I created a function that checks if a given string is a number or not.
The problem is that I take the input with fgets but ,when called, it doesn't stop to take input from user!
I tried to fix adding fflush(stdin) and fflush(stdout) to fix because I read some stuffs online but isn't working :S
getInput calls the fgets that stores the string taken from user input then isInt checks if it's an int or not.
The problem is that the program stucks on the first fgets.
int isInt (char* string)
{
int k=0;
while( string[k] != '\0')
{
if ( isdigit(string[k])==0) //it's k++ here, I copied bad
return 1;
}
return 0;
}
void getInput(char* string)
{
printf("Insert a number : \n");
while (1)
{
fflush(stdout); //Tried to fix
fflush(stdin); //same as above
fgets(string,sizeof(string),stdin); //stucks here
if(isInt(string)==0 )
{
printf("Ok it's a number!\n");
break;
}
else printf("Insert a valid number!\n");;
}
}
fgets() will put newline character read from the stream into the array, so remove that to avoid isInt returning 1 seeing the newline character.
sizeof(string) is the size of pointer, not the size of pointed buffer. You will have to receive the size of buffer separately. For more information, see C sizeof a passed array - Stack Overflow.
fflush(stdin); invokes undefined behavior, so you shouldn't use that.
You have to increment k in the loop in isInt, or it may stuck into an infinite loop. (thanks #Crowman)
#include <string> // for using strchr()
int isInt (char* string)
{
int k=0;
while( string[k] != '\0')
{
if ( isdigit(string[k])==0)
return 1;
k++; // increment k
}
return 0;
}
void getInput(char* string, size_t string_max) // add parameter to get buffer size
{
printf("Insert a number : \n");
while (1)
{
fflush(stdout);
fgets(string,string_max,stdin); // use passed buffer size instead of sizeof()
char* lf = strchr(string, '\n'); // find newline character
if (lf != NULL) *lf = '\0'; // and remove that if it exists
if(isInt(string)==0 )
{
printf("Ok it's a number!\n");
break;
}
else printf("Insert a valid number!\n");
}
}
I think that you should put the fgets inside the while condition:
while(fgets(string,sizeof(string),stdin))
Related
I am trying a simple exercise from K&R to append string2 at the end of string1 using pointers. In case of overflow i.e. buffer of string1 can't contain all of string2 I want to prompt the user to re-enter string2 or exit.
I have written the following code:
#include<stdio.h>
#include<string.h>
#define MAXLINE 1000
int get_input(char *s);
int str_cat(char *s, char *t);
void main()
{
char input1[MAXLINE], input2[MAXLINE], c;
get_input(input1);
check:
get_input(input2);
if((strlen(input1) + strlen(input2) + 2) <= MAXLINE)
{
str_cat(input1, input2);
printf("%s\n", input1);
}
else
{
input2[0] = '\0';
printf("String overflow\n Press: \n 1: Re-enter string. \n 2: Exit.\n");
scanf(" %d", &c);
if(c == 1){
input2[0] = '\0';
get_input(input2);
goto check;
}
}
}
int get_input(char *arr)
{
int c;
printf("Enter the string: \n");
while(fgets(arr, MAXLINE, stdin))
{
break;
}
}
int str_cat(char *s, char *t)
{
while(*s != '\0')
{
s++;
}
while((*s++ = *t++) != '\0')
{
;
}
*s = '\0';
}
Initially, I was using the standard getchar() function mentioned in the book to read the input in get_input() which looked like this:
int get_input(char *arr)
{
int c;
printf("Enter the string: \n");
while((c = getchar()) != '\n' && c != EOF)
{
*arr++ = c;
}
*arr = '\0';
}
I am new and I read this and understood my mistake. I understand that one isn't supposed to use different input functions to read stdin and the '\n' is left in the input stream which is picked by the getchar() causing my condition to fail.
So, I decided to use fgets() to read the input and modified the scanf("%d", &c) as mentioned in the thread with scanf(" %d", c). This does work (kinda) but gives rise to behaviors that I do not want.
So, I have a few questions:
What's a better way to fgets() from reading the input on encountering '\n' than the one I have used?
while(fgets(arr, MAXLINE, stdin))
{
break;
}
fgets() stops reading the line and stores it as an input once it either encounters a '\n' or EOF. But, it ends up storing the '\n' at the end of the string. Is there a way to prevent this or do I have to over-write the '\n' manually?
Even though I used the modified version of scanf(" %d", &c), my output looks like
this: (https://imgur.com/a/RaC2Kc6). Despite that I get Enter the string: twice when prompted to re-enter the second string in case of an overflow situation. Is the modified scanf() messing with my input? And how do I correct it?
