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I am appalled that the ANSI Standard leaves so many issues undefined ...
I'm appalled that the ANSI Standard leaves so many issues undefined. Isn't a Standard's whole job to standardize these things? It has always been a characteristic of C that certain constructs behaved in whatever way a particular compiler or a particular piece of hardware chose to implement them. Thi...
2015-11-18, 968👍, 0💬

People keep saying that the behavior of ...
People keep saying that the behavior of i = i++ is undefined, but I just tried it on an ANSI-conforming compiler, and got the results I expected. A compiler may do anything it likes when faced with undefined behavior (and, within limits, with implementation-defined and unspecified behavior), includi...
2015-11-18, 924👍, 0💬

What is wrong with this code?
What's wrong with this code? char c; while((c = getchar()) != EOF) ... For one thing, the variable to hold getchar's return value must be an int. EOF is an ``out of band'' return value from getchar: it is distinct from all possible char values which getchar can return. (On modern systems, it does no...
2015-11-16, 988👍, 0💬

I have a simple little program that reads characters until EOF ...
I have a simple little program that reads characters until EOF, but how do I actually enter that ``EOF'' value from the keyboard? I see that EOF is defined by to be -1; am I supposed to enter -1? If you think about it, what you enter can't be -1, because ``-1'' is two characters, and getchar is read...
2015-11-16, 961👍, 0💬

Why does the simple line-copying loop while ...
Why does the simple line-copying loop while(!feof(infp)) { fgets(buf, MAXLINE, infp); fputs(buf, outfp); } copy the last line twice? p> In C, end-of-file is only indicated after an input routine has tried to read, and failed. (In other words, C's I/O is not like Pascal's.) Usually, you should just c...
2015-11-13, 953👍, 0💬

Why do all the lines end up containing copies of the last line?
I'm using fgets to read lines from a file into an array of pointers. Why do all the lines end up containing copies of the last line? You have only allocated memory for one line, linebuf. Each time you call fgets, the previous line is overwritten. fgets doesn't do any memory allocation: unless it rea...
2015-11-13, 933👍, 0💬

My programs prompts and intermediate output dont always show up on the screen ...
My program's prompts and intermediate output don't always show up on the screen, especially when I pipe the output through another program. It's best to use an explicit fflush(stdout) whenever output should definitely be visible (and especially if the text does not end with \n). Several mechanisms a...
2015-11-11, 934👍, 0💬

How can I read one character at a time, without waiting for the RETURN key
How can I read one character at a time, without waiting for the RETURN key Alas, there is no standard or portable way to do these things in C. Concepts such as screens and keyboards are not even mentioned in the Standard, which deals only with simple I/O ``streams'' of characters. Input to a compute...
2015-11-11, 1116👍, 0💬

How can I print a character in a printf format string?
How can I print a '%' character in a printf format string? I tried \%, but it didn't work. Simply double the percent sign: %% . The reason it's tricky to print % signs with printf is that % is essentially printf's escape character. Whenever printf sees a %, it expects it to be followed by a characte...
2015-11-09, 929👍, 0💬

Why doesnt long int work?
Why doesn't long int n = 123456; printf("%d\n", n); work? Whenever you print long ints you must use the l (lower case letter ``ell'') modifier in the printf format (e.g. %ld). printf can't know the types of the arguments which you've passed to it, so you must let it know by using the correct format ...
2015-11-09, 889👍, 0💬

I thought that ANSI function prototypes were supposed to guard against argument type mismatches.
I thought that ANSI function prototypes were supposed to guard against argument type mismatches. When a function accepts a variable number of arguments, its prototype does not (and cannot) provide any information about the number and types of those variable arguments. Therefore, the usual protection...
2015-11-06, 960👍, 0💬

How can printf use f for type double, if scanf requires lf?
Someone told me it was wrong to use %lf with printf. How can printf use %f for type double, if scanf requires %lf? It's true that printf's %f specifier works with both float and double arguments Due to the ``default argument promotions'' (which apply in variable-length argument lists values of type ...
2015-11-06, 940👍, 0💬

