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How to read / parse input in C? The FAQ
(1 answer)
Closed 2 years ago.
I am implementing a super simple menu selection procedure with a basic input validity check mechanism. The legal inputs are {1,2,3} so the possible issues would be either a number that is out of this range, or a non integer. My code is shown below. This works fine for the former issue (i.e. when i input "4") but for the latter (when i try to input a char), it prints the invalidity message over and over again rather than waiting for a new input, it's like it skips the scanf line entirely on every iteration except the first.. what am I missing here ?
do{
try = scanf("%d", &selection);
if(try!=1 || selection < 1 || selection > 3){
printf("\nInvalid input. Dear guest, please enter '1', '2', or '3'.\n\nInput:");
}
}while(try!=1 || selection < 1 || selection > 3);
After entering a character which cannot be part of the text representation of a decimal number, say 'A', the input available to the program is the byte sequence 'A' '\n' (the latter being a newline), or perhaps 'A' '\r' '\n' on Windows (carriage return followed by a newline).
When scanf tries to parse a number from these characters it balks already at the 'A' and puts it back in the input stream. Input streams in C guarantee that you can perform at least one ungetc(), that is, put at least one character back into the stream so that it will be the first character read by the next input operation. This simple but ingenious facility makes it a lot easier to process variable-format input: Imagine you parse an expression in some C source code, and both integer literal or a variable name are syntactically allowed as the next token: You can try the number first, and if that fails, the input still contains all of the variable name to process. The work of keeping the first failing character "in mind" and making it available to other parts of the program is encapsulated in the FILE implementation.
This is what happens here. The first failing scanf() puts the 'A' back so that it will be encountered again by the next attempt, ad infinitum. The 'A' must be removed from the input. More specifically, the next "word" should be removed from the input altogether: The user may have entered "kkjkllkjlk", and you don't want 10 error messages for that. You can decide whether you would accept "lklkj2" (and read the 2) but it is simpler to discard the whole word.
You can also decide whether you would accept "1 2 3 2" as 4 valid successive inputs or whether you demand newlines between the numbers; for generality (for example, if the input does not come from a terminal) I would accept all number sequences separated by whitespace of any kind. In this case you simply want to read to the next whitespace:
#include <ctype.h>
// ...
while(!isspace(getchar())) { /* ignore */ }
This should do the trick. It is possible that more whitespace is following this one, including newlines etc., but that's OK: The symbolic input conversions of scanf (like %d) skip leading whitespace.
I think it is neat to let the user exit the program by ending the input (inserting end-of-file by pressing Ctrl-z in a Windows Console, or Ctrl-d on a Posix terminal), so I would test for the special case of scanf() returning EOF. If the input is from a pipe that may actually be essential. The fringe case that a bad input is immediately followed by EOF needs a check for EOF even in the code discarding wrong input (Posix: echo -n "a" | myprog would hang; -n suppresses the usual newline which echo usually appends). Putting it all together, my take on the input loop is this:
while(1) { // break on good input
printf("Please enter your choice of 1, 2 or 3:\n");
try = scanf("%d", &selection);
if(try!=1 || selection < 1 || selection > 3){
if(try == EOF)
{
return 0;
}
printf("\nThe input was not 1,2 or 3. Please try again.\n");
int discard;
{
do{
discard = getchar();
if(discard == EOF) { return 0;} // catches EOF after bad char
}while(!isspace(discard));
}
}
else break;
}
Related
I'm trying to develop a simple text-based hangman game, and the main game loop starts with a prompt to enter a guess at each letter, then goes on to check if the letter is in the word and takes a life off if it isn't. However, when I run the game the prompt comes up twice each time, and the program doesn't wait for the user's input. It also takes off a life (one life if it was the right input, two if it wasn't), so whatever it's taking in isn't the same as the previous input. Here's my game loop, simplified a bit:
while (!finished)
{
printf("Guess the word '%s'\n",covered);
scanf("%c", ¤tGuess);
i=0;
while (i<=wordLength)
{
if (i == wordLength)
{
--numLives;
printf("Number of lives: %i\n", numLives);
break;
} else if (currentGuess == secretWord[i]) {
covered[i] = secretWord[i];
secretWord[i] = '*';
break;
}
++i;
}
j=0;
while (j<=wordLength)
{
if (j == (wordLength)) {
finished = 1;
printf("Congratulations! You guessed the word!\n");
break;
} else {
if (covered[j] == '-') {
break;
}
}
++j;
if (numLives == 0) {
finished = 1;
}
}
}
I assume the problem is scanf thinking it's taken something in when it hasn't, but I have no idea why. Does anyone have any idea? I'm using gcc 4.0.1 on Mac OS X 10.5.