In general, do not mix fgets with scanf. Although it may be a bit bloaty, you will avoid many problems by being consistent with reading input with fgets and then parse it with sscanf. (Note the extra s)
A good way to remove the newline is buffer[strcspn(buffer, "\n")] = 0
Example:
// Read line and handle error if it occurs
if(!fgets(buffer, buffersize, stdin)) {
// Handle error
}
// Remove newline (if you want, not necessarily something you need)
buffer[strcspn(buffer, "\n")] = 0;
// Parse and handle error
int val;
if(sscanf(buffer, "%d", &val) != 1) {
// Handle error
}
// Now you can use the variable val
There is one thing here that might be dangerous in certain situations, and that is if buffer is not big enough to hold a complete line. fgets will not read more than buffersize characters. If the line is longer, the remaining part will be left in stdin.
Given the following code:
#include <stdio.h>
int main()
{
int testcase;
char arr[30];
int f,F,m;
scanf("%d",&testcase);
while(testcase--)
{
printf("Enter the string\n");
fgets(arr,20,stdin);
printf("Enter a character\n");
F=getchar();
while((f=getchar())!=EOF && f!='\n')
;
putchar(F);
printf("\n");
printf("Enter a number\n");
scanf("%d",&m);
}
return 0;
}
I want a user to enter a string, a character and a number until the testcase becomes zero.
My doubts / questions:
1.User is unable to enter a string. It seems fgets is not working. Why?
2.If i use scanf instead of fgets,then getchar is not working properly, i.e whatever character I input in it just putchar as a new line. Why?
Thanks for the help.
Mixing functions like fgets(), scanf(), and getchar() is error-prone. The scanf() function usually leaves a \n character behind in the input stream, while fgets() usually does not, meaning that the next call to an I/O function may or may not need to cope with what the previous call has left in the input stream.
A better solution is to use one style of I/O function for all user input. fgets() used in conjunction with sscanf() works well for this. Return values from functions should be checked, and fgets() returns a null pointer in the event of an error; sscanf() returns the number of successful assignments made, which can be used to validate that input is as expected.
Here is a modified version of the posted code. fgets() stores input in a generously allocated buffer; note that this function stores input up to and including the \n character if there is enough room. If the input string is not expected to contain spaces, sscanf() can be used to extract the string, leaving no need to worry about the newline character; similarly, using sscanf() to extract character or numeric input relieves code of the burden of further handling of the \n.
#include <stdio.h>
int main(void)
{
int testcase;
char arr[30];
char F;
int m;
char buffer[1000];
do {
puts("Enter number of test cases:");
if (fgets(buffer, sizeof buffer, stdin) == NULL) {
/* handle error */
}
} while (sscanf(buffer, "%d", &testcase) != 1 || testcase < 0);
while(testcase--)
{
puts("Enter the string");
/* if string should not contain spaces... */
if (fgets(buffer, sizeof buffer, stdin) == NULL) {
/* handle error */
}
sscanf(buffer, "%29s", arr);
printf("You entered: %s\n", arr);
putchar('\n');
puts("Enter a character");
if (fgets(buffer, sizeof buffer, stdin) == NULL) {
/* handle error */
}
sscanf(buffer, "%c", &F);
printf("You entered: %c\n", F);
putchar('\n');
do {
puts("Enter a number");
if (fgets(buffer, sizeof buffer, stdin) == NULL) {
/* handle error */
}
} while (sscanf(buffer, "%d", &m) != 1);
printf("You entered: %d\n", m);
putchar('\n');
}
return 0;
}
On the other hand, if the input string may contain spaces, fgets() can read input directly into arr, but then the stored string will contain a \n character, which should probably be removed. One way of doing this is to use the strcspn() function to find the index of the \n:
#include <string.h> // for strcspn()
/* ... */
puts("Enter the string");
/* or, if string may contain spaces */
if (fgets(arr, sizeof arr, stdin) == NULL) {
/* handle error */
}
/* replace newline */
arr[strcspn(arr, "\r\n")] = '\0';
printf("You entered: %s\n", arr);
putchar('\n');
/* ... */
Note that a maximum width should always be specified when using %s with the scanf() functions to avoid buffer overflow. Here, it is %29s when reading into arr, since arr can hold 30 chars, and space must be reserved for the null terminator (\0). Return values from sscanf() are checked to see if user input is invalid, in which case the input is asked for again. If the number of test cases is less than 0, input must be entered again.
Finally got the solution how can we use scanf and fgets together safely.
#include <stdio.h>
int main()
{
int testcase,f,F,m;
char arr[30];
scanf("%d",&testcase);
while((f=getchar())!=EOF && f!='\n')
;
while(testcase--)
{
printf("Enter the string\n");
fgets(arr,30,stdin);
printf("Enter a character\n");
F=getchar();
while((f=getchar())!=EOF && f!='\n')
;
putchar(F);
printf("\n");
printf("Enter a number\n");
scanf("%d",&m);
while((f=getchar())!=EOF && f!='\n')
;
}
}
We need to make sure that before fgets read anything,flushout the buffer with simple while loop.
