Here Is code of Basic Structure , But the output is not as expected. There is three scanf function but only two is being executed. Middle one contains garbage value.
#include <stdio.h>
int main()
{
struct book
{
char name;
float price;
int pages;
};
struct book b1,b2,b3;
printf("Enter names , prices & no of pages of 3 books\n");
scanf("%c%f%d",&b1.name,&b1.price,&b1.pages);
scanf("%c%f%d",&b2.name,&b2.price,&b2.pages);
scanf("%c%f%d",&b3.name,&b3.price,&b3.pages);
printf("And this is what you entered\n");
printf("%c%f%d",b1.name,b1.price,b1.pages);
printf("%c%f%d",b2.name,b2.price,b2.pages);
printf("%c%f%d",b3.name,b3.price,b3.pages);
return 0;
}
Simply change
scanf("%c%f%d", &bx.name, &bx.price, &bx.pages);
To
scanf(" %c%f%d", &b1.name, &b1.price, &b1.pages);
After you press Enter, a '\n' is left in stdin, which will later be consumed by "%c". Having read a character('\n'), scanf() expects a floating-point number, as "%f" in the format string indicates. However, instead of getting that desired floating-point number, it meets a character, and it returns sadly. As a result, &bx.price and &bx.pages are not updated, so they remain uninitialised, giving you the garbage values.
With a leading space in scanf(), all whitespace characters(if any) are discarded before reading starts. Since \n is discarded, following reading process will(presumably) be successful.
Also, just a tip: Always check the return value of scanf(), because you'll never know that stuff users will input.
Example code:
#include <stdio.h>
struct book
{
char name;
float price;
int pages;
};
int main()
{
struct book b1, b2, ..., bx;
printf("Enter names, prices & no of pages of x books:\n");
while (scanf(" %c%f%d", &bx.name, &bx.price, &bx.pages) != 3)
{
fputs("Error reading bx. Please try again:\n", stderr);
scanf("%*[^\n] ");
}
......
printf("And this is what you have entered:\n");
printf("%c %f %d", bx.name, bx.price, bx.pages);
......
return 0;
}
Example input & output:
Enter names, prices & no of pages of x books:
asd wedewc efcew
Error reading bx. Please try again:
a 12.34 42
And this is what you have entered:
a 12.340000 42
scanf, usually, reads the characters from standard input stream, which includes your newlines as well as spaces. so try to flush all unwanted inputs left in the stdin stream before you use scanf("%c");
Also remember: do not let %c fall somewhere in the middle of the format string
Hope this could help. unfortunately you should use it before all scanf in "Your" program
printf("Enter names , prices & no of pages of 3 books\n");
fflush(stdin);
scanf("%c%f%d",&b1.name,&b1.price,&b1.pages);
You may also use a space before %c in scanf to read whitespaces.
scanf(" %c"....);
Related
I want the output to print the data that we print. but it is not working as expected and the output is not displaying and it is exiting
#include <stdio.h>
int main() {
char name[20], department[3], section[1];
printf("enter the name of the student:");
scanf("%s", name);
printf("enter your department:");
scanf("%s", department);
printf("enter the section");
scanf("%s", section);
printf("Name:%s \n Department:%s \n Section: %s ", name, department, section);
return 0;
}
Your program has undefined behavior because the arrays are too short and scanf() stores the user input beyond the end of the arrays, especially the last one, section that can only contain an empty string which scanf() cannot read anyway.
Make the arrays larger and tell scanf() the maximum number of characters to store before the null terminator, ie: the size of the array minus 1.
Here is a modified version:
#include <stdio.h>
int main() {
char name[50], department[50], section[50];
printf("enter the name of the student:");
if (scanf("%49s", name) != 1)
return 1;
printf("enter your department:");
if (scanf("%49s", department) != 1)
return 1;
printf("enter the section");
if (scanf("%49s", section) != 1)
return 1;
printf("Name:%s\n Department:%s\n Section: %s\n", name, department, section);
return 0;
}
Note that using scanf with a %s conversion requires that each data item be a single word without embedded spaces. If you want name, department and section to accommodate spaces, which is more realistic for anyone besides Superman Krypton A, you would use %[\n] with an initial space to skip pending whitespace and newlines (or fgets() but in another chapter):
#include <stdio.h>
int main() {
char name[50], department[50], section[50];
printf("enter the name of the student:");
if (scanf(" %49[^\n]", name) != 1)
return 1;
printf("enter your department:");
if (scanf(" %49[^\n]", department) != 1)
return 1;
printf("enter the section");
if (scanf(" %49[^\n]", section) != 1)
return 1;
printf("Name:%s\n Department:%s\n Section: %s\n", name, department, section);
return 0;
}
scanf(" %49[^\n]", name) means skip any initial whitespace, including pending newlines from previous input, read and store and bytes read different from newline, up to a maximum of 49 bytes, append a null byte and return 1 for success, 0 for conversion failure or EOF is end of file is reached without storing any byte. For this particular call, conversion failure can happen if there is an encoding error in the currently selected locale.
