why doesn't this code work?
#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
#include<string.h>
int main(void)
{
// local declarations
int len;
char* pStr;
// statements
printf(" how many characters you want to enter?\n");
scanf("%d", &len);
pStr=(char*)calloc(len+1,sizeof(char));
printf("\n enter your string: ");
gets(pStr);
*(pStr+len)='\0';
printf("\n your string: ");
puts(pStr);
printf(" oops! last character deleted.");
getch();
return 0;
}
although it runs correct, when i use scanf function to read the string, but
why it does not with gets?
scanf("%s", pStr) skips to the first non-whitespace character while gets doesn't.
After the first scanf the trailing newline is still in the input buffer so that when you call gets the result is an empty line unless you entered extra characters after the number.
Note that gets is marked as obsolete due to serious security flaws.
It is recommended that any use of gets(var) is replaced with fgets(var, length, stdin).
Because arrays are zero based, and (assuming the input is valid and the correct length, assumption which your code ought not to make) *(ptr + len) already contains \0 and you are just overwriting it. You meant to overwrite ptr[len-1]
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.
I can't take inputs except string.
If I give inputs in code itself, program is working.
But when I try to take inputs, its not working.
How do I get it to accept given inputs?
For example, if s were "codebook", and from == 'o' and to == 'e', s would become "cedebeek".
#include<stdio.h>
#define MAX 50
void replace(char *s, char from, char to)
{
int i=0;
while(s[i]!='\0')
{
if(s[i]==from)
{
s[i]=to;
}
i++;
}
}
int main()
{
char str[MAX];
char from;
char to;
printf("Enter the string");
scanf("%[^\n]s",&str[0]);
printf("\nEnter the character to be replaced");
scanf("%c",&from);
printf("\nEnter the character to be replaced with");
scanf("%c",&to);
replace(str, from, to);
printf("\nThe modified string is %s",str);
return(0);
}
There are a couple of problems in the posted code. Since arrays decay to pointers to their first elements in most expressions, there is no need for the address operator & in the call to scanf(); also, the trailing s is not part of the scanset conversion specifier:
scanf("%[^\n]", str);
As pointed out by #M.M in the comments, it is not incorrect to use &str[0] here instead of str, but it is more idiomatic, and I personally find it more clear, to use the less cluttered str.
When scanf() returns, a newline character will be left in the input stream, so you should add a leading space in the next call to scanf() to skip over this \n character before reading the user input:
scanf(" %c",&from);
And this call to scanf() will also leave a \n character in the input stream, so again:
scanf(" %c",&to);
Note that you should really specify a maximum width to avoid buffer overflow when reading user input into a string; there is no easy way to do this with MAX, but you can do:
scanf("%49[^\n]", str);
You could further improve code and ensure that input is as expected by checking the values returned by the calls to scanf().
This question already has answers here:
How do you allow spaces to be entered using scanf?
(11 answers)
Closed 6 years ago.
#include <stdio.h>
int main()
{
char name[20];
printf("Enter name: ");
scanf("%s", name);
printf("Your name is %s.", name);
return 0;
}
Output:
Enter name: Dennis Ritchie
Your name is Dennis.
So far I haven't found any specific valid reason for this question. Can anyone help me out?
scanf only read till it gets to space that is why it is not storing after the first space , so your printf function is not faulty , it is the scanf that is not storing the complete string , stopping on encountering first space.
One should never use gets() , unless they completely know what they are doing , because it does not have buffer overflow protection , it continue to read after the buffer ends until it finds a new line or encounter a EOF. You can read more about that here.Please Check This Why is the gets function so dangerous that it should not be used?
You should instead use fgets().
#include <stdio.h>
int main(){
char name[20];
printf("Enter name: ");
fgets(name,20,stdin);
printf("Your name is %s.", name);
return 0;
}
Remember fgets() also reads newline character(the one you get when you press enter) so you should manually remove that.
Also I highly Recommend this answer for using fgets() to its full potential and avoiding common pitfalls.
