So I am trying to write a program that will let me read a user input for data on an MP3 file using a doubly linked list data structure. I got most of the methods and functions to work, but when I am prompting the user to put in input it prints out two lines before the user can input for the first line. So for example
int main()
{
int user_input = 0;
while(!(user_input >= 4))
{
struct MP3_data_node* MP3_data;
MP3_data = (struct MP3_data_node*)malloc(sizeof(struct MP3_data_node));
printf("\nPlease select a number for one of the following instructions:\n");
printf("0: add to list\n1: delete from list\n2: print the list from beginning to end\n3: print the list from end to beginning\n4: exit\n");
scanf("%d", &user_input);
if(user_input == 0)
{
printf("Please provide the artist:");
fgets(MP3_data->artist,50,stdin);
printf("Please provide the album:");
fgets(MP3_data->artist,50,stdin);
printf("Please provide the song title:");
fgets(MP3_data->artist,50,stdin);
printf("Please provide the year the song was released: ");
scanf("%d", &MP3_data->yearReleased);
printf("Please provide the length of the song in seconds: ");
scanf("%d", &MP3_data->runTime);
addToList(MP3_data);
}
...
So it prints out "Please provide the artist:Please provide the album:" and then let's me put the input in, so my question is how do I make it so that it prints:
Please provide the artist: (user input)
Please provide the album: (user input)
etc.
You're doing the right thing (fgets) int the first few prompts, then you switch to scanf which is the source of your problem. Use fgets (and strtol) instead of scanf and you will be fine. (And, the first scanf which causes the problem described in your question.)
The problem is that scanf only reads the digit part of whatever you enter. That means if you type 12Enter, then the scanf reads the 1 and 2 but leaves the Enter in the input buffer for the next call to fgets or scanf. On the other hand, fgets reads everything you type including the Enter, avoiding this problem.
Related
I am working on my assignment and this is the issue that I bumped into. In the assignment, it says that the input value for the middle initals should be this - "L. A.". However, once I run my program it prints some printf functions on the same line, skipping the scanf function. I have went through a lot of topics about that " %c" issue, but I still can not make my program run properly. Some of the variables are from .h file. The actual assignment is bigger, however it is pretty much repetative so I thought if I figure out how to fix this certain issue I will be able to finally finish my assignment.
int main(void){
// Declare variables here:
char ch;
struct Name FullName = { {'\0'} };
struct Address AddressInfo = { 0, '\0', 0, '\0', '\0' };
struct Numbers PhoneInfo = { {'\0'} };
// Display the title
printf("Contact Management System\n");
printf("-------------------------\n");
// Contact Name Input:
printf("Please enter the contact’s first name: ");
scanf("%s", &FullName.firstName);
printf("Do you want to enter a middle initial(s)? (y or n): ");
scanf(" %c", &ch);
if (ch == 'y') {
printf("Please enter the contact’s middle initial(s): ");
scanf(" %s", FullName.middleInitial);
}
printf("Please enter the contact’s last name: ");
scanf(" %s", &FullName.lastName);
// Contact Address Input:
printf("Please enter the contact’s street number: ");
scanf("%d", &AddressInfo.streetNumber);
OUTPUT (I have highlighted input values):
Contact Management System
-------------------------
Please enter the contactÆs first name: *Artem*
Do you want to enter a middle initial(s)? (y or n): *y*
Please enter the contactÆs middle initial(s): *L. A.*
Please enter the contactÆs last name: Please enter the contactÆs street number:
The %s format specifier reads a sequence of characters terminated by whitespace. When you enter L. A., only L. gets read into middleInitial because it stops reading at the space and A. is left in the input buffer. On the next scanf, it immediately reads those buffered characters so it doesn't stop to prompt for anything.
The simplest way to handle this is to leave out the space when inputting, i.e. L.A.. If you want to support whitespace, you'll want to get rid of scanf entirely and read everything a full line at a time using fgets. Note that fgets also reads in the trailing newline, so you'll need to strip that out.
