filtering input to only numbers. C - c

Here's a basic question:
How can I take the input from a user and only accept numbers?
I know I need to use this start:
do{
ch=getchar();
}while (ch != '\n');
But I know it's not enough. This will block every input, including numbers, so I need to break when input is number.
also how do I break not after the first digit of the number?
I tried looking this up with no luck.
thanks!

When you need to perform error checking, do different things based on the input, etc., it's best to read user input line by line and process each line as you see fit.
// Make it large enough for your needs.
#define LINE_LENGTH 200
char line[LINE_LENGTH];
// Keep reding lines of text until there is nothing to read.
while ( fgets(line, sizeof(line), stdin) != NULL )
{
// Process contents of line.
}
If you expect to see only one number per line, you can use sscanf to extract numbers from each line.
while ( fgets(line, sizeof(line), stdin) != NULL )
{
int num;
if ( sscanf(line, "%d", &num) == 1 )
{
// Got a number. Use it.
}
}

The probably easiest way to read in a number is using scanf:
int number;
if (scanf("%d",&number) == 1) {
printf("successfully read number %d\n",number);
} else {
printf("not a number.\n");
}
It skips leading white spaces and then takes characters from stdin as long as these match an integral number format. Note that you still might have to press "enter" before your program will proceed, because the operating system might buffer the input (beyond your control).
Note that a ch=getchar() will take also the first non-digit value from stdin, which can then not be consumed by any further access to stdin anymore. scanf, in contrast, keeps this character in the buffer for later use.

Just check if the input is some value between '0' and '9'
#include <stdio.h>
int main(void)
{
size_t len = 0;
char str[32];
int ch;
while (len < sizeof(str) - 1) {
ch = getchar();
if ((ch >= '0') && (ch <= '9')) {
str[len++] = (char)ch;
} else {
break;
}
}
str[len] = '\0';
puts(str);
return 0;
}

simple, nice and neat solution:), I used to solve similar questions when I started programming, this will let you only input numbers
#include <stdio.h>
int main()
{
char ch;
do{
ch=getchar();
}while (!(ch>=48 && ch<=57));
printf("yay,we got a number : %c !",ch);
return 0;
}

Related

How to accept string input only if it of certain length in C else ask user to input the string again

