I'm facing a problem with my code of a simple login program. The problem I'm facing is when I use a switch case or if statement for the option of logging in as an Admin or a User, the input for username is skipped and goes directly to password, and no matter what I type it gives me my error message. Instead, I want it to receive my username first then the password. It works fine on its own if there is only code for either Admin OR User, only one but not when there are more than one. Please help. Note: I'm using the same functions for both admin and user just to check if it works. The picture shows the output.I'm a C newbie, so minimal jargon perhaps? Code as follows:
#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
char username[18];
char pass[16];
void arequest()
{
printf("\nPlease Enter username:");
fflush(stdin);
gets(username);
printf("\nPlease Enter Password:");
fflush(stdin);
gets(pass);
}
void averify()
{
if (strcmp(username, "admin") == 0)
{
if (strcmp(pass, "apass") == 0)
{
printf("Successful Login");
_getch();
}
else
{
printf("Invalid Password");
_getch;
}
}
else
{
printf("Invalid Username");
_getch();
}
}
int choice;
int main()
{
printf("Welcome to Railway Reservation System");
printf("\n1.Admin \n2.User");
printf("\nPlease Enter your selection:");
scanf_s("%d", &choice);
if (choice == 1)
{
arequest();
averify();
}
else if (choice == 2)
{
arequest();
averify();
}
else
{
printf("Invalid Choice");
_getch();
return main;
}
return 1;
}
output
You are flushing the input stream with fflush(). fflush(stdin) is undefined behavior in most cases, and is at best implementation-dependent. To clear the extra characters from the input stream, consider writing a little function like this:
void clear_stream(void)
{
int c;
while ((c = _getch()) != '\n' && c != EOF)
continue;
}
Remove the calls to fflush(). You do not need to clear the stream after gets(username) since gets() discards the newline. Add a call to clear_stream() after this line in main():
scanf_s("%d", &choice);
There may be extra characters, including a newline, left in the input stream after the call to scanf_s(), and these need to be removed before trying to read user input again. In some cases scanf()_s (and scanf()) will skip over initial whitespaces in reading input, but _getch() and getchar() will not. This illustrates one of the dangers of using scanf().
printf("\nPlease Enter your selection:");
scanf("%d", &choice);
clear_stream();
Also, gets() is considered so dangerous that there is never a reason to use it for anything at all. Use fgets() instead. fgets() does keep the newline, where gets() discards it, so I often write my own version of gets() using fgets() that is safe:
char * s_gets(char *st, int n)
{
char *ret;
int ch;
ret = fgets(st, n, stdin);
if (ret) {
while (*st != '\n' && *st != '\0')
++st;
if (*st)
*st = '\0';
else {
while ((ch = getchar()) != '\n' && ch != EOF)
continue; // discard extra characters
}
}
return ret;
}
The library conio.h is nonstandard, as are the functions _getch() and scanf_s(). You should use the stdio.h functions getchar() and scanf(). The value returned by scanf() is the number of successful assignments, and you should check this to be sure that the input is as expected. In your program, if the user enters a letter at the selection prompt, no assignment is made, and the value of choice remains uninitialized. The code continues without handling this problem. choice could be initialized to some reasonable value, such as int choice = -1;. Alternatively, you can check the return value from scanf() to see if an assignment was made, and proceed accordingly.
I noticed that you are returning 1 from main(). You should return 0 unless there is an error. And, I see that you return main in the event of an invalid choice. Maybe you meant to return 1 here? And it appears that you have forgotten to #include <string.h> for the strcmp() function.
Finally, I don't understand why username, pass, and choice are global variables. This is a bad practice. These should be declared in main() and passed to functions as needed. It would be a good idea to #define the global constants MAXNAME and MAXPASS instead of hard-coding the array dimensions.
