Random letters: How to generate with Rand and how to compare them in C

Question

I’m trying to create a char vector of 10 positions, I want to feed it with 10 random letters and sort down with Bubble Sort. My question is: does Rand generate random characters as well as generate numbers, or does it have any difference? I can compare them using < and >?

#include <stdio.h>
#include <stdlib.h> 
void armazenaAleatorios(char vetor[]){
    int i;
    for(i=0;i<10;i++){
        vetor[i]=rand()%26;
        printf("%c  ",vetor[i]);
    }
}
void bubble_sort(char vetor[], int tamanho){
    int i, j;
    char aux;
    for(i=tamanho-1; i >= 1; i--) {
        for(j=0;j<i; j++){
            if(vetor[j] > vetor[j+1]){
                aux = vetor[j];
                vetor[j] = vetor[j+1];
                vetor[j+1] =  aux;
            }
        }
    }
}
void apresenta(char vetor[]){
    printf("\n\n\n\n\n");
    int i;
    for (i=0; i<10; i++){
        printf("%c  ",vetor[i]);

    }
}
int main(){ 
    //int vetor[5] = {5, 9, 10 ,50 ,1};
    srand((unsigned)time(NULL));
    char vetor[10];
    armazenaAleatorios(vetor);
    bubble_sort(vetor,10);
    apresenta(vetor);
    printf("\n\n\n\n\n");

    system("PAUSE");
}

Answer 1

Your code is working perfectly, just missing add 'a' in each vector char. And your Bubble Sort is ordering in ascending order and not decreasing. To change this just change the > to < no if. Here is the code working:

#include <stdio.h>
#include <stdlib.h> 

void armazenaAleatorios(char vetor[]){
    int i;
    for(i=0; i<10; i++)
        vetor[i]= 'a' + (char)(rand()%26);
}
void bubble_sort(char vetor[], int tamanho){
    int i, j;
    char aux;
    for(i=tamanho-1; i >= 1; i--)
        for(j=0;j<i; j++)
            if(vetor[j] < vetor[j+1]){
                aux = vetor[j];
                vetor[j] = vetor[j+1];
                vetor[j+1] =  aux;
            }
}
void apresenta(char vetor[]){
    printf("\n\n\n\n\n");
    int i;
    for (i=0; i<10; i++)
        printf("%c  ",vetor[i]);
    printf("\n\n\n\n\n");
}
int main(){ 
    srand((unsigned)time(NULL));
    char vetor[10];
    armazenaAleatorios(vetor);
    bubble_sort(vetor,10);
    apresenta(vetor);

    system("PAUSE");
}

Your program was returning crazy characters because you were writing characters from 0 to 26, which represent control characters and are not printable. The code of the letter 'a' is 97, 'b' is 98 and so on, so you just add 97 to the vector[i], that’s all right. By 'a' the compiler will already replace 97 automatically. The two lines below are equivalent:

vetor[i]= 'a' + (char)(rand()%26);
vetor[i]=  97 + (char)(rand()%26);

To generate a random char, just use the Rand() and convert the obtained int to char, this is done by placing (char) in front of what you want to convert. If you do not put it will convert automatically but it is always good to put in the code because it is clear that the conversion is happening!

Char values are numbers! A char has one byte and represents a number from -127 to 128, so comparisons can normally be made as if they were a number! Notice that if you change the printf("%c",vetor[i]) for printf("%d",vetor[i]) the %d will print the number that is stored in the char. The difference is that %c takes the number, queries the ASCII table and prints the letter that equals that number! For example if the char contains the number 97, the %c will print an 'a'. Take a look at the ASCII table to help:

https://pt.wikipedia.org/wiki/ASCII

You can even use a char inside a for:

char i;
for(i=0; i<10; i++)
    vetor[i]= 'a' + (char)(rand()%26);

This will work the same way! But if you’re going to use a char for this remember that your limit is 128 ! Most compilers use char with values between -127 and 128, but in some the value is 0 to 255, because the default does not define exactly which to use. If using char as the best number specify unsigned char.

I hope I’ve helped :)