My Code Does Not Work? Why?

Hi folks,

I have compiled and executed the code listed below, only to find that it crashed my MS-DOS prompt.

I would greatly appreciate any help from someone who knows where the error is. It comiles fine, with no errors or warning.

Code:

---

/* interface.c: The include functions designed to control the I/O of the PowerMath System */
#include <stdio.h>

mathInput()
{
    int x;
    int input;
    char progInput[x];
   
    do
    {
        x=1;
        progInput[x]=getc( stdin );
        progInput[x]=input;
        x=x+1;
    } while (input!=10);
         
}

int main()
{
      mathInput();
     
      return 0;
}

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And what value is x in the red section:

Code:

---

    int x;
    int input;
     char progInput[x];


---

--
Mats

I would like x really to be any number that is wanted. If this is not possible, then do I have to set a limit on the matrix size.

Since this is C, rather than C++, we have to give an array a known size. You can ask the user if you like, but you HAVE to have a constant value for array sizing. Nothing else works. [And for local variables, you must stay within a few megabytes or less, due to limited stack-size].

Edit: Alternatively, you could use dynamic memory allocation and use realloc.

--
Mats

x's value is undefined when first defined. Usually it's some BIG number, which means you exhaust the size of the stack and crash your app.
You need to use dynamic memory or alternatively, input x first THEN create your array.

Quote:

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Originally Posted by matsp

Since this is C, rather than C++, we have to give an array a known size. You can ask the user if you like, but you HAVE to have a constant value for array sizing. Nothing else works. [And for local variables, you must stay within a few megabytes or less, due to limited stack-size].

Edit: Alternatively, you could use dynamic memory allocation and use realloc.

--
Mats

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Why have I been able to do this on most compilers I have used? Eg.

Code:

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int x = 123;
char array[x];

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Non-standards compliant? Talking MS and Mingw - or perhaps C++?

This is standard in C99. Visual Studio doesn't support it. It's not standard in C90 or C++.

Quote:

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Originally Posted by IceDane

Why have I been able to do this on most compilers I have used? Eg.

Code:

---

int x = 123;
char array[x];

---

Non-standards compliant? Talking MS and Mingw - or perhaps C++?

---

It is C99 (which isn't fully supported by MS, but nearly complete in gcc). But it is not valid to change x later on and expect the array to change size.

But my objection is not that it's a run-time sized array, but that the original post doesn't actually set x until inside the do-while loop.

By the way, there are several other problems in the code:
1) the variable input is never set to anything.
2) x is always set to 1, then changed to 2 and set back to 1 in the beginning of the loop.

--
Mats

Quote:

---

Originally Posted by matsp

It is C99 (which isn't fully supported by MS, but nearly complete in gcc). But it is not valid to change x later on and expect the array to change size.

But my objection is not that it's a run-time sized array, but that the original post doesn't actually set x until inside the do-while loop.

By the way, there are several other problems in the code:
1) the variable input is never set to anything.
2) x is always set to 1, then changed to 2 and set back to 1 in the beginning of the loop.

--
Mats

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Ah yeah - now that I think of it, MS cries havoc if I try to declare variables in the midst of my code.

But as to expecting the arrays to change size as x changes - no, that isn't valid, and I didn't think it was. It is a common error with newbies, however. E.g.:

Code:

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int x, y;

x = 3 * y;

y = 4;

// Now they think x should be 3 * 4 = 12.

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I guess that stems from math, or something.