Calling fgets() twice

Question

I am a beginner. I have really struggled with fgets() when using the function twice. I am on Windows with VS Code and C++ extension. My test program is working, but it is IMHO lengthy and complicated for such a simple task. Furthermore, the use of goto is bad practice, I have read. Is there a simpler solution?

#include <iostream>
#include <string.h>
using namespace std;
#define max 5

void killNL(char *str) {            
    int i = 0;                      
    while (*(str+i) != EOF){

            if (*(str+i) == '\n'){
            *(str+i) = '\0';
            *(str+i+1) = EOF;
            }
            i++;
    }
}

int main()
{
    size_t len = 0;
    char string[5] = {0};
    int c = 0;

    begin:
    printf("Enter string: ");
    fgets(string, max, stdin);
    killNL(string);                 //Is used only for the len count when length is 1 less
                                    //than max. It depends on the space that is left for
                                    //'\n' AND '\0' which raises the count to when the 
                                    //(getchar()) != gets activated. Which needs a leftover
                                    //'\n' in the buffer to not go into endless loop.

    len = strlen(string);

    for (int i = 0 ; i <= len ; i++) {
        if (string[i] == '\n'){ printf("\\n \n");}
        else if (string[i] == '\0'){ printf("\\0 \n");}
        else { printf("%c \n", string[i]);}
    }

    askAgain:
    if (len >= max - 1) {
        while (c = (getchar()) != '\n' && c != EOF) {}
    }

    printf("Continue? 'y'/'n' :");
    fgets(string , 3, stdin);

    if (*(string) == 'y'){
        goto begin;
    } else if (*(string) == 'n') {
        goto end;
    } else {
        printf("\nWrong answer!\n");
        goto askAgain;
    }
    
    end:
    printf("END!\n");

    return 0;
} 

I have been searching here, on Youtube and then in a book where I found the killNL function, which finally did make it work. This question has many variants on Stack Overflow, with a combination with sscanf, with reading a file and more. But not this variant, which is why I am posting it despite the fact that it is an old subject.

Answer 1

Well, for starters, you really want to decide whether you are coding in C or C++. You #include <iostream>, but make no use of any iostream functions (presumably you thought about using std::cout or the like). You also specify using namespace std;, but that is not valid in C. Rule: only include necessary headers for the functions used in your code.

We will presume you are wanting to write code in C given your choice of input and output functions. If not, drop a comment and I'm happy to help with C++ as well. Working though your code:

• max 5 -- Don't SKIMP on buffer size. While you would use std::string if writing in C++, when declaring an array to hold input in C, don't skimp. While fgets() will only attempt to write size number of characters (minus 1 to provide room for the nul-terminating character) your purpose in using fgets(), among others, is to read a complete line of input at a time without leaving extraneous characters in the input stream. You do that by providing a sufficiently sized buffer.
• When taking input, you generally loop-continually and then break your read-loop if the input fails stream error or manual EOF (generated by the user pressing Ctrl + d (or Ctrl + z on Windows)), or successful input is received, or one of your exit conditions is satisfied (e.g. the user pressing Enter alone for the string indicating they are done with input)
• In C, to trim the '\n' from the end of the string read by fgets() (e.g. fgets (string, MAXC, stdin)), then you simply call string[strcspn (string, "\n")] = 0; effectively overwriting the '\n' with the nul-terminating character '\0' (which is just ASCII 0)
• Declare your variables in the scope where they are needed, and
• There is absolutely nothing wrong with goto used properly. It is the only way to break nested loops in a single expression, etc..

With that as a backdrop, you can simplify and clean up your code similar to the following. Many of the choices are up to you, but this is one general way to approach

• reading input,
• checking if the input is valid,
• checking if the user generated an EOF,
• checking if the user is done with input,
• checking if the input matches a string you want, and finally
• looping again if the input isn't what is needed.

(a rough interpretation of what it looked like you were wanting to do)

The code (with additional comments) can be:

// #include <iostream>  /* only include needed headers for functions used */
#include <stdio.h>      /* or <cstdio> for modern C++ */
#include <string.h>     /* or <cstring> for modern C++ */

// using namespace std; /* using nothing from std:: namespace */

#define MAXC 1024       /* don't SKIMP on buffer size, UPPERCASE defines */
#define ANS "guess"

int main()
{
    char string[MAXC] = {0};    /* declare string */
    
    /* read-loop continually, break on condition of your choice */
    while (1) {
        fputs ("\nEnter string: ", stdout); /* no conversion, fputs is fine */
        
        /* validate EVERY input based on return of read function */
        if (!fgets (string, MAXC, stdin)) {
            puts ("(user generated manual EOF)");
            return 0;
        }
        
        /* trim '\n' from string */
        string[strcspn (string, "\n")] = 0;
        
        /* exit if empty-string (user just pressed [Enter], or any condition) */
        if (*string == '\0') {
            puts ("(all done)");
            break;
        }
        
        /* check if correct answer ANS given */
        if (strcmp (string, ANS) == 0) {
            puts ("(correct!)");
            break;
        }
        
        puts ("(answer didn't match)");   /* otherwise, loop again */
    }
    
    puts ("(That's All Folks!)");
}

Example Use/Output

$ ./bin/read-loop

Enter string: my dog has fleas
(answer didn't match)

Enter string: why?
(answer didn't match)

Enter string: what if I guess?
(answer didn't match)

Enter string: guess
(correct!)
(That's All Folks!)

Or canceling with a Ctrl + d:

$ ./bin/read-loop

Enter string: (user generated manual EOF)

Or using the exit-condition of the user just pressing Enter alone at the prompt indicating they are done guessing:

$ ./bin/read-loop

Enter string: next we will just press Enter when prompted, to quit input
(answer didn't match)

Enter string:
(all done)
(That's All Folks!)

You can add (or remove) as many of the exit conditions for the read-loop as you like. The key take-aways are:

• loop continually until the user provides the required input,
• always condition exiting your read-loop on the return of your read-function,
• validate EVERY user-input,
• provide the needed exit conditions to control exit from your read-loop, and
• handle any errors that occur as needed.

Let me know if this is what you needed. If not, I'm happy to help further.

Answer 2

This is DavidC.Rankin's code with modification for overflow.

    #include <stdio.h>     
    #include <string.h>     
    
    
    
    #define MAXC 6      //test with minimum storage
    #define ANS "guess"
    
    int c = 8;
    int d = 9;
    int e = 0;
    int len = 0;
    
    
    int main()
    {
        char string[MAXC] = {0};    
        
        /* read-loop continually, break on condition of your choice */
        while (1) {
            fputs ("\nEnter string: ", stdout); 
            
            /* validate EVERY input based on return of read function */
            if (c = !fgets (string, MAXC, stdin)) {     //c = 1 when EOF is entered, else 0.
                puts ("(user generated manual EOF)");   //ctrl + 'z' (manual EOF)
                return 0;
            }
            
            /* trim '\n' from string */
            string[strcspn (string, "\n")] = 0;     //compares string with \n and counts position.
    
            len = strlen(string);                   //needed when input is overflowing
            
            if (len >= (MAXC - 1)) {                //ditto
                 while ((d = getchar()) != '\n' && d != '\0') {}
            }
            
            /* exit if empty-string (user just pressed [Enter], or any condition) */
            if (*string == '\0') {
                puts ("(all done)");
                break;
            }
            
            /* check if correct answer ANS given */
            if (d = strcmp (string, ANS) == 0) {
                puts ("(correct!)");
                break;
            }
            
            puts ("(answer didn't match)");   /* otherwise, loop again */
        }
        
        puts ("(That's All Folks!)");
    
        return 0;
    }