Language: C

#include<stdlib.h>
int main(){
  int i,count[26]={0};
  char now;

  scanf("%c",&now);
//  printf("%c\n",now);

  while( scanf("%c",&now)!=EOF){
//    printf("flag %c\n",now);
    if(now=='a'|| now=='A')count[0]++;
    else if(now=='b'|| now=='B')count[1]++;
    else if(now=='c'|| now=='C')count[2]++;
    else if(now=='d'|| now=='D')count[3]++;
    else if(now=='e'|| now=='E')count[4]++;
    else if(now=='f'|| now=='F')count[5]++;
    else if(now=='g'|| now=='G')count[6]++;
    else if(now=='h'||now=='H')count[7]++;
    else if(now=='i'||now=='I')count[8]++;
    else if(now=='j'||now=='J')count[9]++;
    else if(now=='k'||now=='K')count[10]++;
    else if(now=='l'||now=='L')count[11]++;
    else if(now=='m'||now=='M')count[12]++;
    else if(now=='n'||now=='N')count[13]++;
    else if(now=='o'||now=='O')count[14]++;
    else if(now=='p'||now=='P')count[15]++;
    else if(now=='q'||now=='Q')count[16]++;
    else if(now=='r'||now=='R')count[17]++;
    else if(now=='s'||now=='S')count[18]++;
    else if(now=='t'||now=='T')count[19]++;
    else if(now=='u'||now=='U')count[20]++;
    else if(now=='v'||now=='V')count[21]++;
    else if(now=='w'||now=='W')count[22]++;
    else if(now=='x'||now=='X')count[23]++;
    else if(now=='y'||now=='Y')count[24]++;
    else if(now=='z'||now=='Z')count[25]++;
//    printf("flag2 %c\n",now);
//    printf("flag3 %c\n",now);
  }

  printf("a : %d\n",count[0]);
  printf("b : %d\n",count[1]);
  printf("c : %d\n",count[2]);
  printf("d : %d\n",count[3]);
  printf("e : %d\n",count[4]);
  printf("f : %d\n",count[5]);
  printf("g : %d\n",count[6]);
  printf("h : %d\n",count[7]);
  printf("i : %d\n",count[8]);
  printf("j : %d\n",count[9]);
  printf("k : %d\n",count[10]);
  printf("l : %d\n",count[11]);
  printf("m : %d\n",count[12]);
  printf("n : %d\n",count[13]);
  printf("o : %d\n",count[14]);
  printf("p : %d\n",count[15]);
  printf("q : %d\n",count[16]);
  printf("r : %d\n",count[17]);
  printf("s : %d\n",count[18]);
  printf("t : %d\n",count[19]);
  printf("u : %d\n",count[20]);
  printf("v : %d\n",count[21]);
  printf("w : %d\n",count[22]);
  printf("x : %d\n",count[23]);
  printf("y : %d\n",count[24]);
  printf("z : %d\n",count[25]);


  return 0;
}

Compiler output:
main.c: In function 'main':
main.c:6:3: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
    6 |   scanf("%c",&now);
      |   ^~~~~
main.c:2:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
    1 | #include<stdlib.h>
  +++ |+#include <stdio.h>
    2 | int main(){
main.c:6:3: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
    6 |   scanf("%c",&now);
      |   ^~~~~
main.c:6:3: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:9:28: error: 'EOF' undeclared (first use in this function)
    9 |   while( scanf("%c",&now)!=EOF){
      |                            ^~~
main.c:9:28: note: 'EOF' is defined in header '<stdio.h>'; this is probably fixable by adding '#include <stdio.h>'
main.c:9:28: note: each undeclared identifier is reported only once for each function it appears in
main.c:41:3: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   41 |   printf("a : %d\n",count[0]);
      |   ^~~~~~
main.c:41:3: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:41:3: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:41:3: note: include '<stdio.h>' or provide a declaration of 'printf'