Why this wrong syntax of printf is not producing error in c?

Question

I have used this syntax for printing some variable n. I know that the syntax is wrong but was not able to understand what is happening behind the scenes. It is not even creating an error. Inside the main method:

int fac,p=1,n,i;
printf("Enter the value of n \n");
scanf("%d",&n);
printf("The factorial is \n %d"+n);

I have used +nin last line and that is not the right syntax. Input is 1 Output is:

he factorial is
1

Input is 5 Output is:

actorial is
1

Can someone help me to figure out what is happening behind the scenes? How is that the compiler understanding that +n in print function? From where the 1 is coming in the output?

for every value of n it is giving 1 but if the value is large it is not generating any output. Even not the string. — Ashish sah, Jun 02 '18 at 08:34
There is more output than the "1", how about the text before. Does it change to "he factorial is " if you enter 1? — Yunnosch, Jun 02 '18 at 08:35
Yes in the output i can see changes in static as you can see in the output. The whole text is not printed. — Ashish sah, Jun 02 '18 at 08:38
Please show the exact output your get for 1,2,3,4,5,6,7,8. I think you do not get the point of my comments. Keep in mind that **I** can **not** see the output **you** can see. — Yunnosch, Jun 02 '18 at 08:39
well the ans down is right :) just asking a bit of explanation — Ashish sah, Jun 02 '18 at 08:41
Input 1 | he factorial is 1 , Input 2 | e factorial is 1, Input 5 | actorial is 1 but from where the 1 is coming — Ashish sah, Jun 02 '18 at 08:42
Put those outputs into your question, on per line, aligned a little, to see what I mean and to improve your question. — Yunnosch, Jun 02 '18 at 08:43
Your change was incomplete, also it rendered existing comment/answer ununderstandable. I there rolled it back. — alk, Jun 02 '18 at 08:45
As it stands the question does not makes sense, as the output shown does not match the code. — alk, Jun 02 '18 at 08:48
Read the comment by alk. Check the edits in the history. You are sabotaging your own question. — Yunnosch, Jun 02 '18 at 08:49
@alk In case you are interested, the last edit seems to have fixed the question. — Yunnosch, Jun 02 '18 at 08:53

Mureinik · Accepted Answer · 2018-06-02T08:36:08.970

4

String literals are just pointers to char arrays. When you add an int to a pointer, you perform pointer-arithmetic, and move the pointer. Here, your factorial was 4, so you moved the pointer up 4 characters, so you lose the "The ".

The %d will just take the following value off the stack and interpret it as an int. In your case, this just happens to be 1.

edited Jun 02 '18 at 08:36

answered Jun 02 '18 at 08:31

Mureinik

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Well expecting a bit elaborative explanation. – Ashish sah Jun 02 '18 at 08:35
your ans seems to be correct can you please explain pointer-arithmetic in simple words ? – Ashish sah Jun 02 '18 at 08:40
Ah. A challenge. Good luck @Mureinik , ;-) – Yunnosch Jun 02 '18 at 08:41
1

@Ashishsah - probably a good idea to (re)read that chapter in a book. – Ed Heal Jun 02 '18 at 08:42
@Ashishsah That means it increments the pointer by whatever you input, i.e It would more the starting point of your string forward, read more about it here https://www.tutorialspoint.com/cprogramming/c_pointer_arithmetic.htm – supra28 Jun 02 '18 at 08:42
and what about the 1 which is coming at the last of the output. – Ashish sah Jun 02 '18 at 08:43
@Ashishsah If you have a pointer `p` pointing to an array element `a[i]` (as in `p == &a[i]`), then `p + n` is a pointer to `a[i + n]` (as in `p + n == &a[i + n]`). That's pointer arithmetic. – melpomene Jun 02 '18 at 08:44
Did you read the answer after the last few edits? It answers the question where the "1" is coming from. – Yunnosch Jun 02 '18 at 08:45
@Ashishsah in a nutshell, you move the pointer up and down the elements of the array according to the value of the int. See https://stackoverflow.com/q/394767/2422776 for a more detailed explanation. – Mureinik Jun 02 '18 at 08:45
@Ashishsah That is written in the answer. – supra28 Jun 02 '18 at 08:46
Well sorry did not looked at your edit thanks for your help @Yunnosch – Ashish sah Jun 02 '18 at 08:50

Why this wrong syntax of printf is not producing error in c?

1 Answers1