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Why isnt it being handled properly?
I'm reading strings typed by the user into an array, and then printing them out later. When the user types a sequence like \n, why isn't it being handled properly? Character sequences like \n are interpreted at compile time. When a backslash and an adjacent n appear in a character constant or string...
2015-08-24, 895👍, 0💬

How can I convert numbers to strings (the opposite of atoi)? Is there an itoa function?
How can I convert numbers to strings (the opposite of atoi)? Is there an itoa function? Just use sprintf: sprintf(string, "%d", number); (Don't worry that sprintf may be overkill, potentially wasting run time or code space; it works well in practice.) You can obviously use sprintf to convert long or...
2015-08-24, 1639👍, 0💬

Does C have anything like the `substr (extract substring) routine present in other languages?
Does C have anything like the `substr (extract substring) routine present in other languages? Not as such. To extract a substring of length LEN starting at index POS in a source string, use something like char dest[LEN+1]; strncpy(dest, &amp;source[POS], LEN); dest[LEN] = '\0'; /* ensure \0 term...
2015-08-19, 953👍, 0💬

How do I convert a string to all upper or lower case?
How do I convert a string to all upper or lower case? Some libraries have routines strupr and strlwr or strupper and strlower, but these are not Standard or portable. It's a straightforward exercise to write upper/lower-case functions in terms of the toupper and tolower macros in &lt;ctype.h>;(T...
2015-08-19, 846👍, 0💬

Why do some versions of toupper act strangely if given an upper-case letter?
Why do some versions of toupper act strangely if given an upper-case letter? Why does some code call islower before toupper? In earlier times, toupper was a function-like preprocessor macro and was defined to work only on lower-case letters; it misbehaved if applied to digits, punctuation, or letter...
2015-08-17, 983👍, 0💬

How can I split up a string into whitespace-separated fields?
How can I split up a string into whitespace-separated fields? How can I duplicate the process by which main() is handed argc and argv? The only Standard function available for this kind of ``tokenizing'' is strtok, although it can be tricky to use and it may not do everything you want it to. (For in...
2015-08-17, 1097👍, 0💬

I need some code to do regular expression and wildcard matching.
I need some code to do regular expression and wildcard matching. Make sure you recognize the difference between: * Classic regular expressions, variants of which are used in such Unix utilities as ed and grep. In regular expressions, a dot . usually matches any single character, and the sequence .* ...
2015-08-12, 934👍, 0💬

I am trying to sort an array of strings with qsort, using strcmp as the comparison function, but it is not working.
I am trying to sort an array of strings with qsort, using strcmp as the comparison function, but it is not working. By ``array of strings'' you probably mean ``array of pointers to char.'' The arguments to qsort's comparison function are pointers to the objects being sorted, in this case, pointers t...
2015-08-12, 955👍, 0💬

Now I am trying to sort an array of structures with qsort ...
Now I'm trying to sort an array of structures with qsort. My comparison function takes pointers to structures, but the compiler complains that the function is of the wrong type for qsort. How can I cast the function pointer to shut off the warning? The conversions must be in the comparison function,...
2015-08-10, 991👍, 0💬

How can I sort a linked list?
How can I sort a linked list? Sometimes it's easier to keep the list in order as you build it (or perhaps to use a tree instead). Algorithms like insertion sort and merge sort lend themselves ideally to use with linked lists. If you want to use a standard library function, you can allocate a tempora...
2015-08-10, 902👍, 0💬

How can I sort more data than will fit in memory?
How can I sort more data than will fit in memory? You want an ``external sort,'' which you can read about in Knuth, Volume 3. The basic idea is to sort the data in chunks (as much as will fit in memory at one time), write each sorted chunk to a temporary file, and then merge the files. Your operatin...
2015-08-07, 869👍, 0💬

How can I get the current date or time of day in a C program?
How can I get the current date or time of day in a C program? Just use the time, ctime, localtime and/or strftime functions. Here is a simple example: #include &lt;stdio.h> #include &lt;time.h> int main() { time_t now; time(&amp;now); printf("It's %s", ctime(&amp;now)); return 0; } C...
2015-08-07, 815👍, 0💬