My problem is as follow: I need scanf to read multiple lines of user input. The user can input any combination of characters separated by whitespace or newline characters. My objective is to take each input that is separated by whitspace or newlines, and process it and output via printf. However, I am not allowed to store multiple inputs at once. My issues is this: every time the user presses enter, the scanf reads the entire line since I have scanf in a while loop != EOF. However, I don't want the scanf to read all lines of input until the user presses EOF. Is this possible? Here are some examples below:
Some pseudo code:
User Input:
cat dog mouse rabbit
snake dog
pink bob
joke*/
//Some pseudo code:
char input[100];
while (scanf("%s", input) != EOF) {
printf("%s", input);
}
In summary, I don't want scanf to read when the user presses enter since if it does, then the printf will activate but still allow the user to input stuff in.
Is there a way to bypass this? Note: I have to use scanf and I can't store multiple separate inputs (like dog or cat) in a single array.
Thank you.
Just use a 2-dimensional array, where each row stores a line of the input string. And as for accepting the input, ask the user to enter the number of lines which he/she would be giving and just loop through it.
#include<stdio.h>
void main(){
char inp[10][50];
int i,j,n;
printf("\nenter the number of lines: ");
scanf("%d",&n);
for(i=0;i<n;i++){
scanf(" %[^\n]s",inp[i]);
}
//for printing it out.. follow the same with printf
printf("\nthe strings are:\n");
for(i=0;i<n;i++){
printf("%s\n",inp[i]);
}
}
This question already has answers here:
How do you allow spaces to be entered using scanf?
(11 answers)
Closed 6 years ago.
I'm trying to run the following code in the basic ubuntu gcc compiler for a basic C class.
#include<stdio.h>
struct emp
{
int emp_num, basic;
char name[20], department[20];
};
struct emp read()
{
struct emp dat;
printf("\n Enter Name : \n");
scanf("%s", dat.name);
printf("Enter Employee no.");
scanf("%d", &dat.emp_num);
//printf("Enter department:");
//fgets(dat->department,20,stdin);
printf("Enter basic :");
scanf("%d", &dat.basic);
return dat;
}
void print(struct emp dat)
{
printf("\n Name : %s", dat.name);
printf("\nEmployee no. : %d", dat.emp_num);
//printf("Department: %s", dat.department);
printf("\nBasic : %d\n", dat.basic);
}
int main()
{
struct emp list[10];
for (int i = 0; i < 3; i++)
{
printf("Enter Employee data\n %d :\n", i + 1);
list[i] = read();
}
printf("\n The data entered is as:\n");
for (int i = 0; i < 3; i++)
{
print(list[i]);
}
return 0;
}
I want the name to accept spaces.
The problem comes when I'm entering the values to the structures. I am able to enter the name the first time but the subsequent iterations don't even prompt me for an input.
I've tried using fgets, scanf("%[^\n]",dat.name) and even gets() (I was desperate) but am the facing the same problem every time.
The output for the 1st struct is fine but for the rest is either garbage, the person's last name or just blank.
Any ideas?
When reading a string using scanf("%s"), you're reading up to the first white space character. This way, your strings cannot include spaces. You can use fgetsinstead, which reads up to the first newline character.
Also, for flushing the input buffer, you may want to use e.g. scanf("%d\n") instead of just scanf("%d"). Otherwise, a subsequent fgets will take the newline character and not ask you for input.
I suggest that you experiment with a tiny program that reads first one integer number and then a string. You'll see what I mean and it will be much easier to debug. If you have trouble with that, I suggest that you post a new question.
The problem is that scanf("%[^\n",.. and fgets don't skip over any whitespace that may be left over from the previous line read. In particular, they won't skip the newline at the end of the last line, so if that newline is still in the input buffer (which it will be when the last line was read with scanf("%d",..), the scanf will fail without reading anything (leaving random garbage in the name array), while the fgets will just read the newline.
The easiest fix is to add an explicit space in the scanf to skip whitespace:
printf("\n Enter Name : \n");
scanf(" %19[^\n]", dat.name);
This will also skip over any whitespace at the beginning of the line (and blank lines), so may be a problem if you want to have a name that begins with a space.
