Thread: can u help me understand what this pogram does?

Im using Dev C++ and this program cant run....
This is an example from arrays book from C by dissection

Code:

/* Count each uppercase letters separately */

#include <stdio.h>
#include <conio.h>
#include <ctype.h>

int main(void)
{
    int c, i, letter[26];
    
    for (i = 0; i < 26;++i)
        letter[i] = 0;  //initialize i to zero yep good here
    while ((c = getchar()) != EOF) /*Input character and not equal to escape */
          if (isupper(c))    /* If it is uppercase */
             ++letter [c - 'A']; /* <<<< This is the part i do not understand i think its ASCII */
    
for (i = 0; i < 26; ++i) {
        if (i % 6 == 0) /* yeah must have 6 columns till newline */
           printf ("/n"); 
        printf ("%4c:%3d", 'A' + i, letter [i]); /* i think i get it the first part is A,B,C,D  and the second part addresses the value of letter??? */
        }
        printf ("\n\n"); 
        getchar();
        return 0;
}

What do you mean "can't run" ?

> printf ("/n");
There's a / which should be a \ perhaps.

Before the final getchar() call (you've reached EOF in the while loop remember), you need to do
clearerr(stdin);
otherwise the final getchar() will immediately return with EOF.

> ++letter [c - 'A']; /* <<<< This is the part i do not understand i think its ASCII */
Add some debug, and print the result of c - 'A' and find out.

Unless you are using a strange OS:

Code:

    while ((c = getchar()) != EOF) /*Input character and not equal to escape */

Comment and code are not matching. EOF is an indication that the input stream is at it's end. Usually produced by pressing CTRL-Z (Windows, DOS, CP/M, VAX/VMS, etc) or CTRL-D (Linux, Unix and such) when using the console for input - or indicated by the current file-pointer being equal to the number of bytes in the file if the input is a text-file in your file-system.

--
Mats

This program cant still run and i tried what you did
Only some blank screen appeared

Code:

printf ("\n\n");
        clearerr(stdin);
        getchar();
        return 0;

is this what you mean?

from here:
printf ("&#37;4c:%3d", 'A' + i, letter [i]);
the output of these (taken from the book) are:
A: 75 B: 52 C: 219 D:14 .........
what are these numbers? Idonot understand here
can u help?...

Originally Posted by EdwardElric

This program cant still run and i tried what you did
Only some blank screen appeared

It's waiting for your to type some letters (counting the upper-case ones). Finish by issuing the CTRL-key for "End of file" as I wrote in my previous post.

Code:

printf ("\n\n");
        clearerr(stdin);
        getchar();
        return 0;

Yup.

is this what you mean?

from here:
printf ("%4c:%3d", 'A' + i, letter [i]);
the output of these (taken from the book) are:
A: 75 B: 52 C: 219 D:14 .........
what are these numbers? Idonot understand here
can u help?...

What do you THINK it means, what does the program say that it does?

--
Mats

What part of "what are these numbers" is it that you have a difficulty understanding. Do you have a description of the overall program that tells you WHAT THE PURPOSE OF THE PRORGRAM is?

--
Mats

Originally Posted by EdwardElric

Counting each letter separately
and followed by the numbers stuff....

if u have the book
its on
C by dissection 4th edition page 307, lesson 9.2

No, I don't have that book - and I'm asking you the question so that YOU understand things, not because I don't understand it. If I had the book, I may recommend that you re-read a particular paragraph, but as it stands, I don't so I can't say WHERE in the book, it explains exactly what this program does.

So if the application is counting letters, what do you think the output is?

--
Mats

Originally Posted by matsp

EOF is an indication that the input stream is at it's end. Usually produced by pressing CTRL-Z (Windows, DOS, CP/M, VAX/VMS, etc) or CTRL-D (Linux, Unix and such) when using the console for input - or indicated by the current file-pointer being equal to the number of bytes in the file if the input is a text-file in your file-system.

Note that on some systems or emulators you have to press <enter> after typing EOF, or so I hear.

So, what does this do?

Code:

++letter [c - 'A'];

Let's deal with the indexing code first. c - 'A'. Have you ever seen an ASCII table or chart? http://www.ascii.cl/

An ASCII table shows how each character is represented by a number. 'A' is 65, ' ' (space) is 32, and so on. Every character is actually a number as far as the compiler is concerned. You could use 65 anywhere you could use 'A' -- but don't bother, because 'A' is a lot easier to read than 65.

If you look at the table carefully, you'll notice that the letters 'A' through 'Z' (and 'a' through 'z') appear in a sequence. The values for 'A' to 'Z' are 65 ('A'), 66 ('B'), ... to 90 for 'Z'. So if you subtract 65 ('A') from each of these numbers, you'll get 0, 1, ... 25, which can be used as array indexes. And that's exactly what your code does.

So the c - 'A' gets the index of the array; the rest of the code increments that element of the array. In effect, it adds one to letter[0] if c is 'A', to letter[1] if c is 'B', and so on.