When you read keyboard input with scanf(), the input is read after enter is pressed but the newline generated by the enter key is not consumed by the call to scanf(). That means the next time you read from standard input there will be a newline waiting for you (which will make the next scanf() call return instantly with no data).
To avoid this, you can modify your code to something like:
scanf("%c%*c", ¤tGuess);
The %*c matches a single character, but the asterisk indicates that the character will not be stored anywhere. This has the effect of consuming the newline character generated by the enter key so that the next time you call scanf() you are starting with an empty input buffer.
Caveat: If the user presses two keys and then presses enter, scanf() will return the first keystroke, eat the second, and leave the newline for the next input call. Quirks like this are one reason why scanf() and friends are avoided by many programmers.
Newlines.
The first time through the loop, scanf() reads the character.
Then it reads the newline.
Then it reads the next character; repeat.
How to fix?
I seldom use scanf(), but if you use a format string "%.1s", it should skip white space (including newlines) and then read a non-white space character. However, it will be expecting a character array rather than a single character:
char ibuff[2];
while ((scanf("%.1s", ibuff) == 1)
{
...
}
Break the problem up into smaller parts:
int main(void) {
char val;
while (1) {
printf("enter val: ");
scanf("%c", &val);
printf("got: %d\n", val);
}
}
The output here is:
enter val: g
got: 103
enter val: got: 10
Why would scanf give you another '10' in there?
Since we printed the ASCII number for our value, '10' in ASCII is "enter" so scanf must also grab the "enter" key as a character.
Sure enough, looking at your scanf string, you are asking for a single character each time through your loop. Control characters are also considered characters, and will be picked up. For example, you can press "esc" then "enter" in the above loop and get:
enter val: ^[
got: 27
enter val: got: 10
Just a guess, but you are inputting a single character with scanf, but the user must type the guess plus a newline, which is being consumed as a separate guess character.
scanf(" %c", &fooBar);
Notice the space before the %c. This is important, because it matches all preceding whitespace.
Jim and Jonathan have it right.
To get your scanf line to do what you want (consume the newline character w/o putting it in the buffer) I'd change it to
scanf("%c\n", ¤tGuess);
(note the \n)
The error handling on this is atrocious though. At the least you should check the return value from scanf against 1, and ignore the input (with a warning) if it doesn't return that.
A couple points I noticed:
scanf("%c") will read 1 character and keep the ENTER in the input buffer for next time through the loop
you're incrementing i even when the character read from the user doesn't match the character in secretWord
when does covered[j] ever get to be '-'?
I'll guess: your code is treating a newline as one of the guesses when you enter data. I've always avoided the *scanf() family due to uncontrollable error handling. Try using fgets() instead, then pulling out the first char/byte.
I see a couple of things in your code:
scanf returns the number of items it read. You will probably want to handle the cases where it returns 0 or EOF.
My guess would be that the user is hitting letter + Enter and you're getting the newline as the second character. An easy way to check would be to add a debugging printf statement to show what character was entered.
Your code will only match the first occurrence of a match letter, i.e. if the word was "test" and the user entered 't', your code would only match the first 't', not both. You need to adjust your first loop to handle this.
When you enter the character, you have to enter a whitespace character to move on. This whitespace character is present in the input buffer, stdin file, and is read by the scanf() function.
This problem can be solved by consuming this extra character. This can be done by usnig a getchar() function.
scanf("%c",¤tGuess);
getchar(); // To consume the whitespace character.
I would rather suggest you to avoid using scanf() and instead use getchar(). The scanf()requires a lot of memory space. getchar() is a light function. So you can also use-
char currentGuess;
currentGuess=getchar();
getchar(); // To consume the whitespace character.
I'm trying to develop a simple text-based hangman game, and the main game loop starts with a prompt to enter a guess at each letter, then goes on to check if the letter is in the word and takes a life off if it isn't. However, when I run the game the prompt comes up twice each time, and the program doesn't wait for the user's input. It also takes off a life (one life if it was the right input, two if it wasn't), so whatever it's taking in isn't the same as the previous input. Here's my game loop, simplified a bit:
while (!finished)
{
printf("Guess the word '%s'\n",covered);
scanf("%c", ¤tGuess);
i=0;
while (i<=wordLength)
{
if (i == wordLength)
{
--numLives;
printf("Number of lives: %i\n", numLives);
break;
} else if (currentGuess == secretWord[i]) {
covered[i] = secretWord[i];
secretWord[i] = '*';
break;
}
++i;
}
j=0;
while (j<=wordLength)
{
if (j == (wordLength)) {
finished = 1;
printf("Congratulations! You guessed the word!\n");
break;
} else {
if (covered[j] == '-') {
break;
}
}
++j;
if (numLives == 0) {
finished = 1;
}
}
}
I assume the problem is scanf thinking it's taken something in when it hasn't, but I have no idea why. Does anyone have any idea? I'm using gcc 4.0.1 on Mac OS X 10.5.
