how the following for loop function differently

Question

#include<stdio.h>
void main()
{
    int a,b,c;
    for(b = c = 10; a = "- FIGURE?, UMKC,XYZHello Folks,TFy!QJu ROo TNn(ROo)SLq SLq ULo+UHs UJq TNn*RPn/QPbEWS_JSWQAIJO^NBELPeHBFHT}TnALVlBLOFAkHFOuFETpHCStHAUFAgcEAelclcn^r^r\\tZvYxXyT|S~Pn SPm SOn TNn ULo0ULo#ULo-WHq!WFs XDt!"[b+++21];)
    {
        for(;a-->64;)
        {
            putchar((++c == 'Z') ? (c = c/9) : (33^b&1));
        }
    }
    getch();
}

Above program in c language gives the output as map of India. In the above program outer for loop has 2 slots and the third one is left empty. However I understood how the program works but the doubt is that the condition slot of the outer for loop works as an assignment slot. Syntactically and logically this should be wrong but it works. According to the value in array index, ASCII code of corresponding char is assigned variable a.
How this works?

That looks like an entry from the [IOCCC](http://www.ioccc.org/). It's questionable if those are meant to be understood. — Some programmer dude, Jan 03 '14 at 10:04
@user314723 please only extract relevant part of the code. Obfuscated writing competition entries are of no good neither for you nor for us. — Ivaylo Strandjev, Jan 03 '14 at 10:05
Related: http://stackoverflow.com/questions/1642028/what-is-the-name-of-this-operator — Chris Culter, Jan 03 '14 at 10:08
In obfuscated codes, do not bother about syntax, logic or UB. — haccks, Jan 03 '14 at 10:17
Please, learn the basics of C syntax before asking questions like this. Assignments are also expressions, and in boolean context are tests against 0. — Jim Balter, Jan 03 '14 at 10:24

Michał Trybus · Answer 1 · 2014-01-03T10:24:00.533

The condition slot of the outer for loop assigns to a one of the characters of the string literal, at the same time incrementing b. Because the assignment operator also returns the assigned value, the condition of the outer for loop becomes the value of some of the characters of the string literal. Because strings are '\0'-delimited in C, the condition is true until the expression b++ + 21 reaches the end of string (then the last (extra) character of the string is returned, and it's equal to 0, thus evaluating as false)

In fact, this is an obfuscated and more complex version of a common C idiom for iterating a string, which looks like this:

char *string = "my string";
int i;
for (i = 0; string[i]; ++i)
    /* do something with string[i] */

which can be simplified to:

int i = 0;
for (; string[i++]; )
    /* do something with string[i] */

Moreover, the current character can be extracted to a separate char variable c:

int i = 0;
char c;
for (; c = string[i++]; )
    /* do something with c */

A while loop can be used instead as well:

while (c = string[i++])
    /* do something with c */

how the following for loop function differently

1 Answers1