I have a program that needs to take input until EOF and because this is an assignment, I am constrained to using scanf. I am doing something like this in the main function:
while (1){
int x;
int s=scanf("%d",&x);
if (x==EOF){
break;
}
if(s==0){
fprintf(stderr, "Non integer value input.");
return 1;
}else if(something){...}
}
return 0;
But if I press ^D, the program goes into an infinite loop instead of exiting with 0 and sometimes I just keep seeing ^D on the terminal. How can I fix this?
Another approach I tried looked something like this:
char c;
if (c==EOF){
break;
}
int x=c+0;
PS I also tried doing while(x!=EOF) and while (!EOF) for the loop but that doesn't work either.
Compare the result returned by scanf to EOF, not the out parameter that receives the typed input value.
That is, replace this:
int s = scanf("%d", &x);
if (x == EOF) {
break;
}
With this:
int s = scanf("%d", &x);
if (s == EOF) {
break;
}
Related
I'm trying to create a program that asks to type something and check if it is an integer. If it is an integer, then print "the integer is ...". Else, print "try again" and waits for another input. However, the program prints an infinite number of "try again" if you type in a character. Here's the source code:
#include <stdio.h>
#include <stdbool.h>
int main()
{
int inp;
bool t = 1;
printf("type an integer\n");
while (t) {
if (scanf("%i", &inp) == 1) {
printf("The integer is %i", inp);
t = 0;
} else {
printf("try again");
scanf("%i", &inp);
}
}
}
OP's code fail to consume the offending non-numeric input. It remains in stdin, for the next input function. As it is unfortunately just another scanf("%i", &inp) which fails the same way - infinite loop.
After attempting to read an int, read the rest of the line.
#include <stdio.h>
#include <stdbool.h>
int main() {
int inp;
int scan_count;
printf("Type an integer\n");
do {
scan_count = scanf("%i", &inp); // 1, 0, or EOF
// consume rest of line
int ch;
while ((ch == fgetchar()) != '\n' && ch != EOF) {
;
}
} while (scan_count == 0);
if (scan_count == 1) {
printf("The integer is %i\n", inp);
} else {
puts("End of file or error");
}
}
An even better approach would read the line of user input with fgets(). Example
When you entered a char, the variable inp in scanf("%d", &inp) would get null, since the input that doesn't match the format string. And the character you input would remain in the buffer, so that's the reason both your scanf would not stop.
A simplest way to fix this is modify your second scanf("%i", &inp); to scanf("%c", &c); (don't forget to declare a char c in your main function).
check here while(t) its in an infinite loop because you have to set a condition for t something like while(t==1) or while(t>1) or (t<1) something like that. saying while(t) means that t can be anything and it will continue to run.
There is nothing in to break the while loop.
consider getting rid of the boolean, and simply using a while (1) loop with a break. Also you should be using "%d" to indicate an integer in scanf/printf. And there is no need for the scanf call in the else, since your program would loop back and call scanf again anyway.
#include <stdio.h>
int main() {
int inp = 0;
printf("type an integer\n");
while (1) {
if (scanf("%d", &inp) == 1) {
printf("The integer is %d", inp);
break;
}
else {
printf("try again");
}
}
return 0;
}
I hope this helped.
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.
'when there is no successful assignments' i know that scanf returns 0 to indicate it, but is that the only thing that it does? this is my code:
#include<stdio.h>
int main(void) {
int val,x;
x=scanf("%d",&val);
if(x==1)
printf("success!");
else{
printf("try again\n");
scanf("%d",&val);
}
return 0;
}
if i enter a number, it works fine but if i enter a character scanf doesn't work anymore, this is what i get:
k
try again
process returned 0 (0x0) execution time :2.578 s
press any key to continue.
_
meaning that it doesn't allow me to enter a new value, why is that? is there something wrong in the code? if yes how can i fix it? should i stop using scanf?
When scanf doesn't work, the invalid data is still left in the stream. You'll have to read and discard the data from the stream first before you can enter more data.
#include<stdio.h>
int main(void) {
int val,x;
x=scanf("%d",&val);
if(x==1)
printf("success!");
else{
// Discard everything upto and including the newline.
while ( (x = getchar()) != EOF && x != '\n' );
printf("try again\n");
scanf("%d",&val);
}
return 0;
}
The scanf family of functions are broken-as-specified and should never be used for anything.
The correct way to write this program is to use getline, if available, or fgets otherwise, to read an entire line of user input. Then use strtol to convert the input to a machine integer, taking care to check for errors:
errno = 0;
result = strtol(line, &endptr, 10);
if (endptr == line || *endptr != '\n' || errno)
// invalid input
I was working on this sample exercise, and everything works as I would like it to, but there is one behavior I don't understand.
When providing input: if I make consecutive invalid entries everything seems to work great. But if I enter a number different from 1,2,3 in the case of the first question, or 1,2 in the case of the second question, the program just sits there until a new input is given. If another invalid entry is made, it goes back to the error "invalid entry" message, and if an appropriate number is entered, everything moves along fine.
I do not understand why it stops to wait for a second input...anyone?
Thanks guys.