This answer tells about using scanf to read string.What it says is the following:
int main(){
char string[100], c;
int i;
printf("Enter the string: ");
scanf("%s", string);
i = strlen(string); // length of user input till first space
do{
scanf("%c", &c);
string[i++] = c; // reading characters after first space (including it)
}while (c != '\n'); // until user hits Enter
string[i - 1] = 0; // string terminating
return 0;
}
How this works? When user inputs characters from standard input, they will be stored in string variable until first blank space. After that, rest of entry will remain in input stream, and wait for next scanf. Next, we have a for loop that takes char by char from input stream (till \n) and appends them to end of string variable, thus forming a complete string same as user input from keyboard.
i am a learner of 'C' and written a code, but after i compile it, shows a Debug Error message, here is the code:
#include<stdio.h>
void main()
{
int n,i=1;
char c;
printf("Enter Charecter:\t");
scanf("%s",&c);
printf("Repeat Time\t");
scanf("%d",&n);
n=n;
while (i <= n)
{
printf("%c",c);
i++;
}
}
Pls tell me why this happens and how to solve it
The scanf("%s", &c) is writing to memory it should not as c is a single char but "%s" expects its argument to be an array. As scanf() appends a null character it will at the very least write two char to c (the char read from stdin plus the null terminator), which is one too many.
Use a char[] and restrict the number of char written by scanf():
char data[10];
scanf("%9s", data);
and use printf("%s", data); instead of %c or use "%c" as the format specifier in scanf().
Always check the return value of scanf(), which is the number of successful assignments, to ensure subsequent code is not processing stale or uninitialized variables:
if (1 == scanf("%d", &n))
{
/* 'n' assigned. 'n = n;' is unrequired. */
}
scanf("%s",&c); should be scanf("%c",&c);
The %s format specifier tells scanf you're passing a char array. You're passing a single char so need to use %c instead.
Your current code will behave unpredictably because scanf will try to write an arbitrarily long word followed by a nul terminator to the address you provided. This address has memory allocated (on the stack) for a single char so you end up over-writing memory that may be used by other parts of your program (say for other local variables).
I'm not sure you understood the answer to your other question: Odd loop does not work using %c
These format specifiers are each used for a specific job.
If you want to get a:
character from stdin use %c.
string (a bunch of characters) use %s.
integer use %d.
This code:
char c;
printf("Enter Character:\t");
scanf("%c",&c);
Will read 1 character from stdin and will leave a newline ('\n') character there. So let's say the user entered the letter A in the stdin buffer you have:
A\n
The scanf() will pull 'A' and store it in your char c and will leave the newline character. Next it will ask for your int and the user might input 5. stdin now has:
\n5
The scanf() will take 5 and place it in int n. If you want to consume that '\n' there are a number of options, one would be:
char c;
printf("Enter Character:\t");
scanf("%c",&c); // This gets the 'A' and stores it in c
getchar(); // This gets the \n and trashes it
Here is a working version of your code. Please see inline comments in code for fixes:
#include<stdio.h>
void main()
{
int n,i=1;
char c;
printf("Enter Character:\t");
scanf("%c",&c);//Use %c instead of %s
printf("Repeat Time\t");
scanf("%d",&n);
n=n;//SUGGESTION:This line is not necessary. When you do scanf on 'n' you store the value in 'n'
while (i <= n)//COMMENT:Appears you want to print the same character n times?
{
printf("%c",c);
i++;
}
return;//Just a good practice
}
This question already has answers here:
How do you allow spaces to be entered using scanf?
(11 answers)
Closed 4 years ago.
I'm using Ubuntu and I'm also using Geany and CodeBlock as my IDE.
What I'm trying to do is reading a string (like "Barack Obama") and put it in a variable:
#include <stdio.h>
int main(void)
{
char name[100];
printf("Enter your name: ");
scanf("%s", name);
printf("Your Name is: %s", name);
return 0;
}
Output:
Enter your name: Barack Obama
Your Name is: Barack
How can I make the program read the whole name?
Use:
fgets (name, 100, stdin);
100 is the max length of the buffer. You should adjust it as per your need.
Use:
scanf ("%[^\n]%*c", name);
The [] is the scanset character. [^\n] tells that while the input is not a newline ('\n') take input. Then with the %*c it reads the newline character from the input buffer (which is not read), and the * indicates that this read in input is discarded (assignment suppression), as you do not need it, and this newline in the buffer does not create any problem for next inputs that you might take.