I know that the library function localtime will convert ...
I know that the library function localtime will convert a time_t into a broken-down struct tm, and that ctime will convert a time_t to a printable string. How can I perform the inverse operations of converting a struct tm or a string into a time_t? ANSI C specifies a library function, mktime, which ...
2015-08-05, 817👍, 0💬

How can I add N days to a date? How can I find the difference between two dates?
How can I add N days to a date? How can I find the difference between two dates? The ANSI/ISO Standard C mktime and difftime functions provide some (limited) support for both problems. mktime accepts non-normalized dates, so it is straightforward to take a filled-in struct tm, add or subtract from t...
2015-08-05, 847👍, 0💬

Did C have any Year 2000 problems?
Did C have any Year 2000 problems? No, although poorly-written C programs might have. The tm_year field of struct tm holds the value of the year minus 1900; this field therefore contains the value 100 for the year 2000. Code that uses tm_year correctly (by adding or subtracting 1900 when converting ...
2015-08-03, 903👍, 0💬

I need a random number generator.
I need a random number generator. The Standard C library has one: rand. The implementation on your system may not be perfect, but writing a better one isn't necessarily easy, either. Here is a portable C implementation of the ``minimal standard'' generator : #define a 16807 #define m 2147483647 #def...
2015-08-03, 1313👍, 0💬

How can I get random integers in a certain range?
How can I get random integers in a certain range? The obvious way, rand() % N /* POOR */ (which tries to return numbers from 0 to N-1) is poor, because the low-order bits of many random number generators are distressingly non-random. A better method is something like (int)((double)rand() / ((double)...
2015-07-29, 919👍, 0💬

Each time I run my program, I get the same sequence of numbers back from rand
Each time I run my program, I get the same sequence of numbers back from rand It's a characteristic of most pseudo-random number generators (and a defined property of the C library rand) that they always start with the same number and go through the same sequence. (Among other things, a bit of predi...
2015-07-29, 985👍, 0💬

I need a random true/false value ...
I need a random true/false value, so I'm just taking rand() % 2, but it's alternating 0, 1, 0, 1, 0... Poor pseudorandom number generators (such as the ones unfortunately supplied with some systems) are not very random in the low-order bits. (In fact, for a pure linear congruential random number gen...
2015-07-24, 841👍, 0💬

How can I return a sequence of random numbers which dont repeat at all?
How can I return a sequence of random numbers which dont repeat at all? What you're looking for is often called a ``random permutation'' or ``shuffle.'' One way is to initialize an array with the values to be shuffled, then randomly interchange each of the cells with another one later in the array: ...
2015-07-24, 898👍, 0💬

How can I generate floating-point random numbers?
How can I generate floating-point random numbers? drand48 is a Unix System V routine which returns floating point random numbers (presumably with 48 bits of precision) in the half-open interval [0, 1). (Its companion seed routine is srand48; neither is in the C Standard.) It's easy to write a low-pr...
2015-07-22, 942👍, 0💬

How can I generate random numbers with a normal or Gaussian distribution?
How can I generate random numbers with a normal or Gaussian distribution? There are a number of ways of doing this. 1. Exploit the Central Limit Theorem (``law of large numbers'') and add up several uniformly-distributed random numbers: #include &lt;stdlib.h> #include &lt;math.h> #define NSU...
2015-07-22, 867👍, 0💬

I am porting this program, and it calls a routine drand48, which my library doesnt have. What is it?
I am porting this program, and it calls a routine drand48, which my library doesnt have. What is it? drand48 is a Unix System V routine which returns floating point random numbers (presumably with 48 bits of precision) in the half-open interval [0, 1). (Its companion seed routine is srand48; neither...
2015-07-20, 868👍, 0💬

Is exit(status) truly equivalent to returning the same status from main?
Is exit(status) truly equivalent to returning the same status from main? Yes and no. The Standard says that a return from the initial call to main is equivalent to calling exit. However, a return from main cannot be expected to work if data local to main might be needed during cleanup; A few very ol...
2015-07-20, 895👍, 0💬

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