I didn't intend this to be a full-scale code review when I started, but that is what it turned into. Here is a revised version of your program that implements the suggested changes:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAXNAME 18
#define MAXPASS 16
void clear_stream(void)
{
int c;
while ((c = getchar()) != '\n' && c != EOF)
continue;
}
char * s_gets(char *st, int n)
{
char *ret;
int ch;
ret = fgets(st, n, stdin);
if (ret) {
while (*st != '\n' && *st != '\0')
++st;
if (*st)
*st = '\0';
else {
while ((ch = getchar()) != '\n' && ch != EOF)
continue; // discard extra characters
}
}
return ret;
}
void arequest(char username[MAXNAME], char pass[MAXPASS])
{
printf("\nPlease Enter username:");
s_gets(username, MAXNAME);
printf("\nPlease Enter Password:");
s_gets(pass, MAXPASS);
}
void averify(char username[MAXNAME], char pass[MAXPASS])
{
if (strcmp(username, "admin") == 0)
{
if (strcmp(pass, "apass") == 0)
{
printf("Successful Login");
getchar();
}
else
{
printf("Invalid Password");
getchar();
}
}
else
{
printf("Invalid Username");
getchar();
}
}
int main(void)
{
char username[MAXNAME];
char pass[MAXPASS];
int choice;
printf("Welcome to Railway Reservation System");
printf("\n1.Admin \n2.User");
printf("\nPlease Enter your selection: ");
if (scanf("%d", &choice) == 1) {
clear_stream();
if (choice == 1)
{
arequest(username, pass);
averify(username, pass);
}
else if (choice == 2)
{
arequest(username, pass);
averify(username, pass);
}
else
{
printf("Invalid Choice: %d\n", choice);
getchar();
return 1;
}
} else {
clear_stream(); // stream has not yet been cleared
printf("Nonnumeric input");
getchar();
}
return 0;
}
EDIT
The OP mentioned in the comments that scanf() was causing problems in Visual Studio. Apparently Visual Studio tries to force the use of scanf_s(). The issue with this function is not that it is inherently bad, just that it is nonstandard. One solution might be to use the s_gets() function already added to the code to read the user selection into a character buffer, and then to use sscanf() to extract input. This has an advantage in that there is no need to call the clear_stream() function after s_gets(), because s_gets() cleans up after itself, so the clear_stream() function could now be removed altogether from the program. This can be accomplished with only a small change in main():
char choice_buffer[10];
int choice;
...
if (s_gets(choice_buffer, sizeof(choice_buffer)) &&
sscanf(choice_buffer, "%d", &choice) == 1) {
if (choice == 1)
...
} else {
printf("Nonnumeric input");
getchar();
}
s_gets() reads up to the first 9 characters (in this case) of a line of user input into choice_buffer, which is an array that will hold chars (there is more space in choice_buffer than is needed to hold a single digit choice and a '\0'). If there is an error, s_gets() returns a NULL pointer, otherwise a pointer to the first char of choice_buffer is returned. If the return value of s_gets() was non-NULL, then sscanf() assigns the first int stored in the buffer to choice. If no int is found in the string, sscanf() returns a value of 0, failing the conditional test.
Related
I want to break this loop when the user press enters twice. Meaning, if the user does not enter a character the second time, but only presses enter again, the loop must break.
char ch;
while(1) {
scanf("%c",&ch);
if(ch=='') { // I don't know what needs to be in this condition
break;
}
}
It is not possible to detect keypresses directly in C, as the standard I/O functions are meant for use in a terminal, instead of responding to the keyboard directly. Instead, you may use a library such as ncurses.
However, sticking to plain C, we can detect newline characters. If we keep track of the last two read characters, we can achieve similar behavior which may be good enough for your use-case:
#include <stdio.h>
int main(void)
{
int currentChar;
int previousChar = '\0';
while ((currentChar = getchar()) != EOF)
{
if (previousChar == '\n' && currentChar == '\n')
{
printf("Two newlines. Exit.\n");
break;
}
if (currentChar != '\n')
printf("Current char: %c\n", currentChar);
previousChar = currentChar;
}
}
Edit: It appears that the goal is not so much to detect two enters, but to have the user:
enter a value followed by a return, or
enter return without entering a value, after which the program should exit.
A more general solution, which can also e.g. read integers, can be constructed as follows:
#include <stdio.h>
#define BUFFER_SIZE 64U
int main(void)
{
char lineBuffer[BUFFER_SIZE];
while (fgets(lineBuffer, BUFFER_SIZE, stdin) != NULL)
{
if (lineBuffer[0] == '\n')
{
printf("Exit.\n");
break;
}
int n;
if (sscanf(lineBuffer, "%d", &n) == 1)
printf("Read integer: %d\n", n);
else
printf("Did not read an integer\n");
}
}
Note that there is now a maximum line length. This is OK for reading a single integer, but may not work for parsing longer input.
