Posts Tagged ‘ va_list ’

SQL Aggregates in C


For fun, I would like to implement some of the SQL aggregates in C: SUM (sum), AVG (average), MIN (minimum), MAX (maximum), STDEV (standard deviation), VAR (variance).

To do this in C, I’ll use functions with variable number of arguments instead of a function with an array and a count of the elements in that array. To do this, I need to have a special value for the last parameter in the variable list of arguments to know when to stop getting arguments in the list. For that, I’m going to use the INT_MIN macro in the <climits> C library. For example, to get the average of the numbers 6, 3, and 19, the call would look like AVG(6, 3, 19, INT_MIN).

SUM

Implementing this in C is lame, but I’ll use it as the starting point before getting to the other functions. I would definitely prefer to write 1 + 2 + 3 in C instead of SUM(1, 2, 3, INT_MIN).

Definition

int sum (int first, ...)
{
    register int i = first;
    int sum = 0;
   
    //  declare a variable argument list
    va_list arg_list;
   
    //  initialize the arglist
    va_start (arg_list, first);
   
    //  Sum all arguments
    while(i != INT_MIN)
    {
        sum += i;
        i = va_arg (arg_list, int);
    }
   
    //  End the argument list
    va_end(arg_list);
   
    return sum;
}

Usage

printf ("Sum of (1, 2, 3, 4, 5) = %d\n", sum (1, 2, 3 , 4, 5, INT_MIN));

Output

15

AVG

Instead of writing something like (1+2+3+4)/4 or (4 + 3 + 6 + 1 + 5 + 11 + 19 + 25 + 111 + 55 + 32 + 57)/ 12, I’d rather write AVG(1, 2, 3, 4, INT_MIN) or AVG(4, 3, 6, 1, 5, 11, 19, 25, 111, 55 , 32, 57, INT_MIN) without having to count the number of numbers I’m calculating the average for.

Definition

double avg (int first, ...)
{
    register int i = first;
    int sum = 0, count = 0;
   
    va_list arg_list;
    va_start (arg_list, first);
   
    //  sum and count all arguments
    while(i != INT_MIN)
    {
        sum += i;
        count++;
        i = va_arg (arg_list, int);
    }
   
    va_end(arg_list);
   
    return (double) sum / count;
}

Usage

printf ("Average of (1, 2, 3, 4, 5) = %.2f\n", avg (1, 2, 3 , 4, 5, INT_MIN));

Output

3.00

MIN

Instead of implementing MIN by passing an array and the count of numbers in that array , I’d rather just pass all the numbers to the function without having to worry about creating an array and counting the number of elements inside it.

Definition

int min (int first, ...)
{
    register int i = first;
    int min = first;
   
    va_list arg_list;
    va_start(arg_list, first);
   
    //  Search for any other argument that is less than the first
    //  If you find one, then assign it to min
    while(i != INT_MIN)
    {
        if(i < min)
            min = i;

        i = va_arg(arg_list, int);
    }
   
    va_end(arg_list);
   
    return min;
}

Usage

printf ("Minimum of (10, 23, 3, 41, 15) = %d\n", min (10, 23, 3, 41, 15, INT_MIN));

Output

3

MAX

Gets the maximum number in a set.

Definition

int max (int first, ...)
{
    register int i = first;
    int max = first;
   
    va_list arg_list;
    va_start(arg_list, first);
   
    //  Search for any other argument that is greater than the first
    //  If you find one, then assign it to max
    while(i != INT_MIN)
    {
        if(i > max)
            max = i;

        i = va_arg(arg_list, int);
    }
   
    va_end(arg_list);
   
    return max;
}

Usage

printf ("Maximum of (10, 23, 3, 41, 15) = %d\n", max (10, 23, 3, 41, 15, INT_MIN));

Output

41

STDEV

Calculates the standard deviation for a set of numbers.

Definition

double stdev (int first, ...)
{
    register int i = first;
    int count = 0;
    int sum = 0;
    double mean = 0;
    double deviation = 0;
    va_list arg_list;

    //  Calculate the mean (average)
    va_start(arg_list, first);
   
    while(i != INT_MIN)
    {
        sum += i;
        count++;
        i = va_arg (arg_list, int);
    }
   
    va_end(arg_list);
    mean = (float) sum / count;

    //  Calculate the standard deviation
    i = first;
    va_start(arg_list, first);
   
    while(i != INT_MIN)
    {
        deviation += pow((i - mean), 2);
        i = va_arg (arg_list, int);
    }

    deviation /= count;
    deviation = sqrt(deviation);

    va_end(arg_list);
   
    return deviation;
}

Usage

printf ("Standard Deviation of (2, 4, 4, 4, 5, 5, 7, 9) = %.2f\n", stdev (2, 4, 4, 4, 5, 5, 7, 9, INT_MIN));

Output

2.00

VAR

Calculates the variance for a set of numbers.

