CODEWITHSUNDEEP
cfilescanfgets
OldSchool
OldSchool

Reputation: 2183

unexpected behavior of scanf() and gets()

I want to write a line ( including spaces ) to the file in one go with following code:-

//in main
char ch[40];
FILE *p;
char choice;
p=fopen("textfile.txt","w");
printf("%s\n","you are going to write in the first file" );
while (1)
{
    gets(ch);// if i use scanf() here the result is same,i.e,frustrating
    fputs(ch,p);
    printf("%s\n","do you want to write more" );
    choice=getche();
    if (choice=='n'|| choice=='N')
    {
        break;
    }
}

Result of above code is frustrating to me and is hard to explain.but still i will try. If i enter,say, 

"my name is bayant."

and press enter the statment comes to screen is

"do you want to write more"

it is good till now,but when i prees a key other than 'n' or 'N' (as required by the logic of program to write more line) then the message 

"do you want to write more"

prints again.Now if i press other key than 'n' or 'N' the same line prints on screen.this procedure is followed and prints the statement

"do you want to write more"

4 times,which is the number of words,i.e, 4 in this case.By following this inflexible procedure i get the desired line on my file but if in response to first time printing of statement 

"do you want to write more"

i press 'n' or 'N' then only first word ,i.e, "my" in this case prints on file. so what is the solution to write a complete line on the file in one go?and why gets() and fputs() seems ineffective in this case? thanxxx in advance.

Upvotes: 0

Views: 188

Answers (2)

tesseract
tesseract

Reputation: 901

Do something like this, its a very crude program but should give you an idea

your mistakes, you had only made a pointer to a char in your program, you need to allocate memory for the pointer using malloc, or the other option is just to make an array of chars. which I have done.

#include <stdio.h>
#include <stdlib.h>
int main(void){

char ch[100];
FILE *p;
char choice;
p=fopen("textfile.txt","w");
printf("%s\n","you are going to write in the first file" );
while (1)
{
// gets(ch);// if i use scanf() here the result is same,i.e,frustrating
int c =0;

fgets(ch,100,stdin);
fputs(ch,p);
printf("%s\n","do you want to write more" );
choice=getchar();
if (choice=='n'|| choice=='N')
    {
    break;
    }
while ((c = getchar()) != '\n' && c != EOF);
}
return 0;
}

you program was repeating the printf("%s\n","do you want to write more" ); because the input buffer had a \n written to it, you need to clear the buffer before reading. this line removes the newline from buffer while ((c = getchar()) != '\n' && c != EOF);

check this scanf() leaves the new line char in buffer?

Upvotes: 2

davir
davir

Reputation: 942

If you use

scanf("%s",ch);

(Which I assume is what you mean by "scanf"), this will read one string. If you type

"my name is bayant."

This will result in 4 strings: "my", "name", "is" and "bayant.".

Notice that, from your description, you don't want to read strings, you want to read lines. To read a whole line of text with scanf, you could use:

scanf("%[^\n]", ch);
scanf("%*c");

Which means: Line 1: "read everything until you find a \n character".

Line 2: "read and ignore that '\n' character (which was left in the buffer)".

I should say that this is not a secure solution, as the user can easily overflow the "ch" buffer, but I'm sure you can find better ways to do that if this is a real concern in your particular case.

Upvotes: 2

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