How to accept set of strings as input in C and prompt the user again to re-enter the string if it exceeds certain length. I tried as below
#include<stdio.h>
int main()
{
char arr[10][25]; //maximum 10 strings can be taken as input of max length 25
for(int i=0;i<10;i=i+1)
{
printf("Enter string %d:",i+1);
fgets(arr[i],25,stdin);
}
}
But here fgets accepts the strings greater than that length too.
If the user hits return, the second string must be taken as input. I'm new to C
How to accept string input only if it of certain length
Form a helper function to handle the various edge cases.
Use fgets(), then drop the potential '\n' (which fgets() retains) and detect long inputs.
Some untested code to give OP an idea:
#include <assert.h>
#include <stdio.h>
// Pass in the max string _size_.
// Return NULL on end-of-file without input.
// Return NULL on input error.
// Otherwise return the buffer pointer.
char* getsizedline(size_t sz, char *buf, const char *reprompt) {
assert(sz > 0 && sz <= INT_MAX && buf != NULL); // #1
while (fgets(buf, (int) sz, stdin)) {
size_t len = strlen(buf);
// Lop off potential \n
if (len > 0 && buf[--len] == '\n') { // #2
buf[len] = '\0';
return buf;
}
// OK if next ends the line
int ch = fgetc(stdin);
if (ch == '\n' || feof(stdin)) { // #3
return buf;
}
// Consume rest of line;
while (ch != '\n' && ch != EOF) { // #4
ch = fgetc(stdin);
}
if (ch == EOF) { // #5
return NULL;
}
if (reprompt) {
fputs(reprompt, stdout);
}
}
return NULL;
}
Uncommon: reading null characters remains a TBD issue.
Details for OP who is a learner.
Some tests for sane input parameters. A size of zero does not allow for any input saved as a null character terminated string. Buffers could be larger than INT_MAX, but fgets() cannot directly handle that. Code could be amended to handle 0 and huge buffers, yet leave that for another day.
fgets() does not always read a '\n'. The buffer might get full first or the last line before end-of-file might lack a '\n'. Uncommonly a null character might be read - even the first character hence the len > 0 test, rendering strlen() insufficient to determine length of characters read. Code would need significant changes to accommodate determining the size if null character input needs detailed support.
If the prior fgets() filled its buffer and the next read character attempt resulted in an end-of-file or '\n', this test is true and is OK, so return success.
If the prior fgetc() resulted in an input error, this loops exits immediately. Otherwise, we need to consume the rest of the line looking for a '\n' or EOF (which might be due to an end-of-file or input error.)
If EOF returned (due to an end-of-file or input error), no reason to continue. Return NULL.
Usage
// fgets(arr[i],25,stdin);
if (getsizedline(arr[i], sizeof(arr[i]), "Too long, try again.\n") == NULL) {
break;
}
This code uses a buffer slightly larger than the required max length. If a text line and the newline can't be read into the buffer, it reads the rest of the line and discards it. If it can, it again discards if too long (or too short).
#include <stdio.h>
#include <string.h>
#include <stdbool.h>
#define INPUTS 10
#define STRMAX 25
int main(void) {
char arr[INPUTS][STRMAX+1];
char buf[STRMAX+4];
for(int i = 0; i < INPUTS; i++) {
bool success = false;
while(!success) {
printf("Enter string %d: ", i + 1);
if(fgets(buf, sizeof buf, stdin) == NULL) {
exit(1); // or sth better
}
size_t index = strcspn(buf, "\n");
if(buf[index] == '\0') { // no newline found
// keep reading until end of line
while(fgets(buf, sizeof buf, stdin) != NULL) {
if(strchr(buf, '\n') != NULL) {
break;
}
}
if(feof(stdin)) {
exit(1); // or sth better
}
continue;
}
if(index < 1 || index > STRMAX) {
continue; // string is empty or too long
}
buf[index] = '\0'; // truncate newline
strcpy(arr[i], buf); // keep this OK string
success = true;
}
}
printf("Results:\n");
for(int i = 0; i < INPUTS; i++) {
printf("%s\n", arr[i]);
}
return 0;
}
The nice thing about fgets() is that it will place the line-terminating newline character ('\n') in the input buffer. All you have to do is look for it. If it is there, you got an entire line of input. If not, there is more to read.
The strategy then, is:
fgets( s, size_of_s, stdin );
char * p = strpbrk( s, "\r\n" );
if (p)
{
// end of line was found.
*p = '\0';
return s; (the complete line of input)
}
If p is NULL, then there is more work to do. Since you wish to simply ignore lines that are too long, that is the same as throwing away input. Do so with a simple loop:
int c;
do c = getchar(); while ((c != EOF) && (c != '\n'));
Streams are typically buffered behind the scenes, either by the C Library or by the OS (or both), but even if they aren’t this is not that much of an overhead. (Use a profiler before playing “I’m an optimizing compiler”. Don’t assume bad things about the C Library.)
Once you have tossed everything you didn’t want (to EOL), make sure your input isn’t at EOF and loop to ask the user to try again.
Putting it all together
char * prompt( const char * message, char * s, size_t n )
{
while (!feof( stdin ))
{
// Ask for input
printf( "%s", message );
fflush( stdout ); // This line _may_ be necessary.
// Attempt to get an entire line of input
if (!fgets( s, n, stdin )) break;
char * p = strpbrk( s, "\r\n" );
// Success: return that line (sans newline character(s)) to the user
if (p)
{
*p = '\0';
return s;
}
// Failure: discard the remainder of the line before trying again
int c;
do c = getchar(); while ((c != EOF) && (c != '\n'));
}
// If we get this far it is because we have
// reached EOF or some other input error occurred.
return NULL;
}
Now you can use this utility function easily enough:
char user_name[20]; // artificially small
if (!prompt( "What is your name (maximum 19 characters)? ", user_name, sizeof(user_name) ))
{
complain_and_quit();
// ...because input is dead in a way you likely cannot fix.
// Feel free to check ferror(stdin) and feof(stdin) for more info.
}
This little prompt function is just an example of the kinds of helper utility functions you can write. You can do things like have an additional prompt for when the user does not obey you:
What is your name? John Jacob Jingleheimer Schmidt
Alas, I am limited to 19 characters. Please try again:
What is your name? John Schmidt
Hello John Schmidt.

