Okay so I am doing an assignment for a class in which I have to read the 4 colour bands on a resistor and determine its resistance value. In order to do this I wrote code first asking the user to input the letter which corresponded to a specific band colour. For example R corresponds to red. Here is the code I wrote:
char band1, band2, band3, band4;
float firstband, secondband, thirdband, fourthband;
float value1, value2, value3, value4, finvalue;
printf("Please enter the 1st band: ");
scanf("%c", &band1);
printf("Please enter the 2nd band: ");
scanf("%c", &band2);
printf("Please enter the 3rd band: ");
scanf("%c", &band3);
printf("Please enter the 4th band: ");
scanf("%c", &band4);
The problem is that the when the user is asked to input the letters the request gets jumbled up. It gives me this:
Please enter first band:
Please enter the second band: Please enter the third band:
Please enter the fourth band: Resistance is ...
Only the first band value and the third band value are read, the second and fourth are skipped. I can't seem to find the error in the code.
You are feeding scanf references to single char variables. You seem to want to input strings, which are arrays of char.
Looks like this:
char aString[20];
printf("Enter a string, no more than 19 characters: ");
scanf("%s", aString);
It says "no more than 19 characters" because strings in C are null-terminated.
This is however just written in an easy way to understand. It's error-prone though. As pointed out in the comments, this would be more robust:
if (1 != scanf("%19s", aString))
{
Handle_problem();
}
You didn't do any error checking or input clearing. As a result, the next scanf call "eats up" the invalid characters from the previous one resulting in the call seeming to be "skipped".
In order to fix this, you can use the following function to clear the input buffer (which is what scanf reads from first):
void clear_input() {
int ch = 0;
while(( ch = getchar) != '\n' && ch != '\0' && ch != EOF );
}
Also, it is a good idea to get into the habit of checking scanf's return value; this ensures valid input is given.:
int check = -1;
do {
check = scanf( "%c", &variable );
clear_input();
} while (check != 1);
An alternative to scanf if you only need a single character might be to use getchar( void ). getchar() obtains a single character from the user and returns the value of that character. Even if you take this route, you still have to clear the input buffer.
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.
getchar() is not working in the below program, can anyone help me to solve this out. I tried scanf() function in place of getchar() then also it is not working.
I am not able to figure out the root cause of the issue, can anyone please help me.
#include<stdio.h>
int main()
{
int x, n=0, p=0,z=0,i=0;
char ch;
do
{
printf("\nEnter a number : ");
scanf("%d",&x);
if (x<0)
n++;
else if (x>0)
p++;
else
z++;
printf("\nAny more number want to enter : Y , N ? ");
ch = getchar();
i++;
}while(ch=='y'||ch=='Y');
printf("\nTotal numbers entered : %d\n",i);
printf("Total Negative Number : %d\n",n);
printf("Total Positive number : %d\n",p);
printf("Total Zero : %d\n",z);
return 0 ;
}
The code has been copied from the book of "Yashvant Kanetkar"
I think, in your code, the problem is with the leftover \n from
scanf("%d",&x);
You can change that scanning statement to
scanf("%d%*c",&x);
to eat up the newline. Then the next getchar() will wait for the user input, as expected.
That said, the return type of getchar() is int. You can check the man page for details. So, the returned value may not fit into a char always. Suggest changing ch to int from char.
Finally, the recommended signature of main() is int main(void).
That's because scanf() left the trailing newline in input.
I suggest replacing this:
ch = getchar();
With:
scanf(" %c", &ch);
Note the leading space in the format string. It is needed to force scanf() to ignore every whitespace character until a non-whitespace is read. This is generally more robust than consuming a single char in the previous scanf() because it ignores any number of blanks.
When the user inputs x and presses enter,the new line character is left in the input stream after scanf() operation.Then when try you to read a char using getchar() it reads the same new line character.In short ch gets the value of newline character.You can use a loop to ignore newline character.
ch=getchar();
while(ch=='\n')
ch=getchar();
When you using scanf getchar etc. everything you entered stored as a string (char sequence) in stdin (standard input), then the program uses what is needed and leaves the remains in stdin.
For example: 456 is {'4','5','6','\0'}, 4tf is {'4','t','f','\0'} with scanf("%d",&x); you ask the program to read an integer in the first case will read 456 and leave {'\0'} in stdin and in the second will read 4 and leave {''t','f',\0'}.
After the scanf you should use the fflush(stdin) in order to clear the input stream.
Replacing ch = getchar(); with scanf(" %c", &ch); worked just fine for me!
But using fflush(stdin) after scanf didn't work.
