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.
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 am writing a super simple command line based program in C. It's just a small test and the code is very simple. So what it is meant to do is to ask the user for their name, maths grade, english grade, computing grade. Then it figures out their average grade and also tells them the name they entered. Yes I know this is an extremely simple program, but I'm still doing something wrong.
The problem is, one part of my code will run first telling the user to enter their name and then once they do this and press enter the rest of my code will run all at once and then stop working. It's weird I just don't understand what is wrong.
#include <stdio.h>
int main(int argc, const char * argv[])
{
char chr;
char firstname;
int mathsmark, englishmark, computingmark, averagemark;
printf("What is your name?\n");
scanf("%c", &firstname);
printf("\n");
printf("What is your maths mark?\n");
scanf("%d", &mathsmark);
printf("\n");
printf("What is your english mark?\n");
scanf("%d", &englishmark);
printf("\n");
printf("What is your computing mark?\n");
scanf("%d", &computingmark);
printf("\n");
printf("Your name is: %c", firstname);
printf("\n");
averagemark = (mathsmark + englishmark + computingmark) / 3;
printf("%d", averagemark);
printf("\n");
chr = '\0';
while (chr != '\n') {
chr = getchar ();
}
return 0;
}
One major problem is that you've declared firstname to be a single character long, and when you try to read the name from the console, you're using the %c conversion specifier, which reads the next single character from the input stream and stores it to firstname. The remainder of the name is left in the input stream to foul up the remaining scanf calls.
For example, if you type "Jacob" as a first name, then the first scanf call assigns J to firstname, leaving "acob\n" in the input stream.
The next scanf call attempts to convert "acob\n" to an integer value and save it to mathsmark, which fails ("acob\n" is not a valid integer string). Same thing happens for the next two scanf calls.
The last loop
while (chr != '\n')
{
chr = getchar();
}
finally consumes the rest of "acob\n", which contains the newline character (because you hit Enter after typing the name), causing the loop and program to exit.
How do you fix this?
First, you need to declare firstname as an array of char:
char firstname[SOME_SIZE] = {0};
where SOME_SIZE is large enough to handle all your cases. The you need to change scanf call to
scanf("%s", firstname);
This tells scanf to read characters from the input stream up to the next whitespace character and store the results to the firstname array. Note that you don't need to use the & operator here; under most circumstances, an expression of array type will be converted ("decay") to an expression of pointer type, and the value of the expression will be the address of the first element in the array.
Note that scanf is not very safe, and it's not very robust. If you enter more characters than your buffer is sized to hold, scanf will happily store those extra characters to memory following the array, potentially clobbering something important. You can guard against this by using an explicit field width in the conversion specifier, like
scanf(*%29s", firstname);
but in general it's a pain.
scanf is also not very good at detecting bad input. If you enter "12er" as one of your marks, scanf will convert and assign the "12", leaving the "er" in the stream to foul up the next read.
scanf returns the number of successful assignments, so one way to guard against bad input is to check the return value, like so:
if (scanf("%d", &mathmarks) != 1)
{
printf("Bad input detected for math marks\n");
}
Unfortunately, scanf won't remove bad characters from the stream; you'll have to do that yourself using getchar or similar.
This is a common mistake amongst newer C/C++ developers. The scanf function detects you hitting the ENTER/RETURN key to signal the end of input, but it also catches the \n character as well at the end of the input string, so you essentially get two RETURNS being detected.
Please read up on an example of using fgets and sscanf here:
http://www.linuxforums.org/forum/programming-scripting/67560-problem-scanf.html
It will resolve this issue very quickly for you. In the meantime, I strongly urge you to check out this book:
http://www.amazon.com/Primer-Plus-5th-Stephen-Prata/dp/0672326965
It is the most commonly used C programming book in high school and colleges in North America, and has TONS of examples for you to work through, including this specific program you demonstrated above. The print version has more examples than the e-book, so I would just cough up the $30.00 for the printed version.
Good luck!
You might want to look at a few tutorials. Maybe one on Format specifiers and one on strings in C
scanf() reads data from stdin and stores them as specified by the format specifiers. In this case:
char firstname;
scanf("%c", &firstname);
Read 1 character from stdin and store it to firstname:
>> What is your first name?
Mike
Now firstname == 'M' because scanf() read 1 character as we requested.
What you wanted to do was read a string (a bunch of characters):
char firstname[5]; // an array of characters
scanf("%s", firstname); // store as a string
firstname[4] = '\0'; // Truncate the result with a NULL to insure no overflow
>> What is your first name?
Mike
Now firstname is [M][i][k][e][\0] because scanf() read 1 string, as we requested.
Note the same holds true for printf(), a printf with a %c will give you one character where as a printf() with a %s will give you all the characters until the NULL terminator.
You have (at least) two choices.
char firstname[number_big_enough_to_hold_long_name];
/*or */
char *firstname = malloc(sizeof(char) * number_big_enough_to_hold_long_name);
/* ... code ... */
free(firstname);
Further it would be best to limit width of read. scanf() does not know the size (available space) of firstname.
scanf("%number_big_enough_to_hold_long_names", ...
/* i.e. */
char firstname[32];
if(scanf("%31s", firstname) == EOF) {
perror("bad");
return 1;
}
Further you should check if there is anything left before trying next read. I.e. If someone enters "My Name" then only "My" will end up in firstname and "Name" will be left in input stream.
And getchar() returns an int not a char.
getchar
scanf
And search "ansi c char arrays tutorial" or similar.
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.