#include <stdio.h>
static int getInt(const char *prompt)
{
int value;
printf("%s",prompt);
while (scanf("%d", &value) !=1)
{
printf("Your entry is invalid.\nGive it another try: %s", prompt);
getchar();
scanf("%d", &value);
}
return value;
}
int main() {
int wood_type, table_size, table_price;
printf("Please enter " );
wood_type = getInt("1 for Pine, 2 for Oak, and 3 for Mahogany: ");
printf("Please enter ");
table_size = getInt("1 for large, 2 for small: ");
printf("\n");
switch (wood_type) {
case 1:
table_price = (table_size == 1)? 135:100;
printf("The cost of for your new table is: $%i", table_price);
break;
case 2:
table_price = (table_size == 1)? 260:225;
printf("The cost of for your new table is: $%i", table_price);
break;
case 3:
table_price = (table_size == 1)? 345:310;
printf("The cost of for your new table is: $%i", table_price);
break;
default:
table_price = 0;
printf("The cost of for your new table is: $%i", table_price);
break;
}
}
You most likely need to flush your input buffer (especially with multiple scanf calls in a function). After scanf, a newline '\n' remains in the input buffer. fflush does NOT do this, so you need to do it manually. A simple do...while loop works. Give it a try:
edit:
static int getInt(const char *prompt)
{
int value;
int c;
while (printf (prompt) && scanf("%d", &value) != 1)
{
do { c = getchar(); } while ( c != '\n' && c != EOF ); // flush input
printf ("Invalid Entry, Try Again...");
}
return value;
}
The blank line you get if you enter nothing is the normal behavior of scanf. It is waiting for input (some input). If you want your routine to immediately prompt again in the case the [Enter] key is pressed, then you need to use another routine to read stdin like (getline or fgets). getline is preferred as it returns the number of characters read (which you can test). You can then use atoi (in <stdlib.h>) to convert the string value to an integer. This will give you the flexibility you need.
example:
int newgetInt (char *prompt)
{
char *line = NULL; /* pointer to use with getline () */
ssize_t read = 0; /* number of characters read */
size_t n = 0; /* numer of chars to read, 0 no limit */
static int num = 0; /* number result */
while (printf ("\n %s ", prompt) && (read = getline (&line, &n, stdin)) != -1)
{
if ((num = atoi (line)))
break;
else
printf ("Invalid Input, Try Again...\n");
}
return num;
}
If some invalid input is entered, it stays in the input buffer.
The invalid input must be extracted before the scanf function is completed.
A better method is to get the whole line of input then work on that line.
First, put that input line into a temporary array using fgets(),
then use sscanf() (safer than scanf because it guards against overflow).
#include <stdio.h>
int main(int argc, const char * argv[]) {
char tempbuff[50];
int result, d , value;
do
{
printf("Give me a number: ");
fgets( tempbuff, sizeof(tempbuff), stdin ); //gets string, puts it into tempbuff via stdin
result = sscanf(tempbuff, "%d", &value); //result of taking buffer scanning it into value
if (result < 1){ //scanf can return 0, # of matched conversions,
//(1 in this case), or EOF.
printf("You didn't type a number!\n");
}
}while (result < 1);
//some code
return 0;
}
Knowledge from: http://www.giannistsakiris.com/2008/02/07/scanf-and-why-you-should-avoid-using-it/
I'm Writing a program for Billing System. I'm using do-while loop in my program. And the program is executed according to user input. If the user want to continue the execution, the program will be continue. But I Got a prob in Execution. I was trying my logic in simple do-while loop. The same Problem arises for simple do-while loop also.
Problem is: If the input is yes, the program does not get the further input from user.
That simple do-while loop is:
#include <stdio.h>
main()
{
int c;
char ch;
do
{
printf("enter the no less then 4:");
scanf("%d",&c);
switch(c)
{
case 1:
printf("In 1\n");
break;
case 2:
printf("In 2\n");
break;
case 3:
printf("In 3\n");
break;
}
printf("do u want to continue?:");
ch=getchar();
}while(ch=='y');
}
If i put while(ch != 'n') instead of while(ch=='y') the program working fine. I couldn't understand the problem behind this. Please Help me to rectify this. And Explain about this problem.Thank u in advance.
first run, 3 is printed, user types "y" and presses return
getchar() reads 'y' and program loops
second time, getchar() reads newline character from the previous key press
newline is not 'y' so program does not loop
Several problems:
getchar returns an int, not a char, so ch must be an int just like c.
scanf needs a pointer to go with the %d, so it should be scanf("%d", &c);
The while should rather test for EOF, as in while ((ch = getchar()) != EOF)
Note that the input will contain the newlines, which you should deal with (e.g. ignore).
This should be quite robust:
#include <stdio.h>
int main(void)
{
int c, ch;
for (;;) {
printf ("Enter a number (1, 2 or 3):");
fflush (stdout);
if (scanf ("%d", &c) == 1) {
switch (c) {
case 1:
printf ("In 1\n");
break;
case 2:
printf ("In 2\n");
break;
case 3:
printf ("In 3\n");
break;
}
printf ("Do you want to continue? [y/n]:");
fflush (stdout);
while ((ch = getchar ())) {
if (ch == 'y')
break;
else if (ch == 'n' || ch == EOF)
return 0;
}
} else {
printf ("That was not a number. Exiting.\n");
return 0;
}
}
}
While(ch=='y') or the character whatever in while() it will sent to case 3 as per your coding....y is pressing ,it wil sent to case 3 otherwise it wont work
Instead of reading the answer using getchar, use fgets.
As others explained, the second getchar call gives you the newline, which was typed after the first y.
With fgets, you'll get everything the user typed. Then you can check if it's y (just check the first character, or use strcmp).