How to read multiple lines of string from stdin in C?

I am a novice in C programming. Suppose I want to read multiple lines of string from stdin. How can I keep reading until a line only containing EOL?
example of input
1+2\n
1+2+3\n
1+2+3+4\n
\n (stop at this line)
It seems that when I hit enter(EOL) directly, scanf won't execute until something other than just EOL has been entered. How can I solve that problem?
I'll be really grateful if someone can help me with this. Thank you.
If you want to learn C, you should avoid scanf. The only use cases where scanf actually makes sense are in problems for which C is the wrong language. Time spent learning the foibles of scanf is not well spent, and it doesn't really teach you much about C. For something like this, just read one character at a time and stop when you see two consecutive newlines. Something like:
#include <stdio.h>
int
main(void)
{
char buf[1024];
int c;
char *s = buf;
while( (c = fgetc(stdin)) != EOF && s < buf + sizeof buf - 1 ){
if( c == '\n' && s > buf && s[-1] == '\n' ){
ungetc(c, stdin);
break;
}
*s++ = c;
}
*s = '\0';
printf("string entered: %s", buf);
return 0;
}
to read multiple lines of string from stdin. How can I keep reading until a line only containing EOL?
Keep track of when reading the beginning of the line. If a '\n' is read at the beginning, stop
getchar() approach:
bool beginning = true;
int ch;
while ((ch = getchar()) != EOF) {
if (beginning) {
if (ch == '\n') break;
}
// Do what ever you want with `ch`
beginning = ch == '\n';
}
fgets() approach - needs more code to handle lines longer than N
#define N 1024
char buf[N+1];
while (fgets(buf, sizeof buf, stdin) && buf[0] != '\n') {
; // Do something with buf
}
If you need to read one character at a time then you can with either getchar or fgetc depending upon whether or not you're reading from stdin or some other stream.
But you said you were reading strings, so I'm assuming fgets is more appropriate.
There are primarily two considerations:
maximum line length
whether or not to handle Windows versus non-Windows line endings
Even if you are a beginner--and I won't go into #2 here--you should know you can defend against it. I will at least say that if you compile on one platform and read from stdin from a redirected file from another platform, then you might have to write a defense.
#include <stdio.h>
#include <string.h>
#include <errno.h>
int main (int argc, char *argv[]) {
char buf[32]; // relatively small buf makes testing easier
int lineContinuation = 0;
// If no characters are read, then fgets returns NULL.
while (fgets(buf, sizeof(buf), stdin) != NULL) {
int l = strlen(buf); // No newline in buf if line len + newline exceeds sizeof(buf)
if (buf[l-1] == '\n') {
if (l == 1 && !lineContinuation) {
break; // errno should indicate no error.
}
printf("send line ending (len=%d) to the parser\n", l);
lineContinuation = 0;
} else {
lineContinuation = 1;
printf("send line part (len=%d) to the parser\n", l);
}
}
printf("check errno (%d) if you must handle unexpected end of input use cases\n", errno);
}