My suggestion for you is to define a Macro like:
#define __GETCHAR__ if (getchar()=='\n') getchar();
Then you can use it like:
printf("\nAny more number want to enter : Y , N ? ");
__GETCHAR__;
I agree that it is not the best option, but it is a little bit more elegant.
Add one more line ch = getchar();
between scanf("%d",&x); and ch = getchar();
then your code work correctly.
Because when you take input from user, in this time you press a new line \n after the integer value then the variable ch store this new line by this line of code ch = getchar(); and that's why you program crash because condition can not work correctly.
Because we know that a new line \n is also a char that's why you code crash.
So, for skip this new line \n add one more time ch = getchar();
like,
ch = getchar(); // this line of code skip your new line when you press enter key after taking input.
ch = getchar(); // this line store your char input
or
scanf("%d",&x);
ch = getchar(); // this line of code skip your new line when you press enter key after taking input.
pieces of code work correctly.
I am new to C this is something I have always been confused about
let's say I have a code like this
I only want to use char
char a, b, c;
printf("input first character: ");
scanf(" %c", &a);
printf("input second character: ");
scanf(" %c", &b);
printf("input thrid character: ");
scanf(" %c", &c);
how ever I want to be able to read in space as well; I noticed how this would only read in non-space characters, what if I want to read space as well something like this c=' '; how do I scan this space in;
now by listening to suggestion of using getchar() I wrote this :
#include<stdio.h>
int main(void)
{
char a,b,c;
printf("input the first char:");
a=getchar();
printf("input the second char:");
b=getchar();
printf("input the third char:");
c=getchar();
return 0;
}
how ever something strange happens when I compile and run the program
the program output is like this
input the first char:
input the second char:input the third char:
now it never let me to input the second char it jumped straight to the third request at the end I was only asked to enter 2 inputs which is very strange because the program clearly asked for 3 in the code.
now here is a program I wrote like this I added what is suggested into the code block
int main(void)
{
int totalHeight=0, floorWidth=0, amountOfStories, amountWindowForTop, amountWindowForMiddle, amountWindowForBottom, windowHeight, middleWindowWidth, topWindowWidth, bottomWindowWidth, minimumHeight, minimumWidth;
int betweenDistanceTop, betweenDistanceMiddle, betweenDistanceBottom, edgeDistanceTop, edgeDistanceBottom, edgeDistanceMiddle;
char topFloorWindowContent, middleFloorWindowContent, bottomFloorWindowContent, windowBorder, floorBorder;
int tempMax, tempValue, tempSideDistance, tempBetweenDistance;
printf("please enter how many stories your building would like to have: ");
scanf("%d",&amountOfStories);
minimumHeight=amountOfStories*6+1;
while((totalHeight<minimumHeight)||((totalHeight%amountOfStories)!=1))
{
printf("please enter the totalHeight (minimum %d): ",minimumHeight);
scanf("%d",&totalHeight);
}
printf("please enter how many window building would have for top floor: ");
scanf("%d",&amountWindowForTop);
printf("please enter how many window building would have for middle floors: ");
scanf("%d",&amountWindowForMiddle);
printf("please enter how many window building would have for bottom floor: ");
scanf("%d",&amountWindowForBottom);
tempMax=amountWindowForTop;
if (tempMax<amountWindowForMiddle)
{
tempMax=amountWindowForMiddle;
}
if (tempMax<amountWindowForBottom)
{
tempMax=amountWindowForBottom;
}
while(floorWidth<tempMax)
{
printf("please enter the width of the building (Minimum %d): ",tempMax*4+1);
scanf("%d",&floorWidth);
}
char a, b, c;
printf("a:");
a=getchar();getchar();
printf("b:");
b=getchar();getchar();
printf("c:");
c=getchar();
printf("a=%c, b=%c, c=%c", a, b, c);
return 0;
}
now here is the funny part if I put this block of code in the big program it doesn't work the output is something like this
please enter how many stories your building would like to have: 2
please enter the totalHeight (minimum 13): 2
please enter the totalHeight (minimum 13): 2
please enter the totalHeight (minimum 13): 13
please enter how many window building would have for top floor: 2
please enter how many window building would have for middle floors: 2
please enter how many window building would have for bottom floor: 2
please enter the width of the building (Minimum 9): 9
a:
b:*
c:a=
, b=
, c=
as we can see a b c all read in \n instead of the space * and c didn't even read anything at all Why is that ?