Definition

double var (int first, ...)
{
    register int i = first;
    int count = 0;
    int sum = 0;
    double mean = 0;
    double variance = 0;
    va_list arg_list;

    //  Calculate the mean (average)
    va_start(arg_list, first);
   
    while(i != INT_MIN)
    {
        sum += i;
        count++;
        i = va_arg (arg_list, int);
    }
   
    va_end(arg_list);
    mean = (float) sum / count;

    //  Calculate the variance
    i = first;
    va_start(arg_list, first);
   
    while(i != INT_MIN)
    {
        variance += pow((i - mean), 2);
        i = va_arg (arg_list, int);
    }

    variance /= (count - 1);

    va_end(arg_list);
   
    return variance;
}

Usage

printf ("Variance of (1, 2, 3, 4, 5, 6) = %.2f\n", var (1, 2, 3, 4, 5, 6, INT_MIN));

Output

3.50


Full source code is available here.

OpenGL String (printw)


There are many ways for displaying text in OpenGL. In this post, I’ll use the glutBitmapCharacter function to draw characters, and then will extend it to a more generic function (printw), which will make drawing a string in OpenGL as simple as using printf.

First, I will start by defining the font to be used by the glutBitmapCharacter function. I will choose a 24-point proportional spaced Times Roman font as shown below:

//  A pointer to a font style..
//  Fonts supported by GLUT are: GLUT_BITMAP_8_BY_13,
//  GLUT_BITMAP_9_BY_15, GLUT_BITMAP_TIMES_ROMAN_10,
//  GLUT_BITMAP_TIMES_ROMAN_24, GLUT_BITMAP_HELVETICA_10,
//  GLUT_BITMAP_HELVETICA_12, and GLUT_BITMAP_HELVETICA_18.
GLvoid *font_style = GLUT_BITMAP_TIMES_ROMAN_24;

To draw a character, simply define the raster position using glRasterPos3f (x, y, z), then draw it using glutBitmapCharacter(font_style, character). To extend this to drawing a string at certain coordinates, loop over each character in the string and draw it with glutBitmapCharacter. The additional logic that goes below is in making this as pretty as printf.

This can be done by using C’s va_list type, which is C’s approach to defining functions with variables number of arguments.

  • In the function prototype, place ellipsis (…) as the last argument
  • Define a variable argument list: va_list args;
  • Call va_start on the args list and the first real argument in the function prototype preceding the ellipsis: va_start(args, format);
  • Use _vscprintf to get the number of characters that would be generated if the string pointed to by the list of arguments was printed using the specified format
  • Allocate memory for a string with the specified number of characters
  • Call vsprintf_s to build the string we want from the list of arguments
  • Call va_end to end the use of the variables argument list
  • Draw our beautified string
  • Free allocated memory

Function implementation with full comments shown below:

//-------------------------------------------------------------------------
//  Draws a string at the specified coordinates.
//-------------------------------------------------------------------------
void printw (float x, float y, float z, char* format, ...)
{
    va_list args;   //  Variable argument list
    int len;        // String length
    int i;          //  Iterator
    char * text;    // Text

    //  Initialize a variable argument list
    va_start(args, format);

    //  Return the number of characters in the string referenced the list of arguments.
    // _vscprintf doesn't count terminating '\0' (that's why +1)
    len = _vscprintf(format, args) + 1;

    //  Allocate memory for a string of the specified size
    text = malloc(len * sizeof(char));

    //  Write formatted output using a pointer to the list of arguments
    vsprintf_s(text, len, format, args);

    //  End using variable argument list
    va_end(args);

    //  Specify the raster position for pixel operations.
    glRasterPos3f (x, y, z);

    //  Draw the characters one by one
    for (i = 0; text[i] != '\0'; i++)
    glutBitmapCharacter(font_style, text[i]);

    //  Free the allocated memory for the string
    free(text);
}

Now that printw is defined, using it is similar to printf:

printf(         "char: %c, decimal: %d, float: %f, string: %s", 'X', 1618, 1.618, "text");
printw(x, y, z, "char: %c, decimal: %d, float: %f, string: %s", 'X', 1618, 1.618, "text");

Libraries to include before copying the function ;)

#include <stdio.h>    //  Standard Input\Output C Library
#include <stdarg.h>   //  To use functions with variables arguments
#include <stdlib.h>   //  for malloc
#include <gl/glut.h>  //  Include GLUT, OpenGL, and GLU libraries

Here is a sample source code that contains the printw function. If you have any issues compiling or running the app, check out this section for details about compiling and running an OpenGL app that uses the GLUT library. Below is a screen shot of the demo app:

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