[PDF] [PDF] Interactive Input in C

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Interactive Input in C

A standard task in C programming is to get interactive input from the user; that is, to read in a number or a string typed at the keyboard. One method is to use the function scanf. For example to get an integer, we might use (#include is implied): int answer; /* an integer chosen by the user */ scanf ("%d", &answer); This works fine if the user actually types a number, but is problematic if they make a mistake and type a letter instead. One can do better by checking the return value ofscanf, which is zero if no conversions were assigned, but it is difficult to recover and get corrected input. We advocate an alternative toscanf, described in "Practical C Programming" by Steve

Oualline, which usesfgets. Here"s an example:

char line[100]; /* a line to be read from the terminal */ int answer; /* an integer chosen by the user */ fgets (line, sizeof(line), STDIN); /* read in a line */ sscanf (line, "%d", &answer); /* scan for the integer */ The input procedure has been converted to two steps:

1.fgetsreads in a line terminated by an end-of-line character (e.g., "enter");

2.sscanfscans the line for the desired input (which could include numbers or strings).

Some comments:

•The return value of sscanf is the number of input items assigned. Zero will be returned if an alphabetic character is entered for a "%d" conversion andEOFis returned is nothing if entered before an end-of-line character. •sizeof(line)is the dimension of line (notthe length of the input string). As an argument tofgets, it represents the maximum number of characters to read, including the end-of-line character. •STDINis "standard input," which tellsfgetsto read from keyboard input. If we wanted to read from a file instead, we could do something like:

FILE * input; /* file handle for input file */

char line[100]; /* a line to be read from a file */ input = fopen ("filename.dat", "r"); /* open the file for reading */ fgets (line, sizeof(line), input); /* get the line */ 1 The following code fragment shows how we might use the return fromsscanfto request input until we"re satisfied with the response: line char[100]; /* a line to be read from the terminal */ int answer; /* an integer chosen by the user */ answer = -1; /* set answer to a value that falls through */ while (answer != 0) /* iterate until told to move on */ printf ("\nSample menu:\n"); printf (" [1] Do something"); printf (" [2] Do something else"); printf ("\nWhat do you want to do? [0 for nothing] "); fgets (line, sizeof(line), STDIN); /* read in a line */ sscanf_result = sscanf (line, "%d", &answer); /* get answer */ if ((sscanf_result == 0) | (sscanf_result == EOF)) /* either a non-integer entered or an end-of-line */ printf ("\n *** You have to enter an integer! ***\n"); answer = -1; /* set answer to a value that falls through */ switch (answer) case 0: break; case 1: printf (" Doing something ... "); break; case 2: printf (" Doing something else ... "); break; default: /* keep going if answer is not 0, 1, or 2 */ break; 2quotesdbs_dbs6.pdfusesText_11