’ && Str1[i] <= ‘z’) – Condition is False

Second Iteration: for (i = 1; Str[1]!=’\0′; 1++)
(Str[1] != ‘\0’) condition is TRUE because Str[1] = o
if(Str1[i] >= ‘a’ && Str1[i] <= ‘z’) – Condition is True
Str1[i] = Str1[i] – 32 => 111 – 32
Str1[i] = 79
79 is the ASCII code for capital O

Do the same for remaining iterations

C program to Convert String to Uppercase using For Loop

This program is same as above but this time we are using ASCII values inside the If Statement. I suggest you to refer ASCII Table to understand the codes

/* C program to Convert String to Uppercase without using strupr() */

#include <stdio.h>
#include <string.h>
 
int main()
{
  	char Str1[100];
  	int i;
 
  	printf("\n Please Enter a String that you want to Convert into Uppercase :  ");
  	gets(Str1);
  	
  	for (i = 0; Str1[i]!='\0'; i++)
  	{
  		if(Str1[i] >= 97 && Str1[i] <= 122)
  		{
  			Str1[i] = Str1[i] -32;
		}
  	}

  	printf("\n The given String in Upper Case = %s", Str1);
  	
  	return 0;
}

OUTPUT

C program to Convert String to Uppercase Using While Loop

This program is same as above but this time we are using While loop (Just replacing For Loop with While Loop). I suggest you to refer While Loop to understand the loop Iteration.

/* C program to Convert String to Uppercase without using strupr() */

#include <stdio.h> 

int main()
{
	char str[100];
	int i;
 
	printf("\n Please Enter a String to convert it into Uppercase :  ");
	gets(str);
 
 	i = 0;
	while (str[i] != '\0') 
	{
    	if (str[i] >= 'a' && str[i] <= 'z') 
		{
        	str[i] = str[i] - 32;
    	}
      	i++;
	}
 
	printf("\n The given String in Upper Case = %s", str);
 
	return 0;
}

OUTPUT

Program to Convert String to Uppercase Using Functions

This program is same as above but this time we are using Functions concept to separate the logic from main program.

/* C program to Convert String to Uppercase without using strupr() */

#include <stdio.h> 

void String_Upper(char []);
 
int main()
{
	char str[100];
 
	printf("\n Please Enter a string to convert it into Uppercase : ");
	gets(str);
 
	String_Upper(str);
 
	printf("\n The given String in Upper Case = %s", str);
 
	return 0;
}
 
void String_Upper(char string[]) 
{
	int i = 0;
 
	while (string[i] != '\0') 
	{
    	if (string[i] >= 'a' && string[i] <= 'z') 
		{
        	string[i] = string[i] - 32;
    	}
      	i++;
	}
}

OUTPUT