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The Basics of C Programming

Looping: A Real Example

Let's say that you would like to create a program that prints a Fahrenheit-to-Celsius conversion table. This is easily accomplished with a for loop or a while loop:

#include <stdio.h>

int main()
{
    int a;
    a = 0;
    while (a <= 100)
    {
        printf("%4d degrees F = %4d degrees C\n",
            a, (a - 32) * 5 / 9);
        a = a + 10;
    }
    return 0;
}

If you run this program, it will produce a table of values starting at 0 degrees F and ending at 100 degrees F. The output will look like this:

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   0 degrees F =  -17 degrees C
  10 degrees F =  -12 degrees C
  20 degrees F =   -6 degrees C
  30 degrees F =   -1 degrees C
  40 degrees F =    4 degrees C
  50 degrees F =   10 degrees C
  60 degrees F =   15 degrees C
  70 degrees F =   21 degrees C
  80 degrees F =   26 degrees C
  90 degrees F =   32 degrees C
 100 degrees F =   37 degrees C

The table's values are in increments of 10 degrees. You can see that you can easily change the starting, ending or increment values of the table that the program produces.

If you wanted your values to be more accurate, you could use floating point values instead:

#include <stdio.h>

int main()
{
    float a;
    a = 0;
    while (a <= 100)
    {
        printf("%6.2f degrees F = %6.2f degrees C\n",
            a, (a - 32.0) * 5.0 / 9.0);
        a = a + 10;
    }
    return 0;
}

You can see that the declaration for a has been changed to a float, and the %f symbol replaces the %d symbol in the printf statement. In addition, the %f symbol has some formatting applied to it: The value will be printed with six digits preceding the decimal point and two digits following the decimal point.

Now let's say that we wanted to modify the program so that the temperature 98.6 is inserted in the table at the proper position. That is, we want the table to increment every 10 degrees, but we also want the table to include an extra line for 98.6 degrees F because that is the normal body temperature for a human being. The following program accomplishes the goal:

#include <stdio.h>

int main()
{
    float a;
    a = 0;
    while (a <= 100)
    {
	if (a > 98.6)
        {
            printf("%6.2f degrees F = %6.2f degrees C\n",
                98.6, (98.6 - 32.0) * 5.0 / 9.0);
        }
        printf("%6.2f degrees F = %6.2f degrees C\n",
            a, (a - 32.0) * 5.0 / 9.0);
        a = a + 10;
    }
    return 0;
}

This program works if the ending value is 100, but if you change the ending value to 200 you will find that the program has a bug. It prints the line for 98.6 degrees too many times. We can fix that problem in several different ways. Here is one way:

#include <stdio.h>

int main()
{
    float a, b;
    a = 0;
    b = -1;
    while (a <= 100)
    {
	if ((a > 98.6) && (b < 98.6))
        {
            printf("%6.2f degrees F = %6.2f degrees C\n",
                98.6, (98.6 - 32.0) * 5.0 / 9.0);
        }
        printf("%6.2f degrees F = %6.2f degrees C\n",
            a, (a - 32.0) * 5.0 / 9.0);
        b = a;
        a = a + 10;
    }
    return 0;
}

C Errors to Avoid

  • Putting = when you mean == in an if or while statement
  • Forgetting to increment the counter inside the while loop - If you forget to increment the counter, you get an infinite loop (the loop never ends).
  • Accidentally putting a ; at the end of a for loop or if statement so that the statement has no effect - For example: for (x=1; x<10; x++); printf("%d\n",x); only prints out one value because the semicolon after the for statement acts as the one line the for loop executes.
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