Use and explanation of getchar() function

I am writing a program to read a user input statement and extract all integers from the input. For example, if I enter "h3ll0", the program will output "30". I have used the fgets function to read the user input.
However, I am currently reading about getchar() and would like to know what would be the best way to use getchar() in my program to read user input instead of fgets. I am not really clear on how getchar() works and what situations it can be useful in.
This question is related to a project that specifically asks for getchar() as the method of reading user input. As I was unclear on how getchar() works, I built the rest of the program using fgets to ensure it was working.
#include <stdio.h>
int main()
{
char user_input[100];
int i;
int j = 0;
printf("Please enter your string: ");
fgets(user_input ,100, stdin);
for(i = 0; user_input[i] ; i++)
{
if(user_input[i] >= '0' && user_input[i] <= '9')
{
user_input[j] = user_input[i];
j++;
}
}
user_input[j] = '\0';
printf("Your output of only integers is: ");
printf("%s\n", user_input);
return 0;
}
OP: unclear on how getchar() works
int fgetc(FILE *stream) typically returns 1 of 257 different values.
"If ... a next character is present, the fgetc function obtains that character as an unsigned char converted to an int C11 §7.21.7.1 2
On end-of-file or input error (rare), EOF, is returned.
OP: to use getchar() in my program to read user input instead of fgets.
Create your own my_fgets() with the same function signature and same function as fgets() and then replace.
char *fgets(char * restrict s, int n, FILE * restrict stream);
The fgets function reads at most one less than the number of characters specified by n from the stream pointed to by stream into the array pointed to by s. No additional characters are read after a new-line character (which is retained) or after end-of-file. A null character is written immediately after the last character read into the array. C11 §7.21.7.2 2
Return the same value
The fgets function returns s if successful. If end-of-file is encountered and no characters have been read into the array, the contents of the array remain unchanged and a null pointer is returned. If a read error occurs during the operation, the array contents are indeterminate and a null pointer is returned. §7.21.7.2 3
Sample untested code
#include <stdbool.h>
#include <stdio.h>
char *my_fgets(char * restrict s, int n, FILE * restrict stream) {
bool something_read = false;
int ch = 0;
char *dest = s;
// Room ("reads at most one less") and EOF not returned?
while (n > 1 && (ch = fgetc(stream)) != EOF) {
n--;
something_read = true;
*dest++ = (char) ch;
if (ch == '\n') {
break; // "No additional characters are read after a new-line character"
}
}
// Did code end the while loop due to EOF?
if (ch == EOF) {
// Was EOF due to end-of-file or rare input error?
if (feof(stream)) {
// "If end-of-file is encountered and no characters ... read into the array ..."
if (!something_read) {
return NULL;
}
} else {
// "If a read error ..."
return NULL; // ** Note 1
}
}
// room for \0?
if (n > 0) {
*dest = '\0'; //" A null character is written immediately after the last character"
}
return s;
}
Perhaps improve fgets() and use size_t for n.
char *my_fgets(char * restrict s, size_t n, FILE * restrict stream);
fgets() with n <= 0 is not clearly defined. Using size_t, an unsigned type, at least eliminates n < 0 concerns.
Note 1: or use s = NULL; instead of return NULL; and let the remaining code null terminate the buffer. We have that option as "array contents are indeterminate".
Something like this should work as a clunky replacement to fgets using only getchar. I don't guarantee the accuracy of the error handling.
I don't think you would ever want to use getchar over fgets in an application. Getchar is more limited and less secure.
#include <stdint.h>
void your_fgets(char *buffer, size_t buffer_size)
{
int i;
size_t j;
if (buffer_size == 0)
return ;
else if (buffer_size == 1)
{
buffer[0] = '\0';
return ;
}
j = 0;
while ((i = getchar()) != EOF)
{
buffer[j++] = i;
if (j == buffer_size - 1 || i == '\n')
{
buffer[j] = '\0';
return ;
}
}
buffer[j] = '\0';
}
I am baffled by the comments on this post suggesting that fgets is easier to use. Using fgets unnecessarily complicates the issue. Just do:
#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
int
main(int argc, char **argv)
{
int c;
while( ( c = getchar() ) != EOF ) {
if(isdigit(c) && (putchar(c) == EOF)) {
perror("stdout");
return EXIT_FAILURE;
}
}
return ferror(stdin);
}
There is absolutely no reason to use any additional buffering, or read the input one line at a time. Maybe you'll want to output newlines as they come in, but that would be an implementation detail that is left unspecified in the question. Either way, it's utterly trivial (if(( c == '\n' || isdigit(c)) && (putchar(c) == EOF))). Just read a character and decide if you want to output it. The logic is much easier if you don't think about the input as being more complicated than it is. (It's not line-oriented...it's just a stream of bytes.)
If, for some unknown reason you want to make this tool usable only in an interactive setting and load up your output with excess verbosity, you can easily do something like:
#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
int
main(int argc, char **argv)
{
int c;
do {
int want_header = 1;
printf("Please enter your string: ");
while( ( c = getchar() ) != EOF && c != '\n' ) {
if(! isdigit(c)) {
continue;
}
if(want_header) {
want_header=0;
printf("Your output of only integers is: ");
}
if(putchar(c) == EOF) {
perror("stdout");
return EXIT_FAILURE;
}
}
if( c == '\n')
putchar(c);
want_header = 0;
} while(c == '\n');
return ferror(stdin);
}
but, please, don't do that. (Imagine if grep started by emitting a prompt that said "please enter the regex you would like to search for"!)