When you read keyboard input with scanf(), the input is read after enter is pressed but the newline generated by the enter key is not consumed by the call to scanf(). That means the next time you read from standard input there will be a newline waiting for you (which will make the next scanf() call return instantly with no data).
To avoid this, you can modify your code to something like:
scanf("%c%*c", ¤tGuess);
The %*c matches a single character, but the asterisk indicates that the character will not be stored anywhere. This has the effect of consuming the newline character generated by the enter key so that the next time you call scanf() you are starting with an empty input buffer.
Caveat: If the user presses two keys and then presses enter, scanf() will return the first keystroke, eat the second, and leave the newline for the next input call. Quirks like this are one reason why scanf() and friends are avoided by many programmers.
Newlines.
The first time through the loop, scanf() reads the character.
Then it reads the newline.
Then it reads the next character; repeat.
How to fix?
I seldom use scanf(), but if you use a format string "%.1s", it should skip white space (including newlines) and then read a non-white space character. However, it will be expecting a character array rather than a single character:
char ibuff[2];
while ((scanf("%.1s", ibuff) == 1)
{
...
}
Break the problem up into smaller parts:
int main(void) {
char val;
while (1) {
printf("enter val: ");
scanf("%c", &val);
printf("got: %d\n", val);
}
}
The output here is:
enter val: g
got: 103
enter val: got: 10
Why would scanf give you another '10' in there?
Since we printed the ASCII number for our value, '10' in ASCII is "enter" so scanf must also grab the "enter" key as a character.
Sure enough, looking at your scanf string, you are asking for a single character each time through your loop. Control characters are also considered characters, and will be picked up. For example, you can press "esc" then "enter" in the above loop and get:
enter val: ^[
got: 27
enter val: got: 10
Just a guess, but you are inputting a single character with scanf, but the user must type the guess plus a newline, which is being consumed as a separate guess character.
scanf(" %c", &fooBar);
Notice the space before the %c. This is important, because it matches all preceding whitespace.
Jim and Jonathan have it right.
To get your scanf line to do what you want (consume the newline character w/o putting it in the buffer) I'd change it to
scanf("%c\n", ¤tGuess);
(note the \n)
The error handling on this is atrocious though. At the least you should check the return value from scanf against 1, and ignore the input (with a warning) if it doesn't return that.
A couple points I noticed:
scanf("%c") will read 1 character and keep the ENTER in the input buffer for next time through the loop
you're incrementing i even when the character read from the user doesn't match the character in secretWord
when does covered[j] ever get to be '-'?
I'll guess: your code is treating a newline as one of the guesses when you enter data. I've always avoided the *scanf() family due to uncontrollable error handling. Try using fgets() instead, then pulling out the first char/byte.
I see a couple of things in your code:
scanf returns the number of items it read. You will probably want to handle the cases where it returns 0 or EOF.
My guess would be that the user is hitting letter + Enter and you're getting the newline as the second character. An easy way to check would be to add a debugging printf statement to show what character was entered.
Your code will only match the first occurrence of a match letter, i.e. if the word was "test" and the user entered 't', your code would only match the first 't', not both. You need to adjust your first loop to handle this.
When you enter the character, you have to enter a whitespace character to move on. This whitespace character is present in the input buffer, stdin file, and is read by the scanf() function.
This problem can be solved by consuming this extra character. This can be done by usnig a getchar() function.
scanf("%c",¤tGuess);
getchar(); // To consume the whitespace character.
I would rather suggest you to avoid using scanf() and instead use getchar(). The scanf()requires a lot of memory space. getchar() is a light function. So you can also use-
char currentGuess;
currentGuess=getchar();
getchar(); // To consume the whitespace character.
char c = ' ';
while(c != 'x')
{
c = getchar();
printf("threadFuncParam = %u\n", threadFuncParam);
}
In the above code snippet the print is printing threadFuncParam value twice every time I enter a character where as I expect it to print only once.
As per my understanding very first time it enters while as condition is true and and then wait for user input at getchar call on entering a character it shall print the value and then if input character is not x it shall wait for another input at getchar call, but whenever i enter a character i see 2 prints ion screen for each input. I am not able to understand why?
