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Here, we will write a C program to find the occurrence of a character in a string. Before that, you may go through the following topic in C.

• Strings in C

We will look at two examples to calculate the frequency of characters in a string.

1. For a particular character.
2. For all the character in a string

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C Program to Find the Frequency of Characters in a String (for a particular character)

Both the string and the character whose frequency needed to be found are entered by the user.

The questions: write a program in C to find the frequency of any given character in a given string

#include <stdio.h>

int main()
{
  char str[100], ch;
  int count = 0, i;

  printf("Enter the string: ");
  fgets(str, sizeof(str), stdin);

  printf("Enter a character whose frequency is to be found: ");
  scanf("%c", &ch);

  for (i = 0; str[i] != '\0'; ++i)
  {
    if (ch == str[i])
      ++count;
  }

  printf("\nFrequency of %c = %d", ch, count);
  return 0;
}

Output:

Enter the string: This is simple2code.com
Enter a character whose frequency is to be found: i

Frequency of i = 3

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C Program to Find the Frequency of Characters in a String (for all the characters)

The user needs to enter the string and the number of occurrences for each character will be displayed.

#include <stdio.h>
#include <string.h>

int main()
{
  char str[100], ch;
  int i, j, k, len, count = 0;

  printf("Enter the string: ");
  scanf("%s", str);

  len = strlen(str);

  for (i = 0; i < len; i++)
  {
    ch = str[i];
    for (j = 0; j < len; j++)
    {
      if (ch == str[j])
      {
        count++;
        for (k = j; k < (len - 1); k++)
          str[k] = str[k + 1];

        len--;
        str[len] = '\0';
        j--;
      }
    }

    printf("Frequency of %c = %d\n", ch, count);
    count = 0;
    i--;
  }

  return 0;
}

Output:

Enter the string: simple2code.com
Frequency of s = 1
Frequency of i = 1
Frequency of m = 2
Frequency of p = 1
Frequency of l = 1
Frequency of e = 2
Frequency of 2 = 1
Frequency of c = 2
Frequency of o = 2
Frequency of d = 1
Frequency of . = 1

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In this tutorial, we will learn to write a C program to convert uppercase to lowercase and vice versa. Before that, you may go through the following topic in C.

• String in C

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C Program to Convert Uppercase String to Lowercase String

Explanation: The program asks the user to enter a string. The string can be fully uppercase or partially uppercase. The program converts that string to a fully uppercase string.

We have used for loop to iterate through the string and also, we know that a is equal to 97 and A is equal to 65 (97-32).

#include <stdio.h>
#include <string.h>

int main()
{
  char str[25];
  int i;
  printf("Enter the string: ");
  scanf("%s", str);

  for (i = 0; i <= strlen(str); i++)
  {
    if (str[i] >= 65 && str[i] <= 90)
      str[i] = str[i] + 32;
  }

  printf("Conversion to Lower Case String: %s", str);
  return 0;
}

Output:

Enter the string: sIMpLe2CoDE
Conversion to Lower Case String: simple2code

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C Program to Convert Lowercase String to Uppercase String

Explanation: The program asks the user to enter a string. The string can be fully lowercase or partially lowercase. The program converts that string to a fully lowercase string.

We have used for loop to iterate through the string and also, we know that A is equal to 65 and a is equal to 97 (65+32).

#include <stdio.h>
#include <string.h>

int main()
{
  char str[25];
  int i;
  printf("Enter the string: ");
  scanf("%s", str);

  for (i = 0; i <= strlen(str); i++)
  {
    if (str[i] >= 97 && str[i] <= 122)
      str[i] = str[i] - 32;
  }

  printf("Conversion to uppercase String: %s", str);
  return 0;
}

Output:

Enter the string: simPle2Code.com
Conversion to Uppercase String: SIMPLE2CODE.COM

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