happy_1[1]_% cat kadai8-1.c
#include <stdio.h>
main(){
    char a[2000];
    int i, n;

    printf("Input letters.\n");
    gets( a );/* 改行まで文字列の入力 */

    n = 0 ;
    i = 0 ;
    while( a[i] != 0 ){
       if( a[i]=='r' ){     /* イコールは== */
          n = n + 1 ;
      }
      i = i + 1 ;
    }
    printf("number of letter 'r' in '%s' is %d\n",  a, n);
}

happy_1[2]_% gcc -o kadai8-1 kadai8-1.c -lm
happy_1[3]_% kadai8-1
Input letters.
Merry Christmas!
number of letter 'r' in 'Marry Christmas!' is 3