CS16, 10W, Handout to go with H07 (command line arguments) (printable PDF)

Available online at: http://www.cs.ucsb.edu/~pconrad/cs16/10W/homework/H07/handout

The assignment is available at http://www.cs.ucsb.edu/~pconrad/cs16/10W/homework/H07

This handout is your reading assignment to go with H07—this material is not covered in the textbook.
This is a review of some material covered in lecture on 01/21, and in lab03.

Command line arguments:

Command line arguments allow us to provide input to a C program through the command line.

For example, instead of typing

./gameScore

to run the program foo.c, we type:

./gameScore Steelers 30 Dolphins 20

and the values "Steelers" "30" "Dolphins" and "20" will be available inside the C program—we don't have to use scanf to prompt for them.

Here's how it works:

• argc is the number of arguments on the command line. For example in the case of
  
  ./gameScore Steelers 30 Dolphins 20
  
  argc is equal to 5, because there are five things on the command line.
  
  
• argv is an array of char * values, argv[0], argv[1], argv[2], etc. where each of those has the value of exactly one of the things on the command line.
  
  For example, in this case:
  
  argv[0] has the value "./gameScore"
  argv[1] has the value "Steelers"
  argv[2] has the value "30" (note that this is a string, a char *, not an int)
  argv[3] has the value "Dolphins"
  argv[4] has the value "20" (again, a string, a char *, not an int)

Please turn over for more

Continued from other side

Double Subscripting

Working again with the command line:
./gameScore Steelers 30 Dolphins 20

we see that argv[1] has the value "Steelers", and argv[3] has the value "Dolphins".

We can double subscript these, because argv[1], as a string, is an array of characters followed by a null character.

That is, argv[1][0] is the character 'S', argv[1][1] is 't', argv[1][2] is 'e', etc.

The full string is shown in this table:

Similarly for argv[2], which is "30", we have:

Converting to integer

To convert to integer, we use the function atoi() as shown below.

• We must use #include <stdlib.h> in our program before using atoi()
• We need to check the value of argc first
  • if we try to convert argv[1] using atoi, but there argv[1] doesn't have a value (because argc<2) then we'll get an error (often a "segmentation fault")

For example:

int width, height;
if (argv!=2)
  {
     printf("Usage: ./makeBox width height\n");
     return 1;
  }
width=atoi(argv[1]);
height=atoi(argv[2]);

Converting to double (floating point numbers)

Converting to double works the same as integers, but we use atof instead of atoi.

• Note that atof returns a double, not a float—yet the name is atof, not atod. Go figure.
• You also need to #include stdlib to work with atof. If you don't, you'll get strange results.