Note I also added a length limit of 19 to avoid overflowing the name array -- if the user enters a longer name, the rest of it will be left on the input and be read as the employeee number. You might want to skip over the rest of the line:
scanf("%*[^\n]");
This will read any non-newline characters left on the input and throw them away. You can combine this with the prior scanf, giving you code that looks like:
printf("\n Enter Name : ");
scanf(" %19[^\n]%*[^\n]", dat.name);
printf("Enter Employee no. : ");
scanf("%d%*[^\n]", &dat.emp_num);
printf("Enter department : ");
scanf(" %19[^\n]%*[^\n]", dat.department);
printf("Enter basic : ");
scanf("%d%*[^\n]", &dat.basic);
This will ignore any spurious extra stuff that someone enters on a line, but will still have problems with someone entering letters where numbers are expected, or end-of-file conditions. To deal with those, you need to be checking the return value of scanf.
What you have tried was:-
scanf("%[^\n]",dat.name)
In this you forgot to specify the specifier.
You can try to use this:-
scanf ("%[^\n]%*c", dat.name);
or fgets() if you want to read with spaces.
Note:- "%s" will read the input until whitespace is reached.
Beginner with C here. I am trying to run a loop where strings and ints are entered into various fields of a struct. When prompted for a 'last name', the user can press enter with no other input and the loop should end.
The problem is that with this code, the loop doesnt end (last name and first name entry requests run together on the same line) and the value for salary always comes out wrong (0 or some large number)
while (employee_num <= 2)
{
printf("Enter last name ");
fgets(employee[employee_num].last_name, sizeof(employee[employee_num].last_name), stdin);
if(strlen(employee[employee_num].last_name) == 0)
break;
printf("Enter first name ");
fgets(employee[employee_num].first_name, sizeof(employee[employee_num].first_name), stdin);
printf("Enter title ");
fgets(employee[employee_num].title, sizeof(employee[employee_num].title), stdin);
printf("Enter salary ");
fgets(strng_buffer, 1, stdin);
sscanf(strng_buffer, "%d", &employee[employee_num].salary);
++employee_num;
getchar();
}
If I try this code instead, I am able to exit the loop properly after the first run through it, but cannot exit after that (by pressing enter at the last name portion - perhaps a \n I cant seem to clear?):
char strng_buffer[16];
while (employee_num <= 5)
{
printf("Enter last name ");
fgets(strng_buffer, sizeof(strng_buffer), stdin);
sscanf(strng_buffer, "%s", employee[employee_num].last_name);
if(strlen(employee[employee_num].last_name) == 0)
break;
printf("Enter first name ");
fgets(strng_buffer, sizeof(strng_buffer), stdin);
sscanf(strng_buffer, "%s", employee[employee_num].first_name);
printf("Enter title ");
fgets(strng_buffer, sizeof(strng_buffer), stdin);
sscanf(strng_buffer, "%s", employee[employee_num].title);
printf("Enter salary ");
scanf("%d", &employee[employee_num].salary);
++employee_num;
getchar();
}
I am curious as to how to make this work as intended and what best practice would be for entries like this (ie use of sscanf, fgets, etc)
Thanks in advance!
The Loop breaks prematurely when it encounters the break statement
if(strlen(strng_buffer) == 0)
break;
The uninitialized character buffer strng_buffer, coincidently has null as the first character causing strlen to return 0
I believe you may have intended
if(strlen(employee[employee_num].last_name) == 0)
break;
as the loop terminatorm, and it was a typo in your part causing premature loop exit.
Assuming the fix mentioned by Abhijit, why transform the first into the second? Are you aware that the second behaves differently to the first, because of the addition of sscanf? If your intention was to shorten the first, the second seems quite bulky. Rather than adding sscanf to the situation, why not shorten the first by declaring a struct employee *e = employee + employee_num; and using that repetitively, instead of employee[employee_num]?
One "best practise" regarding fgets is to check it's return value. What do you suppose fgets might return, if it encounters EOF? What do you suppose fgets would return if it's successful?
One "best practise" regarding scanf is to check it's return value. In regards to the return value of scanf, I suggest reading this scanf manual carefully and answering the following questions:
int x = scanf("%d", &employee[employee_num].salary); What do you suppose x will be if I enter "fubar\n" as input?
Where do you suppose the 'f' from "fubar\n" will go?