What i'm trying to do here is that in main thread i am taking single char input from user and in worker thread i am incrementing a counter which gets incremented every second, i print the value everytime user input a char input until user enters 'x' character.
The simple issue here is that the console (input) sends the text to the stdin only when it encounters a enter key or ctr+d.
As a result an extra \n goes into your input stream. This causes your program to read that character in the next iteration.
A simple solution would be to read all white space characters in the stream.
This can be done in multiple ways -
If you need to discard any whitespace characters (including space, tabs, vtabs, newline), you can add
scanf(" ");
before the getchar();
If you need to discard only the newlines which come as a result of pressing enter, you can add
scanf("%*[\n]")
before the getchar();
This will eat up all the \n that come before the next character. But will not eat spaces.
Finally if you want to discard only 1 \n
You can do
scanf("%*1[\n]");
Remember though, in all the cases the scanf should be immediately before the getchar() because scanf would wait till it finds the next non white space character.
All cases figured out with the help of comments from #chux.
If you tried to debug your program (its best way to learn to code), you would spot that the second value is everytime 10. In ASCII table you would find that is code for new line. Which you are pressing after the every character. Then by fast & simple searching you would find this THREAD. Where is the problem described and you would easily solve it.
char c = ' ';
while(c != 'x')
{
printf("threadFuncParam = %u\n", (char)c);
fflush(stdout);
if (scanf(" %c",&c) != 1)
{
// failed
}
}
I think the problem is in the way logic is applied. You print before you check the condition. See this for explanation:
loop 1: getchar() executed say for value 'a'.
printf() executed.
loop 2: Because c='a', condition is true. Now, getchar() is executed with 'x' value.
printf() is again executed.
loop 3: Condition is evaluated false. Loop is terminated.
So basically you should change your logic a little bit.
I'm trying to develop a simple text-based hangman game, and the main game loop starts with a prompt to enter a guess at each letter, then goes on to check if the letter is in the word and takes a life off if it isn't. However, when I run the game the prompt comes up twice each time, and the program doesn't wait for the user's input. It also takes off a life (one life if it was the right input, two if it wasn't), so whatever it's taking in isn't the same as the previous input. Here's my game loop, simplified a bit:
while (!finished)
{
printf("Guess the word '%s'\n",covered);
scanf("%c", ¤tGuess);
i=0;
while (i<=wordLength)
{
if (i == wordLength)
{
--numLives;
printf("Number of lives: %i\n", numLives);
break;
} else if (currentGuess == secretWord[i]) {
covered[i] = secretWord[i];
secretWord[i] = '*';
break;
}
++i;
}
j=0;
while (j<=wordLength)
{
if (j == (wordLength)) {
finished = 1;
printf("Congratulations! You guessed the word!\n");
break;
} else {
if (covered[j] == '-') {
break;
}
}
++j;
if (numLives == 0) {
finished = 1;
}
}
}
I assume the problem is scanf thinking it's taken something in when it hasn't, but I have no idea why. Does anyone have any idea? I'm using gcc 4.0.1 on Mac OS X 10.5.
When you read keyboard input with scanf(), the input is read after enter is pressed but the newline generated by the enter key is not consumed by the call to scanf(). That means the next time you read from standard input there will be a newline waiting for you (which will make the next scanf() call return instantly with no data).
To avoid this, you can modify your code to something like:
scanf("%c%*c", ¤tGuess);
The %*c matches a single character, but the asterisk indicates that the character will not be stored anywhere. This has the effect of consuming the newline character generated by the enter key so that the next time you call scanf() you are starting with an empty input buffer.
Caveat: If the user presses two keys and then presses enter, scanf() will return the first keystroke, eat the second, and leave the newline for the next input call. Quirks like this are one reason why scanf() and friends are avoided by many programmers.
Newlines.
The first time through the loop, scanf() reads the character.
Then it reads the newline.
Then it reads the next character; repeat.
How to fix?
I seldom use scanf(), but if you use a format string "%.1s", it should skip white space (including newlines) and then read a non-white space character. However, it will be expecting a character array rather than a single character:
char ibuff[2];
while ((scanf("%.1s", ibuff) == 1)
{
...
}
Break the problem up into smaller parts:
int main(void) {
char val;
while (1) {
printf("enter val: ");
scanf("%c", &val);
printf("got: %d\n", val);
}
}
The output here is:
enter val: g
got: 103
enter val: got: 10
Why would scanf give you another '10' in there?
Since we printed the ASCII number for our value, '10' in ASCII is "enter" so scanf must also grab the "enter" key as a character.