Credits: chux - Reinstate Monica for suggesting the use of int types and checking for EOF in the first code snippet.
You can store the previous character and compare it with the current character and enter, like this:
char ch = 'a', prevch = '\n';
while(1){
scanf("%c",&ch);
if((ch=='\n') && (ch == prevch)){// don't know what needs to be in this condition
break;
}
prevch = c;
}
Note that the previous character by default is enter, because we want the program to stop if the user hits enter at the very start as well.
Working like charm now
char ch[10];
while(1){
fgets(ch, sizeof ch, stdin);
if(ch[0]=='\n'){
break;
}
}
I am trying to read a numeric only value that will then execute a specific function, depending on its value using the switch statement.
I know I could just use while(option < 0 || option >3 || option != 99), but is there a workaround?
Personal work:
do
{
printf("Please choose an option :\n0. Create Database\n1.Add New Student \n2. Show All Students \n3.Delete Student\n99.Exit\n");
scanf("%d", &option);
} while (!isdigit(option));
Which does not work for some reason.
Your major problem there is that, if scanf fails to read any digits for some reason (such as if you enter x), it will return zero (the number of items successfully scanned), not populate option, and (this is the killer) leave the input pointer in the same place as before you started.
That unfortunately means, when you go back to get another number, it will just attempt reread the problematic data and probably end up in an infinite loop without allowing more input.
The scanf family is meant for formatted data and there's precious little that's more unformatted than user input :-)
Your best bet would be to use a rock-solid input function to get in a line, then just check that line to see if it's valid. Such a function can be found here. Incorporating that into your needs would give you something like:
#include <stdio.h>
#include <string.h>
#define OK 0
#define NO_INPUT 1
#define TOO_LONG 2
static int getLine (char *prmpt, char *buff, size_t sz) {
int ch, extra;
// Get line with buffer overrun protection.
if (prmpt != NULL) {
printf ("%s", prmpt);
fflush (stdout);
}
if (fgets (buff, sz, stdin) == NULL)
return NO_INPUT;
// If it was too long, there'll be no newline. In that case, we flush
// to end of line so that excess doesn't affect the next call.
if (buff[strlen(buff)-1] != '\n') {
extra = 0;
while (((ch = getchar()) != '\n') && (ch != EOF))
extra = 1;
return (extra == 1) ? TOO_LONG : OK;
}
// Otherwise remove newline and give string back to caller.
buff[strlen(buff)-1] = '\0';
return OK;
}
#include <ctype.h> // only needed for isdigit, not for above code.
int main(void) {
int option;
do {
// Only allowed up to two characters (plus '\0').
char buff[3];
int result = getLine(
"Menu\n"
" 0. Create Database\n"
" 1. Add New Student\n"
" 2. Show All Students\n"
" 3. Delete Student\n"
" 99. Exit\n"
"Please choose option: ",
buff, sizeof(buff));
// No point continuing on EOF, input stream is closed.
if (result == NO_INPUT) {
puts("\n*** End of input");
return 1;
}
// All faults just restart operation: too long, too short,
// non-digits.
if (result == TOO_LONG) continue;
if (! isdigit(buff[0])) continue;
if (buff[1] != '\0' && ! isdigit(buff[1])) continue;
// Now get the integer representation and continue unless valid.
sscanf(buff, "%d", &option);
} while (option != 99 && (option < 0 || option > 3));
printf("You chose %d\n", option);
return 0;
}
And, yes, I know I said you should check the return value from scanf but it's not necessary in the case of my posted code since you've already established at that point that the string you're scanning is a valid one- or two-digit number.
Like Carl said:
isdigit is just for check single character, and should NOT be used here to the int value obtained from scanf .
To archive your requirement, we can use scanf to obtain a string, then use strtol convert to int, check the endptr to confirm if there is any invalid character.
Code may like this:
int option;
char str[8];
char *endp;
do{
printf("Please choose an option :\n0. Create Database\n1.Add New Student \n2. Show All Students \n3.Delete Student\n99.Exit\n");
scanf("%7s", str);
option = strtol(str,&endp,10);
} while (*endp);
This is the most complete and safest solution I can come up with using scanf:
int option;
int status;
char tailchar;
status = 2;
tailchar = '\n';
do {
if(tailchar != '\n' || status != 2){
for (int c=getchar(); c!='\n' && c!=EOF; c=getchar());
}
printf("Please choose an option:\n"
" 0. Create Database\n"
" 1. Add New Student \n"
" 2. Show All Students \n"
" 3. Delete Student\n"
"99. Exit\n");
status = scanf(" %d%c", &option, &tailchar);
} while (status != 2 || option < 0 || (option >3 && option != 99));
if(tailchar != '\n'){
for (int c=getchar(); c!='\n' && c!=EOF; c=getchar());
}
It only accepts valid options, and cleans any bad input by the user before asking to choose again.