The problem is that you have not accounted for the null character. It should work with the following.
char name[20] , department[4] , section[2];
The reason this happens is that C requires an extra character for the null character \0 which tells the program when the string ends.
first of all you should respect the size of string ,so you should either convert section to char or increase the size of that string because you have here the problem of '\0' character...so the rule is : the size of string is the size what you need + 1 for '\0' NULL character
and her is two program i tried to Modification you program for two scenarios :
#include <stdio.h>
int main(){
/// any size you like just respect the NULL character
char name[20],department[4],section[23];
printf("enter the name of the student:");
scanf("%s",name);
printf("enter your department:");
scanf("%s",department);
printf("enter the section");
scanf ("%s",section);
printf("Name:%s \n Department:%s \n Section:%s ", name,department,section);
return 0;
}
and case of char :
#include <stdio.h>
int main(){
char name[20],department[4];
char section;
printf("enter the name of the student:");
scanf("%s",name);
printf("enter your department:");
scanf("%s",department);
printf("enter the section");
///don't forget this space before %c it is important
scanf (" %c",§ion);
printf("Name:%s \n Department:%s \n Section:%c ", name,department,section);
return 0;
}
I watched for a long time and finally found the problem.
This is problem: char section[1];.
You declared the size is too short.
It looks like this after you declared it: section[0] = '\0';.
If you scanf a, the array data like section[0] = 'a';, and then it automatically add '\0' somewhere, so you got a memory leaking.
So replace char section[1]; to char section[2];.
I will not insist in the reasons of the other answers (that state other problems in your code than the one you are asking for) but I'll limit my answer to the reasons you don't get any output before the first prompt (there's no undefined behaviour before the third call of printf if you have input short enough strings to not overflow the arrays --- the last is impossible as long as you input one char, because to input one char you heed at least space for two)
I want the output to print the data that we print. but it is not working as expected and the output is not displaying and it is exiting
stdio works in linebuffer mode when output is directed to a terminal, which means that output is written to the terminal in the following cases:
The buffer is filled completely. This is not going to happen with a sort set of strings.
There is a \n in the output string (which there isn't, as you want the cursor to remain in the same line for input as the prompt string)
As there is no \n in your prompts, you need to make printf flush the buffer at each call (just before calling the input routines) You have two ways of doing this.
Calling explicitly the function fflush(3), as in the example below:
printf("enter the name of the student:");
fflush(stdout); /* <-- this forces flushing the buffer */
if (scanf(" %49[^\n]", name) != 1)
return 1;
configuring stdout so it doesn't use buffers at all, so every call to printf forces a write to the standard output.
setbuf(stdout, NULL); /* this disables buffering completely on stdout */
/* ... later, when you need to print something */
printf("enter the name of the student:"); /* data will be printed */
if (scanf(" %49[^\n]", name) != 1)
return 1;
But use this facilities only when it is necessary, as the throughput of the program is degraded if you disable the normal buffering of stdio.
If I try to run this code then it doesn't ask me the value of s2.name. Why is it so?
#include<stdio.h>
int main()
{
struct student
{
char name;
int roll;
int age;
};
struct student s1;
struct student s2;
printf("Enter name of the student: ");
scanf("%c", &s1.name);
printf("%c", s1.name);
printf("\n");
printf("Enter name of the student: ");
scanf("%c", &s2.name);
printf("%c", s2.name);
return 0;
}
When you input a single character and press the Enter key, you are actually inputting two characters: The character in your input and a newline from the Enter key.
The second scanf reads this newline.
Or if you give multiple characters as input to the first name, then the second character will be read by the second scanf.