Sure enough, looking at your scanf string, you are asking for a single character each time through your loop. Control characters are also considered characters, and will be picked up. For example, you can press "esc" then "enter" in the above loop and get:
enter val: ^[
got: 27
enter val: got: 10
Just a guess, but you are inputting a single character with scanf, but the user must type the guess plus a newline, which is being consumed as a separate guess character.
scanf(" %c", &fooBar);
Notice the space before the %c. This is important, because it matches all preceding whitespace.
Jim and Jonathan have it right.
To get your scanf line to do what you want (consume the newline character w/o putting it in the buffer) I'd change it to
scanf("%c\n", ¤tGuess);
(note the \n)
The error handling on this is atrocious though. At the least you should check the return value from scanf against 1, and ignore the input (with a warning) if it doesn't return that.
A couple points I noticed:
scanf("%c") will read 1 character and keep the ENTER in the input buffer for next time through the loop
you're incrementing i even when the character read from the user doesn't match the character in secretWord
when does covered[j] ever get to be '-'?
I'll guess: your code is treating a newline as one of the guesses when you enter data. I've always avoided the *scanf() family due to uncontrollable error handling. Try using fgets() instead, then pulling out the first char/byte.
I see a couple of things in your code:
scanf returns the number of items it read. You will probably want to handle the cases where it returns 0 or EOF.
My guess would be that the user is hitting letter + Enter and you're getting the newline as the second character. An easy way to check would be to add a debugging printf statement to show what character was entered.
Your code will only match the first occurrence of a match letter, i.e. if the word was "test" and the user entered 't', your code would only match the first 't', not both. You need to adjust your first loop to handle this.
When you enter the character, you have to enter a whitespace character to move on. This whitespace character is present in the input buffer, stdin file, and is read by the scanf() function.
This problem can be solved by consuming this extra character. This can be done by usnig a getchar() function.
scanf("%c",¤tGuess);
getchar(); // To consume the whitespace character.
I would rather suggest you to avoid using scanf() and instead use getchar(). The scanf()requires a lot of memory space. getchar() is a light function. So you can also use-
char currentGuess;
currentGuess=getchar();
getchar(); // To consume the whitespace character.
I've got a program here which contains a do-while loop within a specified void method. I'm trying to exit the loop within the function, so that the do-while loop actually works as it is supposed to. Except after I run the program and one of the cases occurs, the program continues to run despite my while statement stating that it should only work while(userInput != 1).
I cannot use global variables to solve this problem, as my assignment limits me on using such techniques, thus any help would be much appreciated!
Here is a snippet of my code:
void functionTest()
{
int gameOver = 0;
int userInput;
do
{
printf("please enter a number 1-3");
scanf("%d",&userInput);
switch(userInput)
{
case 1:
printf("You entered %d",userInput);
gameOver = 1;
break;
case 2:
printf("You entered %d",userInput);
gameOver = 1;
break;
case 3:
printf("You entered %d",userInput);
gameOver = 1;
break;
}
}
while(gameOver!= 1);
}
}
The problem probably lies when you use scanf(). Something that you're inputting before hitting enter is not 1, 2 or 3. Could you tell us exactly what you type when it asks you to enter a choice?
Sometimes, the standard output needs to be flushed before using a fresh scanf(). Try fflush(stdout) before the scanf line.
See older question 1 and older question 2.
EDIT:
I can reproduce the problem easily enough if I enter anything apart from "1","2" or "3"...
I would suggest, you do the following before executing the switch statement:
Add fflush(stdout) before scanf()
Accept the input as a string (%s) instead of a number. (char [] needed)
Trim the string of trailing and leading white spaces.
Convert to number using a library function
Then switch-case based on that number
The problem is that if other characters (that aren't part of an integer) are present in the input stream before an integer can be read, scanf() fails and unusable data is never cleared out... which leads to an infinite loop (where scanf() repeatedly fails to read the same characters as an integer, over and over).
So you need to read off the invalid characters when scanf() fails, or as part of the format.
A simple fix would be to change your scanf from:
scanf("%d",&userInput);
to:
scanf("%*[^0-9]%d",&userInput);
to read (and discard) any characters in the input stream that aren't digits 0-9 before reading your integer... but that still doesn't check whether scanf fails for any other reason (like a closed input stream).
You could replace it with something like this:
int scanfRes,c;
do {
scanfRes = scanf("%d",&userInput); /* try to read userInput */
/* ..then discard remainder of line */
do {
if ((c = fgetc(stdin)) == EOF)
return; /* ..return on error or EOF */
} while (c != '\n');
} while (scanfRes != 1); /* ..retry until userInput is assigned */
..which will retry scanf() until the field is assigned, discarding the remainder of the line after each attempt, and exiting the function if fgetc() encounters an error or EOF when doing so.