If it's ungetc'd back onto stdin, what would your next employee's last name be?
int x = scanf("%d", &employee[employee_num].salary); What do you suppose x will be if I run this code on Windows and press CTRL+Z to send EOF to stdin?
int x = scanf("%d %d", &y, &z); What would you expect x to be, presuming scanf successfully puts values into the two variables y and z?
P.S. EOF can be sent through stdin in Windows by CTRL+Z, and in Linux and friends by CTRL+D, in addition to using pipes and redirection to redirect input from other programs and files.
The problem is that fgets returns the string with the line break (\n) included. So, even the user presses return without entering info, the string won't be empty. Also, your buffer size for salary is too small.
So, either you strip out the \n on every fgets or you change your check to:
if(strlen(employee[employee_num].last_name) == 1) break;
Also, when you're getting the buffer, change 1 to something bigger, like
fgets(strng_buffer, 10, stdin);
However, if you do want to strip out the \n from each fgets, you can do something like:
employee[employee_num].last_name[strlen(employee[employee_num].last_name)-1] = 0;
You can do this for every string or, better yet, create a function that does it.
EDIT: if you can guarantee that the user will press enter after each input then you can safely assume this. However if it's not always the case it's possible that the last character is not \n and just stripping this way might cause problems.
The following is the code for a simple menu. If the user enters an invalid option (4 or 'o') the menu should be reprinted displaying an error message. The code works when the user inputs a number, however it fails (looping indefinitely) when the user inputs a letter or string into option. My guess is that memory is being overwritten in the looping cases generating the strange behavior. Could someone help me to fix it?
Thank You
int inmenu = 1;
while (inmenu){
//Menu
printf("User: %s %s\n", user.firstname, user.lastname);
printf("0) Exit\n1) List Friends\n2) Add Friend\n3) Delete Friend\nWhat would you like to do? ");
int option;
scanf("%i", &option);
if(option == 0)
inmenu = 0;
else if (option == 1)
defaultPrint(friends, numfriends, NULL);
else if (option == 2){
//addfriend
char *name = (char *) malloc(sizeof(char) * 256);
int birthdate;
printf("Enter first name: ");
scanf("%s", name);
printf("Enter last name: ");
scanf("%s", name);
printf("Enter birthdate: ");
scanf("%i", &birthdate);
}
else if (option == 3){
//deletefriend
defaultPrint(friends, numfriends, NULL);
int n;
printf("What friend would you like to delete? ");
scanf("%i", &n);
}
else
printf("ERROR: Invalid option %i\n", option);
}
Test input:
0) Exit
1) List Friends
2) Add Friend
3) Delete Friend
What would you like to do? 4
ERROR: Invalid option 4
0) Exit
1) List Friends
2) Add Friend
3) Delete Friend
What would you like to do?
(correct behavior)
0) Exit
1) List Friends
2) Add Friend
3) Delete Friend
What would you like to do? o
What would you like to do? ERROR: Invalid option 4
0) Exit
1) List Friends
2) Add Friend
3) Delete Friend
...
(incorrect behavior keeps printing the same last 5 lines in an infinite loop)
Why use scanf? How about use getchar, check the range, see if it's 0-4, and if not, error message.
The cause of your problem is that scanf() doesn't consume the input stream if it doesn't match the requested format.
This means that when you call:
scanf("%i", &option);
If the user enters something other than a number (like "o"), then that user input remains in the input stream - so when you loop around and call scanf() again, the "o" is still there, and still doesn't match the requested format.
This is why scanf() is not particularly suitable for taking user input - it's designed for consuming well-formatted data files. Instead, since you have a line-oriented user interface, you should read an entire line from the user with fgets(), then try parsing it with sscanf(). If it doesn't parse correctly, you can report an error and then read a new line from the user.
When you enter a number and press enter, scanf reads the number but leaves the newline in the buffer, when the loop loops, scanf will read the newline from the buffer, see that it doesn't match the format, and leave it there. This will repeat forever.
A quick fix could be to add the newline to the format:
scanf("%i\n", &number);
but this will fail if the user doesn't enter exactly the data you expect, newline isn't the only thing that will cause the behaviour, anything that doesn't match the format is left in the buffer.