Read here about the scanset and the assignment suppression operators.
Note you can also use gets but ....
Never use gets(). Because it is impossible to tell without knowing the data in advance how many characters gets() will read, and because gets() will continue to store characters past the end of the buffer, it is extremely dangerous to use. It has been used to break computer security. Use fgets() instead.
Try this:
scanf("%[^\n]s",name);
\n just sets the delimiter for the scanned string.
Here is an example of how you can get input containing spaces by using the fgets function.
#include <stdio.h>
int main()
{
char name[100];
printf("Enter your name: ");
fgets(name, 100, stdin);
printf("Your Name is: %s", name);
return 0;
}
scanf(" %[^\t\n]s",&str);
str is the variable in which you are getting the string from.
The correct answer is this:
#include <stdio.h>
int main(void)
{
char name[100];
printf("Enter your name: ");
// pay attention to the space in front of the %
//that do all the trick
scanf(" %[^\n]s", name);
printf("Your Name is: %s", name);
return 0;
}
That space in front of % is very important, because if you have in your program another few scanf let's say you have 1 scanf of an integer value and another scanf with a double value... when you reach the scanf for your char (string name) that command will be skipped and you can't enter value for it... but if you put that space in front of % will be ok everything and not skip nothing.
NOTE: When using fgets(), the last character in the array will be '\n' at times when you use fgets() for small inputs in CLI (command line interpreter) , as you end the string with 'Enter'. So when you print the string the compiler will always go to the next line when printing the string. If you want the input string to have null terminated string like behavior, use this simple hack.
#include<stdio.h>
int main()
{
int i,size;
char a[100];
fgets(a,100,stdin);;
size = strlen(a);
a[size-1]='\0';
return 0;
}
Update: Updated with help from other users.
#include <stdio.h>
// read a line into str, return length
int read_line(char str[]) {
int c, i=0;
c = getchar();
while (c != '\n' && c != EOF) {
str[i] = c;
c = getchar();
i++;
}
str[i] = '\0';
return i;
}
Using this code you can take input till pressing enter of your keyboard.
char ch[100];
int i;
for (i = 0; ch[i] != '\n'; i++)
{
scanf("%c ", &ch[i]);
}
While the above mentioned methods do work, but each one has it's own kind of problems.
You can use getline() or getdelim(), if you are using posix supported platform.
If you are using windows and minigw as your compiler, then it should be available.
getline() is defined as :
ssize_t getline(char **lineptr, size_t *n, FILE *stream);
In order to take input, first you need to create a pointer to char type.
#include <stdio.h>
#include<stdlib.h>
// s is a pointer to char type.
char *s;
// size is of size_t type, this number varies based on your guess of
// how long the input is, even if the number is small, it isn't going
// to be a problem
size_t size = 10;
int main(){
// allocate s with the necessary memory needed, +1 is added
// as its input also contains, /n character at the end.
s = (char *)malloc(size+1);
getline(&s,&size,stdin);
printf("%s",s);
return 0;
}
Sample Input:Hello world to the world!
Output:Hello world to the world!\n
One thing to notice here is, even though allocated memory for s is 11 bytes,
where as input size is 26 bytes, getline reallocates s using realloc().
So it doesn't matter how long your input is.
size is updated with no.of bytes read, as per above sample input size will be 27.
getline() also considers \n as input.So your 's' will hold '\n' at the end.
There is also more generic version of getline(), which is getdelim(), which takes one more extra argument, that is delimiter.
getdelim() is defined as:
ssize_t getdelim(char **lineptr, size_t *n, int delim, FILE *stream);
Linux man page
If you need to read more than one line, need to clear buffer. Example:
int n;
scanf("%d", &n);
char str[1001];
char temp;
scanf("%c",&temp); // temp statement to clear buffer
scanf("%[^\n]",str);
"%s" will read the input until whitespace is reached.
gets might be a good place to start if you want to read a line (i.e. all characters including whitespace until a newline character is reached).
"Barack Obama" has a space between 'Barack' and 'Obama'. To accommodate that, use this code;
#include <stdio.h>
int main()
{
printf("Enter your name\n");
char a[80];
gets(a);
printf("Your name is %s\n", a);
return 0;
}
scanf("%s",name);
use & with scanf input