Understanding Increment and Decrement operators in C

Understanding Increment and Decrement operators in C



Almost all programmers who work in C-based languages use the ++ and -- operators .Let's look at some interesting aspects of these operators in C.Increment/decrement operators are good examples to explain the curt (concise)syntax of


C language :

void strcpy(char *t,const char *s){
while(*t++=*s++);
}



In this strcpy function,the while loop continues copying characters from source to target ,till it reaches the end of the string ('\0' character ).

Here is another example ;can you guess the output of this code segment?

char str[]="oh",*p=str;
while(*p)
++*p++;
printf(str);


How is the expression ++*p++ treated? The postfix increment/decrement is a part of the operators with highest precedence ;the dereference and prefix increment/decrement is at the next precedence level and their associativity is from right to left .So ,the expression is treated as:(++(*(p++))).p is a pointer to string "oh".In the while loop,p++ is executed first,but it's effect does not take place till the statement execution completes.The result of p++(which is still the value of p) is dereferenced with *.For the first character it is 'o'
Now ,Prefix ++ increments the character value ,so its value is 'p' .The same happens for the next character ,'h'
,which becomes 'i'.Now the end of the character is reached ,so the while loop terminates .The printf now prints the modified string .which is "pi" .



we can use ++/-- operators to illustrate the 'maximum munch ' rule in C lexical analysis .Simply stated ,the lexical analyser (lexer ) looks for a match for the longest token .For example ,consider the expression a+b
,where a and b are integer varables.Given this as input in a C program ,the lexer might give VARIABLE (a),PLUS(+),VARIABLE(B) as output ,which is a valid sequence of tokens for compilation to succeed .

Now ,how about the expression a++b? .It is treated as a++b ,and not (a+(+b)),and results a compiler error.How about a+++b ?. It becomes ((a++)+b),so it's fine.Expression a++++b?.It is ((a++) (++b)),so it is a compiler error .Hmm how about a+++++b?.It is ((a++)++)(+b)),which is again compiler error .Note that,in all these cases ,if we use explicit white space or parenthesis ,we can get valid expressions .Note that ,in all these cases ,if we use explicit white-space or parenthesis ,we can get valid expressions :(a+(+b)),((a++)+b),
((a++) + (+b)),and ((a++) + (++b).If we try adding more +s after this ,we cannot get a valid expression for integer arguments.
Today,C-based languages (C++,Java ,etc) rule the programming world;because of this ,almost all programmers are aware of the ++ and -- operators and how powerful they are in expressing operations .What even experienced programmers do not know is that the C family was the first to introduce ++ and -- operators ! KEN THOMPSAN ,while implementing the B compiler ,noticed that increment operations generate more compact code than adding 1 to a variable and storing it back .In other words ,an increment was one instruction while adding 1 and and storing it back was more than one instruction .
So he implemented the prefix ++ (and --)operators ,and generalised it by adding a postfix version .Interestingly ,this short code -generation advantage is still true for recent languages .For example,in java ,integer increment would generate one instruction compared to the four instructions for adding 1 to the varable and storing it back .

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