Thanks to all for the help.
A simple hack is to write a function to interpret the newline character. Call clear() after each scanf's
void clear (void){
int c = 0;
while ((c = getchar()) != '\n' && c != EOF);
}
Refer to this question for further explaination: C: Multiple scanf's, when I enter in a value for one scanf it skips the second scanf
This question already has an answer here:
How to read / parse input in C? The FAQ
(1 answer)
Closed 6 years ago.
I am trying to write a simple program which will read two input lines, an integer followed by a string. However, it doesn't seem to work for me.
int main()
{
int i;
char str[1024];
scanf("%d", &i);
scanf("%[^\n]", str);
printf("%d\n", i);
printf("%s\n", str);
return 0;
}
Immediately after entering the integer and pressing "Enter", the program prints the integer. It doesn't wait for me to enter the string. Whats wrong? Whats the correct way to program this?
What you need to know
The problem with %[^\n] is that it fails when the first character to be read is the newline character, and pushes it back into the stdin.
The Problem
After you enter a number for the first scanf, you press Enter. %d in the first scanf consumes the number, leaving the newline character ('\n'), generated by the Enter keypress, in the standard input stream (stdin). %[^\n] in the next scanf sees this \n and fails for the reason given in the first paragraph of this answer.
Fixes
Solutions include:
Changing scanf("%d", &i); to scanf("%d%*c", &i);. What %*c does is, it scans and discards a character.
I wouldn't recommend this way because an evil user could trick the scanf by inputting something like <number><a character>\n, ex: 2j\n and you'll face the same problem again.
Adding a space (any whitespace character will do) before %[^\n], i.e, changing scanf("%[^\n]", str); to scanf(" %[^\n]", str); as #Bathsheba mentioned in a comment.
What the whitespace character does is, it scans and discards any number of whitespace characters, including none, until the first non-whitespace character.
This means that any leading whitespace characters will be skipped when inputting for the second scanf.
This is my recommendation: Clear the stdin after every scanf. Create a function:
void flushstdin(void)
{
int c;
while((c = getchar()) != '\n' && c != EOF);
}
and call it after every scanf using flushstdin();.
Other issues:
Issues unrelated to your problem include:
You don't deal with the case if scanf fails. This can be due to a variety of reasons, say, malformed input, like inputting an alphabet for %d.
To do this, check the return value of scanf. It returns the number of items successfully scanned and assigned or -1 if EOF was encountered.
You don't check for buffer overflows. You need to prevent scanning in more than 1023 characters (+1 for the NUL-terminator) into str.
This can be acheived by using a length specifier in scanf.
The standards require main to be declared using either int main(void) or int main(int argc, char* argv[]), not int main().
You forgot to include stdio.h (for printf and scanf)
Fixed, Complete Program
#include <stdio.h>
void flushstdin(void)
{
int c;
while((c = getchar()) != '\n' && c != EOF);
}
int main(void)
{
int i;
char str[1024];
int retVal;
while((retVal = scanf("%d", &i)) != 1)
{
if(retVal == 0)
{
fputs("Invalid input; Try again", stderr);
flushstdin();
}
else
{
fputs("EOF detected; Bailing out!", stderr);
return -1;
}
}
flushstdin();
while((retVal = scanf("%1023[^\n]", str)) != 1)
{
if(retVal == 0)
{
fputs("Empty input; Try again", stderr);
flushstdin();
}
else
{
fputs("EOF detected; Bailing out!", stderr);
return -1;
}
}
flushstdin();
printf("%d\n", i);
printf("%s\n", str);
return 0;
}
This simply, will work:
scanf("%d %[^\n]s", &i, str);
Instaed of scanf() use fgets() followed by sscanf().
Check return values of almost all functions with a prototype in <stdio.h>.
#include <stdio.h>
int main(void) {
int i;
char test[1024]; // I try to avoid identifiers starting with "str"
char tmp[10000]; // input buffer
// first line
if (fgets(tmp, sizeof tmp, stdin)) {
if (sscanf(tmp, "%d", &i) != 1) {
/* conversion error */;
}
} else {
/* input error */;
}
// second line: read directly into test
if (fgets(test, sizeof test, stdin)) {
size_t len = strlen(test);
if (test[len - 1] == '\n') test[--len] = 0; // remove trailing ENTER
// use i and test
printf("i is %d\n", i);
printf("test is \"%s\" (len: %d)\n", test, (int)len);
} else {
/* input error */;
}
return 0;
}
I currently have a homework assignment and I used gets.