The way to solve the first problem is easy: Tell scanf to read and discard leading white-space (which newline is) by adding a single space in front of the format, like
scanf(" %c", &s2.name);
// ^
// Note space here
The way to solve the second problem is to read strings instead, which means you have to turn your name members into arrays and then use the "%s" format (preferably with a specified width so you don't read to many characters).
You are basically inputting two characters:
- the one that you type
- the newline character `\n` because you hit `enter`
A solution to this problem is clearing stdin after reading in the first "name":
#include<stdio.h>
int main()
{
struct student
{
char name;
int roll;
int age;
};
struct student s1;
struct student s2;
printf("Enter name of the student: ");
scanf("%c", &s1.name);
printf("%c", s1.name);
printf("\n");
fflush(stdin); //only works on windows, clears the input buffer
printf("Enter name of the student: ");
scanf("%c", &s2.name);
printf("%c", s2.name);
return 0;
}
Another way to clear the input buffer is:
while (getchar() != '\n');
This reads in all characters from the input buffer, because as soon as input is read by functions like getchar() or scanf(), the input is removed from stdin.
EDIT:
When you input values using getchar(), scanf(), etc., then the symbols that you type into stdin (most of the times via a keyboard) are stored in the input buffer at first (stdin). getchar() or any similar function then takes the values that it should read in, out of the input buffer. For example:
scanf("%d", &var);
scanf("%d", &var2);
If I input 5x, the characters in the input buffer are '5', 'x' and \n (because you hit the enter key). The first scanf() then takes out the '5', as it fits the format string %d. After that, the characters in the input buffer are 'x' and \n. In this case scanf returns 1, because it read in one value correctly.
When it then continues to the second one, scanf() won't even let you type anything, as there is already something in the input buffer stdin. It won't store any data however, because the first "item" in stdin ist 'x'. That doesn't fit the format string %d. The compiler then doesn't continue to read. In this case scanf would return 0, as no value was read in correctly.
I am trying to get a name in an array " char name[100][100]". I tried doing many thing like these, but none work.Can you help me?
The code: Its a simple student's grade system i think, but only prints "" when trying to save a name.
#include <stdio.h>
#include <string.h>
void insert();
char name[100][100];float f[20];int z;
int main()
{
int x=0;
do{
printf("<1> Insert student\n");
printf("=> ");
scanf("%d",&x);
printf("\n*************************************************************\n");
switch(x){
case 1:
insert();
break;
default: printf("NO");
break;
}
}while(insert >=0 );
return 0;
}
void insert()
{
int x;
int y=0;
float n1,n2,p;
printf("How many students?: ");
scanf("%d",&y);
for(x=0;x<y;x++){
printf("Insert name: ");
fgets(name[x], 100, stdin);
int len = strlen(name[x]);
if (name[x][len-1] == '\n') {
name[x][len-1] = '\0';
}
printf("name[%d] = \"%s\"\n", x, name[x]);
printf("Insert first grade: ");
scanf("%f",&n1);
printf("Insert second grade: ");
scanf("%f",&n2);
printf("Insert final grade: ");
scanf("%f",&p);
f[x] = (n1 * 0.3)+(n2 * 0.3)+(p * 0.4);
z++;
}
for(x=0;x<z;x++){
if(f[x] < 6){
printf("the final grade of %s is: %.2f \n",name[x], f[x]);}
else{printf("the final grade de %s es: %.2f \n",name[x], f[x]);}
}
}
You should bear in mind that fgets() returns the new-line as well, if there's enough space in the buffer. You might want to take it out:
#include <stdio.h>
#include <string.h>
int main()
{
char name[100][100];
int y = 5;
int x = 0;
for (x = 0; x < y; x++) {
printf("Insert name: ");
fgets(name[x], 100, stdin);
int len = strlen(name[x]);
if (name[x][len-1] == '\n') {
name[x][len-1] = '\0';
}
printf("name[%d] = \"%s\"\n", x, name[x]);
}
}
Why are you using that %[^\t\n] string format? You should just go with a %s string format if you want to read a string (or, better, a %100s to limit the number of characters read).
scanf("%100s",name[x]) works just fine, but will mess things up when you try to use spaces (i.e. the scanf() will read one word at a time).
To avoid that, you can use the second option, that is fgets(). But, in this case, you need to pay attention to the final \n character that is appended to the string. To prevent the newline character from ending your string, you can simply do the following:
name[x][strlen(name[x])-1] = 0;
The previous code simply replaces the \n character with a null byte, thus ending the string and "ignoring" the newline.
