CODEWITHSUNDEEP
c
pranav
pranav

Reputation: 53

why to put space between %d and %c while inputting there values using scanf

im a beginner to C,, and i was writing a code to print a square of a user entered character.

usually when we need to input two integers (say x and y) using scanf() we write this scanf("%d%d", &x, &y) but according to the needs of my code i am supposed to input one integer (say m) and a character (say ch).

I wrote it as scanf("%d%c", &x, &ch) but it has an error, when i execute the program it only asks the integral value to be entered and then it just stop executing.

I searched for this and i found that i need to put space between %d and %c as scanf("%d %c", &x, &ch);

Can anyone explain this why we need to put space between this?

Upvotes: 5

Views: 2826

Answers (2)

R Sahu
R Sahu

Reputation: 206717

Meaning of whitespace characters in the format string (from http://www.cplusplus.com/reference/cstdio/scanf/?kw=scanf):

Whitespace character: the function will read and ignore any whitespace characters encountered before the next non-whitespace character (whitespace characters include spaces, newline and tab characters -- see isspace). A single whitespace in the format string validates any quantity of whitespace characters extracted from the stream (including none).

When your code is:

scanf("%d%c", &x, &ch)

and you enter

10

in your console:

  1. 10 is read and stored in x.
  2. the newline character is read and stored in ch.

If you use

scanf("%d %c", &x, &ch)

and you enter

10
  1. 10 is read and stored in x.
  2. the newline character and other whitespace characters are consumed. The program waits for a non-whitespace character. Once a line of input is entered and at least one non-whitespace character is present in the line, the first non-white character of the line is read into ch. This is because stdin is typically line buffered.

Upvotes: 12

AnT stands with Russia
AnT stands with Russia

Reputation: 320739

Almost all format specifiers in scanf automatically skip all whitespace before trying to read anything. That's, for example, how %d works (as well as %s, %f and most other specifiers). However, %c is an exception from this behavior. %c does not skip whitespace. If your input data contains a whitespace character, %c will happily read that character into your ch variable.

This is most likely what happens in your case. If you enter

5 #

to create a 5x5 square of #s, the %d specifier will read 5 into x and stop reading at the space character. Then %c will read that space character into ch. And then your program will just proceed, leaving the # unread. It is quite possible that the rest of your program actually works as intended. It's just that since ch is a space character, it displays a 5x5 square of space characters, which are invisible :) To make your scanf in its original form work as intended you have to input your data as

5#

But a much better way to make sure that your scanf actually reads the # character into ch is to explicitly ask scanf to skip all whitespace before reading ch. In scanf this is done by inserting any whitespace character (space, newline, tab etc.) into the format string. This 

scanf("%d %c", &x, &ch);

will force scanf to skip that space after 5 and read the # into ch. This will also work for inputs like this

5              #

and even like this

5
  #

because scanf will automatically skip all whitespace until it hits the #. Note also that even with the updated format sting whitespace in the input is not required, meaning that this input

5#

will still work with the updated scanf.

You can also use these variants

scanf("%d\t%c", &x, &ch);
scanf("%d\n%c", &x, &ch);
scanf("%d     %c", &x, &ch);
scanf("%d \n \t %c", &x, &ch);

to achieve the same thing. They are all equivalent. The first variant (with a space) just looks better.

Upvotes: 4

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