I am trying to use getchar() to remove characters from the input buffer. In the following code, the user is asked to input a choice to select, and then depending on the choice, another input is required, either type int or type char (string).
In the int case, getcar() is not needed and scanf takes in input correctly. But in the char case, scanf fails to get input without using getchar() beforehand. Is there a reason why that is?
printf("Available Ciphers:\n1) Caesar Cipher\n2) Vigenere Cipher\nSelected Cipher: ");
if(scanf("%d", &choice) != 1){
printf("Error: Bad selection!\n");
exit(EXIT_SUCCESS);
} else if (choice != 1 && choice != 2){
printf("Error: Bad Selection!\n");
exit(EXIT_SUCCESS);
//If the choice entered is correct, then run the following.
} else {
if(choice == 1){
printf("Input key as nuumber: ");
if(scanf("%d", &caesarkey) != 1){ //Why is getchar() not needed here?
printf("Error: Bad Key!\n");
exit(EXIT_SUCCESS);
}
//morecode here
} else if (choice == 2){
printf("Input key as string: ");
while(getchar() != '\n'); //Why is this needed here?
/*Uses scanf and not fgets, since we do not want the
key to contain the newline character '\n'. This is
due to the fact that the newline character is not
considered in the function that encrypts and decrypts
plaintext and ciphertext.*/
if(scanf("%[^\n]s", vigencipherkey) != 1){
printf("Error, Cannot read inputted key!\n");
exit(EXIT_SUCCESS);
}
//More code here..
}
}
It seems that you are scanning for a string rather than an int, and as such, you are passing in an int rather than the address of an int.
Change this line
if(scanf("%[^\n]s", vigencipherkey) != 1){
To
if (scanf("%d", &vigencipherkey) != 1) {
In order to read the remainder of the line input by the user, you can use this function:
int flush_line(void) {
int c;
while ((c = getchar()) != EOF && c != '\n')
continue;
return c;
}
Notes:
c must be defined as int to accommodate for all values of the type unsigned char and the special negative value EOF.
you should test for '\n' and EOF otherwise you will have an endless loop on premature end of file without a trailing newline, such as would occur if you redirect the input of your program from an empty file.
you can test for end of file by comparing the return value of flush_line() with EOF.
I have just started off with C programming and while I was trying to write a programme to accept only y or n characters I came across that
#include <stdio.h>
#include <stdlib.h>
int main()
{
char ch;
printf("Do you want to continue\n");
for (;;)
{
ch=getchar();
if (ch=='Y' || ch=='y')
{
printf("Sure!\n");
break;
}
else if (ch=='N'||ch=='n')
{
printf("Alright! All the best!\n");
break;
}
else
{
printf("You need to say either Yes/No\n");
fflush(stdin);
}
}
return(0);
}
When I run this code, and type in any other character other than Y/y or N/n, I receive the last printf statement (You need to say either Yes/No) as output twice.
I understand that this is happening because it considers enter, i.e, '\n' as another character.
Using fflush doesn't help as it's an infinite loop.
How else can I modify it so that the last statement is displayed only once?
You can use a loop to read any characters left using getchar():
ch=getchar();
int t;
while ( (t=getchar())!='\n' && t!=EOF );
The type of ch should int as getchar() returns an int. You should also check if ch is EOF.
fflush(stdin) is undefined behaviour per C standard. Though, it's defined for certain platforms/compilers such as Linux and MSVC, you should avoid it in any portable code.
Another option - use scanf ignoring white spaces.
Instead of ch=getchar();, just need scanf( " %c", &ch );
With this you can also get rid of fflush(stdin);
Like is said in my comment you should use int ch instead of char ch because the return type of getchar which is int.