How can I exclude floating point numbers from getting the input and send error message to the user?

I need to code a program that only accepts integer from 1-10(excluding characters and floating point numbers). I am using fgets. It runs but I cannot exclude floating point numbers. This is part of my code:
char choice[256];
int choice1;
fgets(choice, 256, stdin);
choice1 = atoi(choice);
if (choice1 > 0 && choice1 <= 10)
{
switch (choice1)
{
case 1:
...
case 10:
Help?
You can use strtol() to do the conversion instead of atoi(). This will give you a pointer to the first character that wasn't part of the number. If that character isn't a blank, the number wasn't integer.
EDIT
Something like below may help. You need to change as per your requirement. See the manpage of strtol
#include <stdio.h>
#include <stdlib.h>
int main (void)
{
int choice1;
char *endptr, choice[256];
fgets (choice, 256, stdin);
choice1 = strtol (choice, &endptr, 10);
if (endptr != NULL && *endptr != '\n')
{
printf ("INVALID\n");
}
printf ("%d\n", choice1);
return 0;
}
The endptr will hold the location of the location of the first invalid character. Comparison with \n is required because the fgets will also store the newline in the buffer. You might want to process this in some other way. The above code shows an outline.
Or you might like to manually iterate on the string and discard it depending on the contents. May be something like below will work.
fgets (choice, 256, stdin);
for (i=0; choice[i] != '\0' || choice[i] != '\n'; i++)
{
if (!isdigit (choice[i]))
{
flag = 0;
break;
}
}
When you use fgets if the line is terminated with a newline character, it will be stored in the string.
You can take help do while loop.
int c;
do
{
c = getchar();
if(atoi(c) > 0 && atoi(c) <=9)
{
// append character to character array(string)
// For user to under stand what he has entered you can use putchar(c);
}
}while(c!=13)
This is not exact solution but you can do something like this. unfortunately I don't have installed c compiler on my machine so I have not tasted this code.