The problem with your code is this: when you read the first char (a), you press enter (\n) for insert the next char, so now on stdin there is a \n that you haven't readed. When you try to read the next character, (b) the program read the previous \n from stdin and does not allow you to read the next char. So, when you read a char with getchar() and then press enter on the keyboard, you need a second getchar() for remove the \n.
Here is a sample code that could solve your issue:
#include<stdio.h>
int main(void) {
char a, b, c;
printf("a:");
a=getchar();getchar();
printf("b:");
b=getchar();getchar();
printf("c:");
c=getchar();
printf("a=%c, b=%c, c=%c", a, b, c);
return 0;
}
For the edited you posted, you need to put what is called "the stdin cleaner" before taking the value for a,b,c:
while(getchar()!='\n');
it just reomove all characters till \n.
Please, take note that when programs like the one you posted has a lot of input from keyboard, sometime you get this issue because there are extra chars in stdin. So the general answer for this issue will be try to figure out where these extra chars (mostly there is an extra \n somewhere) could be and use a function like the one i mentioned to remove so that you can continue reading from stdin.
You should use getchar()
a = getchar();
scanf will not scan anything until you give any data so it is not useful to scan ' ' char.
for this you have to use getchar() fun for char or gets() for string, it will scan data until you give enter. it will comes out even if either you have not provided any char, or simple ' ' char.
If u want to read the character with space then you may use the gets() functtion.
char str[10];
str=gets();
try scanf("%c", &ch).
as stated in scanf format specifier:
"Whitespace character: the function will read and ignore any whitespace characters encountered before the next non-whitespace character (whitespace characters include spaces, newline and tab characters -- see isspace). A single whitespace in the format string validates any quantity of whitespace characters extracted from the stream (including none)."
getchar() gets unexpected result because in windows, newline(when you hit Enter in console) is two characters.
I need to use scanf to get a character and a string which would store the user's answer. (yes/no)
The code below skips scanf("%c", &elem).
while ( !strcmp ("yes", option))
{
printf("enter the elements \n\n");
elem=getchar();
printf("you have entered %c\n",elem);
enqueue(st, elem);
printf("please enter yes or no ");
scanf("%s[^\n]",option);
}
./out
enter the elements
a
you have entered a
enqueue elem= a
please enter yes or no yes
enter the elements
you have entered
enqueue elem=
You don't have any scanf("%c", &elem) in your code... btw the problem is with the enter for scanf. When you get an input by scanf, an enter character stays in the input buffer which will be read by your getchar() function in the second round. one simple way to solve it is to add a dummy getchar after your scanf line:
while ( !strcmp ("yes",option))
{
printf("enter the elements \n\n");
elem=getchar();
printf("you have entered %c\n",elem);
enqueue(st,elem);
printf("please enter yes or no ");
scanf("%s[^\n]",option);
getchar();
}
You can find more information about how to clear your input buffer here: How to clear input buffer in C?
I can recommend you consider two things:
For getting only a character, I personally found it much more easier to use getch and getche function in Windows, and equivalent of them for GCC-compatible environments. You can find samples of it online or on this line [What is Equivalent to getch() & getche() in Linux?
Always flush the input buffer after you read your input to prevent any similar problems to happen.
The input functions check the input buffer, which you can find at 0xb8000000, and check the first input there. If the buffer is empty, they wait for the user to enter the input, otherwise, they check the first element in the buffer and then examine that to what they expect to read. If they succeed, they read it and remove it from buffer. Otherwise, they fail to give you your input and depending on the function, the result is different.
For Example, consider the following line:
scanf("%d %d %f", &a, &b &c);
and give the input as:
a 2 4
The scanf will return 0, which means it reads zero inputs so 'a', 2, and 4 remains in your buffer. So your buffer looks like: [a, 2, 4]. As a result if you add the following line:
scanf("%c", &ch);
scanf will try to get a character from the buffer, and it reads character 'a' and put it in variable ch. So it doesn't get any input from user. And you end up with having 2 and 4 on your buffer again.
When you are pressing Enter/Return key to enter the element then a \n character is also passed to the buffer along with the element. This \n is read by your getchar on next call.
To consume this \n place this line after the getchar();
int ch;
while((ch = getchar()) != EOF && ch != '\n');
Take care mixing scanf() format specifiers "%c", "%s" and "%[]".
