Printing vertical histogram - c

Question

I am trying to print a vertical histogram that counts the frequency of each digit input by the user.
I store the frequency first as follows:
int a[10]; //array
int c; //store input from getchar
int i; //loop variable
for(i=0;i<10;i++)
{
a[i]=0; //initialize to 0
}
while((c=getchar())!=EOF) //read character
{
++a[c-'0'];
}
Next, I try to draw a vertical histogram.
for(i=10;i>0;i--) //asssumed max limit of frequency is 10
{
int j; //iterate through the array
for(j=0;j<10;j++)
{
if(a[j]==i) //if frequency of any element in array matches i
{
printf("* \t");
--a[j]; //decrement array element frequency value
}
else
printf(" \t"); //no action
}
printf("\n"); //next line
}
The problem is when I try to print the histogram, I get a blank screen. I have tested it with horizontal histogram and it works.

Answer 1

Taking exactly your code, formatting it for readability, and adding a check to ensure that only digits are used to avoid breaking things further, it worked. The check was not necessary for functionality, but it's a good idea to avoid writing out of the bounds of the array.
#include <stdio.h>
#include <ctype.h>
int main() {
int a[10]; //array
int c; //store input from getchar
int i; //loop variable
for(i=0;i<10;i++)
{
a[i]=0; //initialize to 0
}
while((c=getchar())!=EOF) //read character
{
if (isdigit(c))
++a[c - '0'];
}
for(i=10;i>0;i--) //asssumed max limit of frequency is 10
{
int j; //iterate through the array
for(j=0;j<10;j++)
{
if(a[j]==i) //if frequency of any element in array matches i
{
printf("* \t");
--a[j]; //decrement array element frequency value
}
else
printf(" \t"); //no action
}
printf("\n"); //next line
}
}
Result:
$ ./a
0123456789666338592
*
* *
* * * * * *
* * * * * * * * * *
For reference, this was compiled for Cygwin.
I've also modified your source code in a few ways to correspond with some of the practices that I follow:
#include <stdio.h>
#include <ctype.h>
int main()
{
int a[10] = {0}; // While use a for to initialize to 0 when you can do it easily?
int i;
char c;
while((c = getchar()) != EOF)
{
if (isdigit(c)) // Never trust the user
a[c - '0']++; // Changed for readability, make it clear that we are incrementing the value, not the pointer.
}
for(i = 10; i > 0; --i)
{
int j;
for(j = 0; j < 10; ++j)
{
// Made flow a little clearer
if(a[j] >= i)
putchar('*');
else
putchar(' ');
putchar('\t');
}
putchar('\n');
}
}
What you do is of course up to you, I just personally find this more consistent.