Input and output

• Input and output of a program

  We have already looked at how to output results to a screen with printf() statement.

  Sometimes, you want to get inputs from human (e.g., how many times you want to run the simulation), and run the program with the inputs. So far, we have been modifying the source code to change values of variables or constants, recompile, and run.

• scanf()
  A function to get inputs from keyboard.
  • simple example:

    #include <stdio.h>
    
    int main(void) {
      int    popSize;
      double probability;
    
      printf("Type in a population size: ");
      scanf("%d", &popSize);
      printf("Type in a probability: ");
      scanf("%lf", &probability);
    
      printf("You typed in %d and %lf\n", popSize, probability);
      return 0;
    }
    

  • Ampersand (&) before the variable's name is required. It means the address (in memory space) of the variable. We will cover this later.
  • Multiple input per scanf()
    

    printf("Type 1 integer and 1 floating-point number: ");
    scanf("%d %lf", &popSize, &probability);
        

    When a user is typing in the inputs, the user can use whatever white spaces (spaces, tabs, or new lines).

    You can type:
    100 0.01
    Or
    100 0.01
    or
    100
    0.01

• stdout and stderr
  • unix (and Mac OS-X) has 2 output channels: standard output (stdout) and standard error (stderr).

    If you type ls, you are seeing the message printed to stdout on the screen.
    When gcc complains, it is printing to stderr on the screen.

    Analogy: FM radio has two output channels (right and left).

  • By default, these two channels are connected to the screen.

    Analogy: This is similar to combining right and left channel (mono mode) and connecting to a single speaker (screen).

  • printf() statement is printing to stdout channel.
  • You can re-direct where the channel is connected.
    For example, you might want to output to a file, instead of to a screen.

    Analogy: you can connect another speakers or headphones to change the location of outputs.

    • ``>'' Redirection of stdout
          ./a.out > fileName1
    • ``2>'' Redirection of stderr
          gcc source.c 2> errFileName
    • To redirect both channels to a file
          ./a.out > bothOutput 2>&1
• fprintf()
  • You can specify which channel you want to print with fprintf().

    fprintf(stderr, "ERROR: value of popSize is %d, should be positive\n", popSize);
      

    The first argument is the ``channel'' (file pointer) where you want to print. So the following 2 statements are equivalent.

    fprintf{stdout, "Print this\n");
    printf("Print this\n");
    

  • In addition to stdout and stderr, you can create other output channels.
    Useful when you want to collect different types of data to separate files during the simulation (e.g., one file for changes in allele frequency, another file for changes in population size).

    #include <stdio.h>
    
    int main (void) {
      FILE *outFP1;
    
      outFP1=fopen("outFileName1", "w");
      if(!outFP1) {  /* important to check that the file was opened successfully */
        fprintf(stderr, "Can't open the file: %s\n", file);
        exit (-1);
      } 
    
      fprintf(outFP1, "Write this to a file\n");
    
        :
    
      fclose(outFP1);
      return 0;
    }
    

    The 2nd argument for fopen() is mode
    

    "w"	write (you can write to this file)
    "r"	read only
    "a"	append mode (When you print out, they will be appended to the end of the file.)

  You can have several open channel within a program (requires different names for the file pointer).
• Exercises:
  

  Make a program which will

  1. Ask a user to input two integer values.
  2. Then pass the two values to a function which will find the largest common divisor of the two integers. You have to make this function.
  3. Print the anwer.
  4. Go back to the step1, and repeat these steps forever.
     You can quit this program by pushing Ctrl and c keys simultaneously.