EDIT:
The thing you need to understand is that the standard input (i.e. the keyboard input usually) is handled as if it were a file (in fact, you can use functions like fgets(), as if you were reading a normal file). So, as it happens with normal files, each line ends with a special character, \n. Every time you enter an input, and you press "Enter", a newline character is appended to your input.
There's a couple of things you need to know to understand what it is that you're doing wrong:
Some functions (like fgets()) read a line until a newline character is found. The newline character is also read, and returned in the string that was just read.
Other functions (like scanf()) also read lines until some special characters (such as or \n) are found. But, in this case, the final character is not read.
And, last: every time you open a file, the process keeps count of the number of characters you have read from the beginning of the file (or, to put it in an easier (and more correct) way, it "stores" a "pointer" to the next character that should be read).
With this being said, let's have a look at what happens with your program: first, the number of students is read (using scanf()), and, then, a name is read (using fgets()).
So, your input "file", looks like:
4\n
^
John Smith\n
...
The ^ is a pointer to the next character that should be read (and isn't, obviously, part of the input).
After the scanf() (which, as I mentioned, won't read the \n), the situation will be the following:
4\n
^
John Smith\n
...
Now, when you read the next line using fgets(), the "pointer" is already pointing to a newline character, and will therefore assume (correctly!) that the line has ended. The string you are reading is therefore "\n", instead of "John Smith\n".
The easiest way to fix this problem is to read, after every scanf(), single characters from standard input until a newline character is encountered.
scanf ( ... );
while (getc(stdin)!='\n');
Usually reading a single character should be enough, but in some cases (e.g. 4 \n) a single getc() isn't effective.
Basically, whenever a character is read from the file, the "pointer" is updated.
I really hope this cleared things up a bit. It isn't that easy to understand these details at first but, as you get more experience, things will definitely become clearer!
Here is the code
printf("\n");
printf("Enter a integer vaule:");
scanf("%d" , &num3);
printf("You entered: %015d", num3);
printf("Enter a float value:");
scanf("%f", &deci3);
printf("You entered: %15.2f", deci3);
printf("\n");
the output is
Enter a integer vaule:4.4
You entered: 000000000000004
Enter a float value:You entered: 0.40
The problem is this code is not stopping at
printf("Enter a float value:");
and this scanf
scanf("%f", &deci3);
seems to be getting its value from the previous scanf
The %d conversion stops wherever the integer stops, which is a decimal point. If you want to discard the input there, do so explicitly… getc in a loop, fgets, or such. This also allows you to validate the input. The program should probably complain about 4.4.
The scanf function works this way per the specification:
An input item shall be defined as the longest sequence of input bytes (up to any specified maximum field width, which may be measured in characters or bytes dependent on the conversion specifier) which is an initial subsequence of a matching sequence. [Emphasis added.]
In your example, the following C string represents the contents of stdin when the first scanf call requests input: "4.4\n".
For this initial call, your format string consists of a single specifier, %d, which represents an integer. That means that the function will read as many bytes as possible from stdin which satisfy the definition of an integer. In your example, that's just 4, leaving stdin to contain ".4\n" (if this is confusing for you, you might want to check out what an integer is).
The second call to scanf does not request any additional input from the user because stdin already contains ".4\n" as shown above. Using the format string %f attempts to read a floating-point number from the current value of stdin. The number it reads is .4 (per the specification, scanf disregards whitespace like \n in most cases).
To fully answer your question, the problem is not that you're misusing scanf, but rather that there's a mismatch between what you're inputting and how you're expecting scanf to behave.
If you want to guarantee that people can't mess up the input like that, I would recommend using strtol and strtod in conjunction with fgets instead.
This works, but it dont complains if you type 4.4 for the int
#include <stdio.h>
int main() {
char buffer[256];
int i;
float f;
printf("enter an integer : ");
fgets(buffer,256,stdin);
sscanf(buffer, "%d", &i);
printf("you entered : %d\n", i);
printf("enter a float : ");
fgets(buffer,256,stdin);
sscanf(buffer, "%f", &f);
printf("you entered : %f\n", f) ;
return 0;
}
use a fflush(stdin) function after the fist scanf(), this will flush the input buffer.