To clean stdin you could do something like the following:
#include <stdio.h>
#include <stdlib.h>
int main(void){
int ch,cleanSTDIN;
printf("Do you want to continue\n");
for (;;)
{
ch = getchar();
while((cleanSTDIN = getchar()) != EOF && cleanSTDIN != '\n');
if (ch=='Y' || ch=='y')
{
printf("Sure!\n");
break;
}
else if (ch=='N'||ch=='n')
{
printf("Alright! All the best!\n");
break;
}
else
{
printf("You need to say either Yes/No\n");
}
}
return(0);
}
Any way a do while will probably do the job for you:
#include <stdio.h>
#include <stdlib.h>
int main(void){
char ch;
int check;
do {
printf("Do you want to continue: ");
if ((scanf("%c",&ch)) == 1){
while((check=getchar()) != EOF && check != '\n');
if ((ch == 'y') || (ch == 'Y')){
printf("Alright! All the best!\n");
break;
} else if((ch == 'n') || (ch == 'N')){
printf("You choosed %c\n",ch);
break;
}else{
printf("You need to say either Yes/No\n");
}
}else{
printf("Error");
exit(1);
}
}while (1);
return 0;
}
Output1:
Do you want to continue: g
You need to say either Yes/No
Do you want to continue: y
Alright! All the best!
Output2:
Do you want to continue: n
You choosed n
Or we can simply use another break; statement after the last printf().
My teacher has asked me to "Fool proof" my code from any sort of misuse, So I have come up with an
program that can remove any empty values (by disallowing them entirely)
Here is the Un-foolproofed code
#include <stdio.h>
#include <conio.h>
int main()
{
char text[16];
printf("Type something: ");
fgets(text,16, stdin);
printf("You typed: %s",text);
getch();
}
I have made some simple adjustments to ensure there is no error, however, i cannot get the if filter to work properly, as it still allows the NULL input
#include <stdio.h>
#include <conio.h>
int main()
{
char text[16];
int loop;
do
{
printf("Type something: ");
fgets(text,16, stdin);
if( text[0] == '\0')
{
printf("Try again");
system("cls");
loop=1;
}
else
{
loop = -1;
}
}
while(loop > 0);
printf("You typed: %s",text);
getch();
}
I've tried google and i cannot get a solid answer, this probably is some very simple line of code, but sadly i have no idea what it is.
Edit: it's fixed, the if statement should be:
if (text[0] == '\n')
Using the return value from fgets() is the best first step to fool-proofing user I/O.
char text[16];
printf("Type something: ");
if (fgets(text, sizeof text, stdin) == NULL) {
if (feof(stdin)) Handle_stdin_is_closed(); // no more input
if (ferror(stdin) Handle_IOerror(): // very rare event, more common with files
}
// Test is input is is only a '\n'
if (text[0] == '\n')
printf("Try again");
// Look for long line.
size_t len = strlen(text);
if (len + 1 == sizeof text && text[len - 2] != '\n') HandleLongLine();
The next step is to look for scan errors. Let's assume code is to read a long.
errno = 0;
char *endptr;
long = strtol(text, &endptr, 10);
if (errno) Handle_NumericOverflow();
if (text == endptr) Handle_InputIsNotNumeric();
while (isspace((unsigned char) *endptr)) endptr++;
if (*endptr != '\0') Handle_ExtraTextAfterNumber();
Although this is a lot of code, robust handling of hostle user input is best spun off to a helper function where lots of tests can be had.
char * prompt = "Type something: ";
long number;
int stat = GetLong(stdin, prompt, &number); // put all tests in here.
if (stat > 0) Handle_SomeFailure();
if (stat < 0) Handle_EOF();
printf("%ld\n", number);
fgets reads a whole line including the newline into the buffer and 0-terminates it.
If it reads something and then the stream ends, the read line will not have a newline.
If the line does not fit, it won't contain a newline.
If an error occurs before it successfully reads the first character, it returns NULL.
Please read the man-page for fgets: http://man7.org/linux/man-pages/man3/fgets.3.html
According to the fgets() man page
char *fgets(char *s, int size, FILE *stream);
//fgets() returns s on success, and NULL on error or when end of file
//occurs while no characters have been read.
so, you can check the return value of fgets()
n = fgets(text,16, stdin);
if that value is NULL, then nothing have been read.
you can do this by checking the value of n in a for loop,
if( n == NULL)
{
printf("Try again");
system("cls");
loop=1;
}
else
{
loop = -1;
}