The professor said I should be using getchar instead.
What is the difference?
How would I change my code to use getchar? I can't seem to get it right.
code:
#include <stdio.h>
#include <string.h>
#include <strings.h>
#define STORAGE 255
int main() {
int c;
char s[STORAGE];
for(;;) {
(void) printf("n=%d, s=[%s]\n", c = getword(s), s);
if (c == -1) break;
}
}
int getword(char *w) {
char str[255];
int i = 0;
int charCount = 0;
printf("enter your sentence:\n"); //user input
gets(str);
for(i = 0; str[i] != '\0' && str[i] !=EOF; i++){
if(str[i] != ' '){
charCount++;
} else {
str[i] = '\0'; //Terminate str
i = -1; //idk what this is even doing?
break; //Break out of the for-loop
}
}
printf("your string: '%s' contains %d of letters\n", str, charCount); //output
strcpy(w, str);
// return charCount;
return strlen(w); //not sure what i should be returning.... they both work
}
gets() was supposed to get a string from the input and store it into the supplied argument. However, due to lack of preliminary validation on the input length, it is vulnerable to buffer overflow.
A better choice is fgets().
However, coming to the usage of getchar() part, it reads one char at a time. So basically, you have to keep reading from the standard input one by one, using a loop, until you reach a newline (or EOF) which marks the end of expected input.
As you read a character (with optional validation), you can keep on storing them in str so that, when the input loop ends, you have the input string ready in str.
Don't forget to null terminate str, just in case.
I created a program to make a diamond out of *'s. I am looking for a way to check if the type of input is an integer in the C language. If the input is not an integer I would like it to print a message.
This is what I have thus far:
if(scanf("%i", &n) != 1)
printf("must enter integer");
However it does not display the message if it's not an integer. Any help/guidance with this issue would be greatly appreciated!
you can scan your input in a string then check its characters one by one, this example displays result :
0 if it's not digit
1 if it is digit
you can play with it to make your desired output
char n[10];
int i=0;
scanf("%s", n);
while(n[i] != '\0')
{
printf("%d", isdigit(n[i]));
i++;
}
Example:
#include <stdio.h>
#include <string.h>
main()
{
char n[10];
int i=0, flag=1;
scanf("%s", n);
while(n[i] != '\0'){
flag = isdigit(n[i]);
if (!flag) break;
i++;
}
if(flag)
{
i=atoi(n);
printf("%d", i);
}
else
{
printf("it's not integer");
}
}
Use fgets() followed by strtol() or sscanf(..."%d"...).
Robust code needs to handle IO and parsing issues. IMO, these are best done separately.
char buf[50];
fgets(buf, sizeof buf, stdin);
int n;
int end = 0; // use to note end of scanning and catch trailing junk
if (sscanf(buf, "%d %n", &n, &end) != 1 || buf[end] != '\0') {
printf("must enter integer");
}
else {
good_input(n);
}
Note:
strtol() is a better approach, but a few more steps are needed. Example
Additional error checks include testing the result of fgets() and insuring the range of n is reasonable for the code.
Note:
Avoid mixing fgets() and scanf() in the same code.
{ I said scanf() here and not sscanf(). }
Recommend not to use scanf() at all.
strtol
The returned endPtr will point past the last character used in the conversion.
Though this does require using something like fgets to retrieve the input string.
Personal preference is that scanf is for machine generated input not human generated.
Try adding
fflush(stdout);
after the printf. Alternatively, have the printf output a string ending in \n.
Assuming this has been done, the code you've posted actually would display the message if and only if an integer was not entered. You don't need to replace this line with fgets or anything.
If it really seems to be not working as you expect, the problem must be elsewhere. For example, perhaps there are characters left in the buffer from input prior to this line. Please post a complete program that shows the problem, along with the input you gave.
Try:
#include <stdio.h>
#define MAX_LEN 64
int main(void)
{ bool act = true;
char input_string[MAX_LEN]; /* character array to store the string */
int i;
printf("Enter a string:\n");
fgets(input_string,sizeof(input_string),stdin); /* read the string */
/* print the string by printing each element of the array */
for(i=0; input_string[i] != 10; i++) // \0 = 10 = new line feed
{ //the number in each digits can be only 0-9.[ASCII 48-57]
if (input_string[i] >= 48 and input_string[i] <= 57)
continue;
else //must include newline feed
{ act = false; //0
break;
}
}
if (act == false)
printf("\nTHIS IS NOT INTEGER!");
else
printf("\nTHIS IS INTEGER");
return 0;
}
[===>] First we received input using fgets.Then it's will start pulling each digits out from input(starting from digits 0) to check whether it's number 0-9 or not[ASCII 48-57],if it successful looping and non is characters -- boolean variable 'act' still remain true.Thus returning it's integer.