fgets and scanf

Hello,

I'm aware that scanf is a bit "nasty" because it often leaves "undigested" characters in the input stream. What about fgets used to input a string from stdin? I tried entering more characters than I had specified in fgets and nothing seemed to remain in the input stream. Is my observation correct?

No .

Quote:

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Originally Posted by kkk

....
Is my observation correct?

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I am not sure about the general trend I am seeing here(not with you), but I would like to point out although it is Computer Science it is not science as in trying to figure out how the universe works. You are not exploring like Capt. Kirk here.....
LOOK AT THE DOCUMENTATION. It tells you what a function does and how it will behave. You do not need to observe and infer here.

Quote:

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Originally Posted by AndrewHunter

I am not sure about the general trend I am seeing here(not with you), but I would like to point out although it is Computer Science it is not science as in trying to figure out how the universe works. You are not exploring like Capt. Kirk here.....
LOOK AT THE DOCUMENTATION. It tells you what a function does and how it will behave. You do not need to observe and infer here.

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And I suppose the corollary to that is: if you are going to write a function to test out the behavior of a function, you should be careful to write your program to actually test the behavior of the function (which apparently didn't happen here).

Quote:

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Originally Posted by AndrewHunter

LOOK AT THE DOCUMENTATION. It tells you what a function does and how it will behave. You do not need to observe and infer here.

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I did consult the man pages and I did try to look it up on the internet. There was nothing about what fgets does with the characters it doesn't "put" in the string.

That's why I asked. Sorry if it bothered you.

Caroline

Quote:

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Originally Posted by kkk

I did consult the man pages and I did try to look it up on the internet. There was nothing about what fgets does with the characters it doesn't "put" in the string.

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There aren't any such characters, which may be why the man page is silent about it. (Or at any rate, there aren't any characters that fgets processes but doesn't put in the string.) Look:

Quote:

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Originally Posted by man fgets

The fgets() function reads at most one less than the number of characters
specified by n from the given stream and stores them in the string s.

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It does what it says on the tin: it reads no more characters than can fit. The remaining characters must needs be still unread. (ETA: If you want an explanation of why your code is doing whatever it is that it's doing that leads you to believe that fgets discards characters, you'd have to provide the code.)

How did you come by this observation? To become good at programming, you should be able to test such ideas with practice snippets of code. For example, I figured out the answer to your question with this:

Code:

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// quick and dirty example

#include <stdio.h>

#define MAX_LEN 10

int main(void)
{
    char str_input[MAX_LEN];
    int left_over = 0;

    // read user input, 9 characters max
    printf("Enter string:  ");
    fgets(str_input,MAX_LEN,stdin);
    // quick and sloppy null termination at the end of the character array
    str_input[MAX_LEN-1] = '\0';

    // at runtime, we will enter 15 characters, followed by enter (newline)

    // now we'll print our string:
    printf("\nString:  %s\n\n",str_input);

    // ... and print out anything left over in the input buffer, up to a newline
    printf("Left over characters:  ");
    while((left_over = getchar()) != '\n')
        printf("%c",left_over);

    printf("\n\n");
    return 0;
}

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And here is my output:

Code:

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Enter string:  ABCDEFGHIJLKMNO

String:  ABCDEFGHI

Left over characters:  JLKMNO

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Therefore, we see that when the data in the input buffer exceeds the limit of "fgets()", the rest of the data continues to sit in the buffer.

Code:

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#include <stdio.h>
#include <stdlib.h>

#define MAXSIZE 5

char numbers[MAXSIZE + 1];

int myatoi(const char *str)
{
    int val = 0;
   
    for (int i = 0; str[i] >= '0' &&  str[i] <= '9'; ++i) {
        val = val * 10 + (str[i] - '0');
    }
    return val;
}

void get_strnr2()
{
    fgets(numbers, MAXSIZE, stdin);
}


int main (int argc, const char * argv[])
{
   
    int result, c;
 
    printf("\nEnter a number:");
    get_strnr2();
    result = myatoi(numbers);
    printf("\nYou entered: %i", result);
   
    while ((c = getchar()) != '\n') {
        putchar(c);
    }
    return 0;
}

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Try to test the code above and enter a number containing more than 5 digits. The leftover characters don't get printed... Why? I'm running Mac OS X Lion and Xcode 4.

Because they are. Try changing this erroneous line:

Code:

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printf("\nYou entered: %i", result);

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to something more correct, like

Code:

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printf("\nYou entered: %i>>result stops here", result);

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What is your output when you enter excessive digits?

EDIT: Disregard. Tabstop went where I was headed!

Quote:

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Originally Posted by tabstop

Because they are. Try changing this erroneous line:

Code:

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printf("\nYou entered: %i", result);

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to something more correct, like

Code:

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printf("\nYou entered: %i>>result stops here", result);

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Yep, that's what I overlooked, thanks. And why did I overlook it? Because I didn't get a message in the XCode debug window that the program terminated... Strange. That's why I thought it was waiting for input... Silly me!

EDIT: The program does terminate but after some 30 seconds or so of being idle after the last output...