Using scanf to accept user input

gcc 4.4.2
I was reading an article about scanf. I personally have never checked the return code of a scanf.
#include <stdio.h>
int main(void)
{
char buf[64];
if(1 == scanf("%63s", buf))
{
printf("Hello %s\n", buf);
}
else
{
fprintf(stderr, "Input error.\n");
}
return 0;
}
I am just wondering what other techniques experienced programmers do when they use scanf when they want to get user input? Or do they use another function or write their own?
Thanks for any suggestions,
EDIT =========
#include <stdio.h>
int main(void)
{
char input_buf[64] = {0};
char data[64] = {0};
printf("Enter something: ");
while( fgets(input_buf, sizeof(input_buf), stdin) == NULL )
{
/* parse the input entered */
sscanf(input_buf, "%s", data);
}
printf("Input [ %s ]\n", data);
return 0;
}
I think most programmers agree that scanf is bad, and most agree to use fgets and sscanf. However, I can use fgets to readin the input. However, if I don't know what the user will enter how do I know what to parse. For example, like if the user was to enter their address which would contain numbers and characters and in any order?
Don't use scanf directly. It's surprisingly hard to use. It's better to read an entire line of input and to then parse it (possibly with sscanf).
Read this entry (and the entries it references) from the comp.lang.c FAQ:
http://c-faq.com/stdio/scanfprobs.html
Edit:
Okay, to address your additional question from your own edit: If you allow unstructured input, then you're going to have to attempt to parse the string in multiple ways until you find one that works. If you can't find a valid match, then you should reject the input and prompt the user again, probably explaining what format you want the input to be in.
For anything more complicated, you'd probably be better off using a regular expression library or even using dedicated lexer/parser toolkits (e.g. flex and bison).
I don't use scanf() for interactive user input; I read everything as text using fgets(), then parse the input as necessary, using strtol() and strtod() to convert text to numeric values.
One example of where scanf() falls down is when the user enters a bad numeric value, but the initial part of it is valid, something like the following:
if (scanf("%d", &num) == 1)
{
// process num
}
else
{
// handle error
}
If the user types in "12e4", scanf() will successfully convert and assign the "12" to num, leaving "e4" in the input stream to foul up a future read. The entire input should be treated as bogus, but scanf() can't catch that kind of error. OTOH, if I do something like:
if (fgets(buffer, sizeof buffer, stdin))
{
int val;
char *chk;
val = (int) strtol(buffer, &chk, 10);
if (!isspace(*chk) && *chk != 0)
{
// non-numeric character in input; reject it completely
}
else
{
// process val
}
}
I can catch the error in the input and reject it before using any part of it. This also does a better job of not leaving garbage in the input stream.
scanf() is a great tool if you can guarantee your input is always well-formed.
scanf() has problems, in that if a user is expected to type an integer, and types a string instead, often the program bombs. This can be overcome by reading all input as a string (use getchar()), and then converting the string to the correct data type.
/* example one, to read a word at a time */
#include <stdio.h>
#include <ctype.h>
#define MAXBUFFERSIZE 80
void cleartoendofline( void ); /* ANSI function prototype */
void cleartoendofline( void )
{
char ch;
ch = getchar();
while( ch != '\n' )
ch = getchar();
}
main()
{
char ch; /* handles user input */
char buffer[MAXBUFFERSIZE]; /* sufficient to handle one line */
int char_count; /* number of characters read for this line */
int exit_flag = 0;
int valid_choice;
while( exit_flag == 0 ) {
printf("Enter a line of text (<80 chars)\n");
ch = getchar();
char_count = 0;
while( (ch != '\n') && (char_count < MAXBUFFERSIZE)) {
buffer[char_count++] = ch;
ch = getchar();
}
buffer[char_count] = 0x00; /* null terminate buffer */
printf("\nThe line you entered was:\n");
printf("%s\n", buffer);
valid_choice = 0;
while( valid_choice == 0 ) {
printf("Continue (Y/N)?\n");
scanf(" %c", &ch );
ch = toupper( ch );
if((ch == 'Y') || (ch == 'N') )
valid_choice = 1;
else
printf("\007Error: Invalid choice\n");
cleartoendofline();
}
if( ch == 'N' ) exit_flag = 1;
}
}
I make a loop call fgets until the end of the line is read, and then call sscanf to parse the data. It's a good idea to check whether sscanf reaches the end of the input line.
I rarely use scanf. Most of the times, I use fgets() to read data as a string. Then, depending upon the need, I may use sscanf(), or other functions such as strto* family of functions, str*chr(), etc., to get data from the string.
If I use scanf() or fgets() + sscanf(), I always check the return values of the functions to make sure they did what I wanted them to do. I also don't use strtok() to tokenize strings, because I think the interface of strtok() is broken.

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