Correct usage of "%[^\n]": there is no s. If leading whitespace in not wanted to be saved, include a leading space as in " %[^\n]"
char option[100];
// scanf("%s[^\n]", option);
scanf(" %[^\n]", option);
// or better
scanf(" %99[^\n]", option);
// or pedantic
switch (scanf(" %99[^\n]", option)) {
case EOF: HandleEOForIOError(); break;
case 0: HandleNoData(); break; // might not be possible here.
case 1: HandleSuccess();
Correct usage of "%c". If leading whitespace in not wanted to be save, include a leading space as in " %c". This may be the case in OP's code so the preceding inputs Enter or '\n' is consumed.
char elem;
scanf(" %c", &elem);
Correct usage of "%s". Leading whitespace is not saved with or without a leading space.
char option[100];
scanf("%99s", option);
// Following works the same.
scanf(" %99s", option);
New here and I have a very simple question. I am making a simple program in C that requires the user to enter a choice of what to do with a char. After they enter the result, the program goes back to the menu. However it seems to take some sort of ghost input as if the char has some unknown value. I need to set the char back to its default state.
Code:
/* display menu for user */
void menu() {
printf("\n- - - Phone Book Database - - -\n");
printf("\nSelect an action:\n\n");
printf("\tc:\tCreate a database entry.\n");
printf("\ts:\tSearch the database entries.\n");
printf("\td:\tDelete a database entry.\n");
printf("\tq:\tQuit program.\n\n");
printf("Enter choice: ");
menu_choice = getchar();
if(menu_choice != 'c' && menu_choice != 's'
&& menu_choice != 'd' && menu_choice != 'q') {
printf("\n\n\tInvalid choice.\n");
menu();
}
//fflush(stdin);
}
Here is an example output:
- - - Phone Book Database - - -
Select an action:
c: Create a database entry.
s: Search the database entries.
d: Delete a database entry.
q: Quit program.
Enter choice: c
Enter name: test
Enter address: test
Enter number: 3
- - - Phone Book Database - - -
Select an action:
c: Create a database entry.
s: Search the database entries.
d: Delete a database entry.
q: Quit program.
Enter choice:
Invalid choice.
- - - Phone Book Database - - -
Select an action:
c: Create a database entry.
s: Search the database entries.
d: Delete a database entry.
q: Quit program.
Enter choice: q
Entering c as an input calls the following function
/* creates a new record in array */
void create_record() {
char name[MAX];
char address[MAX];
int number;
rec_num++; /* add 1 to marker for record placement */
printf("\nEnter name: ");
scanf("%s", name);
printf("\nEnter address: ");
scanf("%s", address);
printf("\nEnter number: ");
scanf("%d", &number);
strcpy(record[rec_num].name, name);
strcpy(record[rec_num].address, address);
record[rec_num].number = number;
}
Looks like you have your answer already, and reviewing, it is correct; using getchar() will read one character from stdin
printf("Enter choice: ");
menu_choice = getchar();
When I get to this prompt at the console and type c<enter key> it's causing two char's to go to stdin: 'c' and '\n'
The first time getchar(); runs, it picks up just the 'c' leaving the newline character. On the second iteration the '\n' is picked up without waiting on anything from the user; thus is seems as though there is "ghost input".
I just wanted to point out, whenever you're getting input from the user and the results aren't what you've expected it doesn't hurt to just dump it back to verify what's happening, something like:
if(menu_choice != 'c' && menu_choice != 's'
&& menu_choice != 'd' && menu_choice != 'q') {
printf("\n\n\tYou entered %c (%d).\n", menu_choice, menu_choice);
printf("\tThat's an invalid choice.\n");
menu();
}
Would have shown you:
You entered
(10).
That's an invalid choice.
Between the fact it was on a different line, and checking that decimal result against an ASCII table you'd see that 1010 is the newline character, which could help lead you to a result.
Edit:
1 option for consuming a newline -
printf("what character?");
c = getchar(); // Get the character from stdin
getchar(); // consume the newline character that was left
a second option:
printf("what character?");
scanf("%c ", &c); // Get the character from stdin, note the space after the %c
// that will allow any special characters including \t and \n
If you just want to get the character and then end at & consume the '\n', you have to add and escape the newline char:
printf("what character?");
scanf("%c\\n", &c);
As others have pointed out, it's the newline character. Just add another
(void) getchar(); /* read and discard newline */
where ever you only want to read one character.
The return char '\n' is probably being left in stdin so that next time you come around it getchar() is fetching this.
Try looking at the value that is fetched with getchar() either with a debugger or just by printing it out.
It may be that getchar() isn't a great choice for your application and that scanf() would do a better job, that way all of stdin will be fetched and it should be empty next time you come around. Obviously you will need to provide scanf with more than a single char to write too, as in your other code.
You can then simply look at the first char in the array to get your char.
A stray character, almost certainly '\n', is left on your input buffer after your input commands.
To remove it, you can add:
fflush(stdin)
...right before your getchar() call.