#include <stdio.h>
 
void histogram(char *);
 
int main() {
	char str[50];
 
	printf("\n\tDigite uma frase: ");
	fgets(str,50,stdin);
	histogram(str);
 
}
 
void histogram(char *frase) {
	int num=0, num2=0, az=0;
	char alfabeto[]="abcdefghijklmnopqrstuvwxyzçABCDEFGHIJKLMNOPQRSTUVWXYZÇ";
 
	for(num = 0; frase[num] != 0; num++) {
        for (num2 = 0; alfabeto[num2] != '\0'; ++num2){
            if (frase[num] == alfabeto[num2]){
                az++;
                break;
            }
        }
	}
 
	printf("\n\n\tTemons %d letras\n\n", az);
}
 

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texto de exemplo