I'm writing this to get student information (full name, id and gpa for the last 3 trimester, so I used structures and a for loop to plug in the information however, after 1st excution of the for loop (which means at student 2) my 1st and 2nd input are shown on screen together. How could I prevent this from happening in a simple and easy way to understand? ( P.S: I already tried to put getchar(); at the end of the for loop and it worked, however; I'm not supposed to use it 'cause we haven't learnt in class)
The part of the c program where my error happens:
#include <stdio.h>
struct Student {
char name[30];
int id;
float gpa[3];
};
float averageGPA ( struct Student [] );
int main()
{
int i;
float average;
struct Student studentlist[10];
i=0;
for (i; i<10; i++)
{
printf("\nEnter the Student %d full name: ", i+1);
fgets(studentlist[i].name, 30, stdin);
printf("Enter the Student %d ID: ", i+1);
scanf("\n %d", &studentlist[i].id);
printf("Enter the Student %d GPA for the 1st trimester: ", i+1);
scanf("%f", &studentlist[i].gpa[0]);
printf("Enter the Student %d GPA for the 2nd trimester: ", i+1);
scanf("%f", &studentlist[i].gpa[1]);
printf("Enter the Student %d GPA for the 3rd trimester: ", i+1);
scanf("%f", &studentlist[i].gpa[2]);
}
average = averageGPA(studentlist);
printf("\n\nThe average GPA is %.2f", average);
return 0;
}
float averageGPA (struct Student studentlist[])
{
int i;
float total = 0.0, average = 0.0;
for (i=0; i<10; i++)
{
total = studentlist[i].gpa[0] + studentlist[i].gpa[1] + studentlist[i].gpa[2];
}
average = total / 30 ;
return average;
}
Computer output:
Enter the Student 1 full name: mm
Enter the Student 1 ID: 12
Enter the Student 1 GPA for the 1st trimester: 3
Enter the Student 1 GPA for the 2nd trimester: 4
Enter the Student 1 GPA for the 3rd trimester: 3
Enter the Student 2 full name: Enter the Student 2 ID: <<<<< Here is the problem!!
Try eating the newline after the last scanf:
scanf("%f ", &studentlist[i].gpa[2]);
^
This is very much like your getchar solution. It's actually superior to getchar, since it only discards whitespace.
But you have to use getchar() to discard the newline character that is still in the input buffer after your last scanf("%f"), which according to given format converts a float and leave in the buffer all other chars.
If you can't use getchar(), use another fgets() at the end of the loop.. but of course getchar() would be better
Edit for explanation: whenever you type on your keyboard characters go in a input buffer waiting to be processed by your application. getchar() just "consumes" one character from this buffer (returning it), waiting for a valid char if the buffer is empty. scanf("%f") only "consumes" characters resulting in a float. So, when you type "5.12<enter>", scanf reads and removes from buffer "5.12" leaving "<enter>". So the next fgets() already finds a newline in the buffer and returns immediately; that's why you should use getchar(): ignoring its returning value you successfully discard "<enter>" from the buffer. Finally, please note that if in the buffer there is only "<enter>", scanf("%f") discards it (since it cannot be converted in a float) and waits for another input blocking application.
One last note: input stream is buffered by your OS default policy, in the sense that application does not receive any character until you type "<enter>".
Use scanf in following way to read the student name:
scanf(" %[^\n]",studentlist[i].name);
The first space in the format specifier is important. It negates the newline from previous input. The format, by the way, instructs to read until a newline (\n) is encountered.
[Edit: Adding explanation on request]
The format specifier for accepting a string is %s. But it allows you to enter non-whitespace characters only. The alternative way is to specify the characters that are acceptable (or not acceptable, based on the scenario) within square brackets.
Within square brackets, you can specify individual characters, or ranges, or combination of these. To specify characters to be excluded, precede with a caret (^) symbol.
So, %[a-z] would mean any character between a and z (both included) will be accepted
In your case, we need every character other than the newline to be accepted. So we come up with the specifier %[^\n]
You will get more info on these specifiers from the web. Here's one link for convenience: http://beej.us/guide/bgc/output/html/multipage/scanf.html
The space in the beginning actually 'consumes' any preceding white space left over from previous input. You can refer the answer here for a detailed explanation: scanf: "%[^\n]" skips the 2nd input but " %[^\n]" does not. why?
I would just say no to scanf(). Use fgets() for all input fields and convert to numeric with atoi() and atof().