C programming notes - School of Physics

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C programming notes file:///F:/my_docs/web_phys2020/Cprogrammingnotes.html 26 of 40 19/03/2007 10:06 AM The ampersand character "&" is a unary operator that returns the address of the thing to its right. Recall that a C function can not alter the value of its arguments (but there is nothing stopping it altering the value that is pointed to by one of its arguments!). scanf details The scanf function is used for reading a line of input from the standard input stream (usually the users terminal) and extracting from it one or more integers, floating point numbers, character strings, etc. The first argument to scanf is a format description, which describes how the input line should be decoded. For example, if you want to read an integer, the format would be "%i" (note that this allows you to enter numbers in decimal, hexadecimal or octal (e.g., 123, 0x123, 0123)). If you want to read a floating point number, the format would be "%f". If you want to read an integer followed by a floating point number, the format woud be "%d %f". If the two numbers were separated by a comma on the input line, you would use "%d, %f". The arguments after the first are pointers to where the data should be stored. Examples (where i and j are integers, x and y are floats, and fruit and colour are char arrays): Input data scanf function call to read the data 123 scanf("%i", &i); 123 0x456 scanf("%i %i", &i, &j); 123. 456. scanf("%i. %i.", &i, &j); 123.1 456.1 scanf("%f %f", &x, &y); e=2.71 pi=3.14 scanf("e=%f pi=%f", &x, &y); apples red scanf("%s %s", fruit, colour); Guido ate 2 apples scanf("Guido ate %i %s", &i, fruit); It is important to check the return value of the scanf function - this is the number of items that were successfully extracted from the input. It may be less than the number you expected, or even zero. If this happens, the input stream pointer is left at the point at which the error occurred, and you can try reading again. scanf can also return the special integer constant EOF to indicate an end-of-file condition. Variations on scanf fscanf is very similar to scanf except it takes a FILE argument ("FILE" is a structure that is defined in the include file - you don't have to worry about what precisely "FILE" is, just follow the examples) to specify which input stream to read from, e.g., FILE *fp; float x, y; fp = fopen("input.dat", "r"); fscanf(fp, "%f %f", &x, &y); "sscanf" takes input from a character array rather than an input stream, e.g., char input="1.23 4.56"; sscanf(input, "%f %f", &x, &y); Improving the ease of interaction with your programs - GNU readline While it is possible to use "scanf" for reading input typed by a person, you can greatly improve the user experience by using the GNU readline package.
C programming notes file:///F:/my_docs/web_phys2020/Cprogrammingnotes.html 27 of 40 19/03/2007 10:06 AM The GNU readline package provides some very convenient features such as line editing and history. This means, e.g., that you can use the arrow keys to move within a line, and between previously entered lines. You can also use, e.g., CTRL-A to go to the beginning of the line, CTRL-E to go to the end, and CNTRL-R to search backwards for particular strings. See "man readline" (on a GNU/Linux computer) for a complete description. Here is an example program showing the use of GNU readline. Arrays In mathematics and physics you often need to group numbers together into arrays, i.e., 1, 2, or higher-dimensional structures. Arrays are declared in C as follows (for example): int counts[100]; float temperature[1024]; double mat[2][3]; In this example, "count" is an array that can hold 100 integers, "temperature" is an array that can hold 1024 floats, and "mat" is a 2x3 array (or matrix) of doubles. Note carefully that the first element of an array in C is element number 0 (rather than 1 as in FORTRAN). While this may be confusing to die-hard FORTRAN programmers, it is really a more natural choice. A side-effect of this choice is that the last element in an array has an index that is one less that the declared size of the array. This is a source of some confusion, and something to watch out for. So, "counts[0]" is the first element of the array "counts", defined above. And "counts[99]" is the last element. There are 100 elements in total. Initialising arrays To initialise an array to have particular numerical values, specify the initial values in a list within curly brackets. For example: int primes[100] = {2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541}; float temp[1024] = {5.0F, 2.3F}; double trouble[] = {1.0, 2.0, 3.0}; In this example, "primes" is initialised with the values of the first 100 primes (trust me!), and the first two elements (temp[0] and temp[1]) of "temp" are initialised to 5.0F and 2.3F respectively. The remaining elements of "temp" are set to 0.0F. Note that we use the trailing "F" on these numbers to indicate that they are floats, not doubles. The array "trouble" in the above example contains three double numbers. Note that its length is not explicitly declared, C is smart enough to calculate the length for you. Static arrays that are not explicitly initialised are set to zero. Automatic arrays that are not explicitly initialised, have undefined values. Multidimensional arrays Multidimensional arrays are declared and referenced in C by using multiple sets of square brackets. For example, int table[2][3][4]; "table" is a 2x3x4 array, with the rightmost array subscript changing most rapidly as you move through memory (the
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C programming notes - School of Physics

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