Characters, Values, and Playing with Your Brain

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I’ve messed with characters as values quite a few times in my code. Keeping in mind that the char data type is really a tiny integer value, you can perform all kinds of tricks — stuff that drives non-programmers crazy.

For example, I often do math using character values. Consider that char variable a is a digit, 0 through 9:

b = a - '0';

This statement subtracts character '0' from the value of variable a. Seems crazy, but what’s really going on is the ASCII code value for '0' (48 decimal, 0x30 hex) is subtracted from variable a. Because variable a holds a digit, the result is the value 0 through 9, which is stored in variable b. This technique is one way to convert character numbers into their integer values.

Likewise, you can convert a letter of the alphabet to an integer value 0 through 26 for A through Z:

b = a - 'A';

All this nonsense works because to the computer, a character is really a visual representation of a value. This interpretation leads to some interesting possibilities, as shown in the following code.

2021_08_07-Lesson.c

#include <stdio.h>

int main()
{
    int t['A'];

    printf("The array contains %ld elements\n",sizeof(t)/sizeof(int));

    return(0);
}

At Line 5, int array t[] is defined, which contains 'A' elements. This statement is valid in C because the 'A' is converted by the compiler into its integer value. To see how many elements are in the array, run the program:

The array contains 65 elements

If you look up the value of 'A' on an ASCII table (see this month’s Exercise), you discover that it’s 65. So the declaration int t['A'] creates an array of 65 elements, the ASCII value of character 'A'.

By the way, the array size is determined by dividing the amount of memory occupied by the array, sizeof(t), by the size of the array’s data type, integer sizeof(int). The result yields the number of elements in the array.

Using characters as integers is a fun trick, one that can throw off a beginning C programmer. The problem with this trick is that it obfuscates the code. It’s much easier to write int t[65] than int t['A']. But it’s cute, so I thought I’d point it out.

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