What printf format should I use for a typedef like size_t when I don't know whether it is long or some other type?
What printf format should I use for a typedef like size_t when I don't know whether it is long or some other type? Use a cast to convert the value to a known, conservatively-sized type, then use the printf format matching that type. For example, to print the size of a type, you might use printf("%lu...
2015-11-04, 890👍, 0💬

How can I print numbers with commas separating the thousands? What about currency formatted numbers?
How can I print numbers with commas separating the thousands? What about currency formatted numbers? The functions in &lt;locale.h> begin to provide some support for these operations, but there is no standard C function for doing either task. (In Standard C, the only thing printf does in respons...
2015-11-02, 1030👍, 0💬

Why doesnt the call scanf work?
Why doesn't the call scanf("%d", i) work? The arguments you pass to scanf must always be pointers: for each value converted, scanf ``returns'' it by filling in one of the locations you've passed pointers to. To fix the fragment above, change it to scanf("%d", &i) .
2015-11-02, 959👍, 0💬

Why does the call char scanf work?
Why does the call char s[30]; scanf("%s", s); work? I thought you always needed an & on each variable passed to scanf. You always need a pointer; you don't necessarily need an explicit &amp;. When you pass an array to scanf, you do not need the &, because arrays are always passed to func...
2015-10-30, 858👍, 0💬

Why doesnt this code work?
Why doesn't this code: double d; scanf("%f", &amp;d); work? Unlike printf, scanf uses %lf for values of type double, and %f for float.%f tells scanf to expect a pointer-to-float, not the pointer-to-double you gave it. Either use %lf, or declare the receiving variable as a float.
2015-10-30, 844👍, 0💬

Why doesnt that code work?
Why doesn't the code short int s; scanf("%d", &s); work? When converting %d, scanf expects a pointer to an int. To convert to a short int, use %hd .
2015-10-28, 802👍, 0💬

How can I specify a variable width in a scanf format string?
How can I specify a variable width in a scanf format string? You can't; an asterisk in a scanf format string means to suppress assignment. You may be able to use ANSI stringizing and string concatenation to construct a constant format specifier based on a preprocessor macro containing the desired wi...
2015-10-26, 841👍, 0💬

How can I read data from data files with particular formats?
How can I read data from data files with particular formats? How can I read ten floats without having to use a jawbreaker scanf format like "%f %f %f %f %f %f %f %f %f %f"? How can I read an arbitrary number of fields from a line into an array? In general, there are three main ways of parsing data l...
2015-10-26, 787👍, 0💬

When I read numbers from the keyboard with scanf ...
When I read numbers from the keyboard with scanf and a "%d\n" format, like this: int n; scanf("%d\n", &amp;n); printf("you typed %d\n", n); it seems to hang until I type one extra line of input. Perhaps surprisingly, \n in a scanf format string does not mean to expect a newline, but rather to re...
2015-10-23, 843👍, 0💬

I am reading a number with scanf and d, and then a string with gets
I'm reading a number with scanf and %d, and then a string with gets(): int n; char str[80]; printf("enter a number: "); scanf("%d", &amp;n); printf("enter a string: "); gets(str); printf("you typed %d and "%s"\n", n, str); but the compiler seems to be skipping the call to gets()! If, in response...
2015-10-23, 800👍, 0💬

Why does everyone say not to use scanf? What should I use instead?
Why does everyone say not to use scanf? What should I use instead? scanf has a number of problems, its %s format has the same problem that gets() has --it's hard to guarantee that the receiving buffer won't overflow. More generally, scanf is designed for relatively structured, formatted input (its n...
2015-10-21, 914👍, 0💬

I am using scanf c to read a Y/N response
I'm using scanf %c to read a Y/N response, but later input gets skipped. You wanted scanf %c to read a single character, and it tried to, but when you tried to type that single character at it, before the rest of the input system would accept it, you had to hit the RETURN key, too. scanf read only t...
2015-10-21, 838👍, 0💬

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