Why isn't the compound assignment taken into account when displaying the latter 'tot'?

Question

#include <stdio.h>

main() {

    int pro;        
    int dot;        
    int tot;        
    char prelude[] = "\nNow we shall do some simple mathematics\n\n";

    printf("%s", prelude);

    pro = 3;        
    dot = 5;
    tot = pro + dot;        
    printf("%d", tot);        
    dot += 23;        
    printf("\n%d\n", tot);        
    return 0;
}

Answer 1

Because C does not offer automatic data binding. When you use the assignment operator (or Addition assignment in your case), you are assigning the VALUE of the statement on the right hand side of the expression.

variable = statement means that statement will be calculated to some value, and this value will be assigned to variable according to variable's data type.

This means that you are assigning a value, not an actual statement to that variable.

To explain it better:

tot = pro + dot; 

does the following:

1. I need to assign something that I must calculate to the variable tot.
2. Something is a statement, pro + dot.
3. This statement is an addition, an operation that takes two operands.
4. Operand 1 is pro, it evaluates to 3. (means that C will replace pro with 3 in that statement)
5. Operand 2 is dot, it evaluates to 5.
6. All of the operands are evaluated.
7. The statement is: 3 + 5;
8. This evaluates to 8, an integer.
9. The expression has become tot = 8;
10. Assign the value 8 to tot. This means, go to the memory address of the variable represented by tot, and write the integer 8 (in accordance with the C standard/machine architecture).

If you do

dot += 23;

C understands this:

dot += 23; //  can be translated to `dot = dot + 23;'

Like before:

1. dot means 8, 23 means 23.
2. statement: 8 + 23
3. The statement evaluates to 31. dot = 31;, meaning write the integer 31 to the memory of dot.

Now tot , is unaffected by that. Why? Because tot is a space in your memory, that holds the value 8. It doesn't know that 8 was created by adding 2 other variables. It's just 8. So changing the other variables, will not affect this one.

If you do dot += 23;, you are changing the memory of the variable dot, not that of tot.

What you were expecting, is called data binding, and is a much higher level feature than what C offers.