Usage of scanf in while loop

Question

I have to read a series of input from a file using scanf. But there are different outcomes in 2 situations. Code1- Reading an integer and a char array.

char plaintext[20];
int started = 0;
int x;


while (scanf("%i,%19[^\n]",&x,plaintext) == 2)
{
    if (started == 1)
        printf(",\n");
    else 
            started = 1;
    printf("i read a line from a file");
}
printf("\n");

This works perfectly fine. The scanf reads every line in the file and the printf() outputs the required line for each line input.

Code2-Reading only a char array

char plaintext[20];
int started = 0;

while (scanf("%19[^\n]",plaintext) == 1)
{
    if (started == 1)
        printf(",\n");
    else 
            started = 1;
    printf("i read a line from a file");
}
printf("\n");

Here, the scanf reads only the first line and prints "i read a line from a file" only once.Why??

One solution is using %*c in scanf in Code 2. But why Code 1 works fine, but not Code 2.

A newlines left in the previous input will be rejected with the next entry. "%19[^\n]" --> "%19[^\n]%*c" — BLUEPIXY
– BLUEPIXY, Commented Apr 30, 2017 at 6:28

Ed Heal · Accepted Answer · 2017-04-30 06:27:56Z

2

You need to change

scanf("%19[^\n]",plaintext)

so that it reads the new line character that is in the buffer after reading the first line.

Try

scanf("%19[^\n] ",plaintext)

The space reads the new line character at the end of the line

answered Apr 30, 2017 at 6:27

Ed Heal

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4 Comments

HN Learner Over a year ago

Sir could we use '\r' instead of space there?

Ed Heal Over a year ago

scanf - From this page A sequence of white-space characters (space, tab, newline, etc.; see isspace(3)). This directive matches any amount of white space, including none, in the input. - So yes

Riten R Over a year ago

But why such new line character reading is not required in Code1 but is necessary in Code2.

Ed Heal Over a year ago

The %i will consume the white space in code1. Code 2 %19[^\n] does not consume the new line character

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