File Input/output in C Basic C C programmings Input and output to and from files is identical to that at the command line, except the fprintf and fscanf functions are used and they require another argument. This additional argument is called a file pointer. In order to write two floating point numbers to a file, you first need to declar the file pointer with the FILE type, and you need to open it, as in float x=1, y=2; FILE *file; file = fopen(‘‘file.txt’’,’’w’’); fprintf(file,’’%f %f\n’’,x,y); fclose(file); The function fprintf is identical to the printf function, except now we see it has another argument file, which is a pointer to the file. Before you use the file variable, you need to open the file with file = fopen(‘‘file.txt’’,’’w’’); This opens up the file ‘‘file.txt’’ and the ‘‘w’’ which is the mode and indicates how the file will be used. The following three modes are allowed: Mode String Open for reading “r” Open for writing “w” Open and append “a” When you are done with the file, you close it with fclose(file); The ‘‘r’’ mode is used when you would like to open a file for reading. To read two floating point numbers from a file, you would use the fscanf function, which is identical to scanf, except that it takes the file pointer as its first argument, as in float x, y; FILE *file; file = fopen(‘‘file.txt’’,’’r’’); fscanf(file,’’%f %f\n’’,&x,&y); fclose(file); You can check to make sure your files are opened correctly (that is, that they exist), by checking to make sure file is not the predefined NULL pointer. We’ll discuss this pointer in more detail later, but for now, to ensure that your file was opened correctly, you use if(!file) printf(‘‘File did not open correctly!\n’’); Handout 7 13/03/03 7 Note that in order to use the FILE type, we need to include the standard C header file with #include<stdio.h> The fscanf and fprintf functions can be used to print to and read from the terminal using the stdin, stdout,and stderr file pointers defined in stdio.h. To write to the standard output of the terminal using fprintf, then, you would use fprintf(stdout,’’%f %f\n’’,x,y); which is identical to using printf(’’%f %f\n’’,x,y); To write to the standard error, you would use fprintf(stderr,’’%f %f\n’’,x,y); To read from the terminal’s standard input with fscanf, you would use fscanf(stdin,’’%f %f’’,&x,&y); which is identical to using scanf(’’%f %f’’,&x,&y); Again, in order for the compiler to know what stdin, stdout, and stderr are, you need to include stdio.h.