C Expert Programming Chapter 1 C: Through the Fog of Time and Space 1.4 K&R C

/*When: 1978
*Who: Steve Johnson
*Event: Writing pcc, a portable C compiler
*Influence: Formed the basis of a generation of C compilers.
*/
/*The evolution of the C language is shown in Figure 1-2.
*1973-3 (early C)--->1976-9 (K&R C)--->1983-9 (ANSI C)(1)
*--->1967 (Simula 67)(2)
*1983 (Ada) (3)
*(1)(2)(3) together form 1985-9 (C++)
*/
/* Software Creed
*An unusual bug
The C language inherited a feature from Algol-68, the compound assignment operator, which allows a recurring operation to be written only once instead of twice, and gives the code generator a hint,That is, operand addressing can be so compact. The original way of writing the compound assignment operator is to write the assignment operator first, and then write the operator. There is a trick in the lexical analyzer of the B language that makes the implementation of =op more efficient than the op= form currently used.Simpler but confusing in this form, it's easy to add
b=-3; //subtract 3 from b
and
b= -3; //assign -3 to b
Confused.
Therefore, this feature has been modified to the form currently in use.As part of the modification, the code formatter program indent
was also modified to determine the obsolete form of the compound assignment operator, and to swap the two to convert it to the corresponding standard
form.This is a very bad decision.
No formatter should modify anything other than whitespace in the program.Unpleasantly, this approach introduces a bug where just about anything (as long as it's not a variable) swaps places with the assignment operator if it appears after the assignment operator.
*/
/* This bug may cause syntax errors, such as
*epsilon=.0001;
*will be replaced with
*epsilon.=0001;
*thisIf the statement fails to pass the compiler, you will immediately find the error
*value=!open; //value is set to the logical inverse of open
*will be silently exchanged to
*value!=open; //Compare unequal value with open
*This statement will compile without changing the value of value
*/
#include
int main() {
    int b = 5;
    
    b =- 3; /*从b中减去3*/
    printf("b = %d\n",b);
b= -3; /*Assign -3 to b*/
printf("b = %d\n", b);
double epsilon=.0001;
printf("epsilon = %f\n", epsilon);
//can't pass the compiliation
//epsilon.=0001;
printf("epsilon = %f\n", epsilon);
int value = 3, open = 4;
bool result = value =! open; /*value is set to the logical inverse of open*/
printf("!open = %d\n", !open);
printf("result = %d\n", result);
result = value != open; /*value compares unequally with open*/
printf("result = %d\n", result);
     
    return 0; 
}

/* output:

*/ /*It seems that the previous problem does not exist now*/