Homework
• Done the reading?
– K&R 1.1-1.9
– Glass Chapters 1 and 2
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Applied for cs240? (If not, keep at it!)
Gotten a UNIX account? (If not, keep at it!)
Making progress on HW1? Due next class!
HW2 will be on line by next class.
1
gcc
• At UNIX prompt, type “gcc hello.c –o hello”
• If you get any compiler error messages:
– Figure out what they mean
– Study and correct your source code
– Repeat “gcc hello.c –o hello” until you get no messages
• If you get no output except a new prompt:
– The compiler has accepted your source code
– You should now have a file named “hello”
2
Running your program “hello”
• At UNIX prompt, type “hello”
• If you get the printout “Hello World!” and a
new prompt, your program ran successfully
• If not,
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Study any UNIX error messages for clues
Study your source code for logical errors
Probably logical errors - compiler didn’t catch
Fix your source code and recompile / rerun
3
Handing in Assignments
• Turn in assignments as hard copy of typescript file
% script
(Start recording typescript file)
Script started, file is typescript
% ls –l
(list directory entries)
% cat hello.c
(display source file)
% gcc hello.c –o hello
(compile source file)
% hello
(run executable file)
% exit
script done on Thu Aug 22 14:30:02 2002
% lpr typescript
(printout typescript file)
4
Debugging a C program error
• There is a big difference between:
– The program compiling correctly
– The program doing what you want it to do
• You hope the compiler will catch your errors
– These errors will be easier to find
• If the compiler does not catch your errors
– These errors will be harder to find
5
Compiler Error Messages
• A compiler error message may direct you to
a specific error in your program
• A compiler error message may be vague
about what the error is and why it is an error
• Some compilers are better than others at
providing useful error messages!
6
Compiler Error Messages
• Introduced error is a missing closing brace:
#include <stdio.h>
int main ( )
{
printf("Hello, World!");
return 0;
/* missing “}” */
• % gcc hello.c –o hello
hello.c: In function `main':
hello.c:6: parse error at end of input
• Not a very helpful message – Gotta figure it out!
7
Variables
• Defined Data Type, Name, and (= value)
int lower = 0;
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/* Note: “=“ and “;” */
lower case by convention for readability
An executable statement
Memory location assigned to hold the value
Value can be changed as program executes
lower = 20;
/* Legal */
8
Symbolic Constants
• Defined Name and Value
#define LOWER 0
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/* Note: No “=“ or “;” */
UPPER CASE by convention for readability
Not an executable statement
No memory location assigned to hold value
Value can’t be changed as program executes
LOWER = 20;
/* NOT Legal */
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Example Program (K&R, P 15)
#include <stdio.h>
#define LOWER 0
/* Symbolic Constants */
#define UPPER 300
#define STEP 20
/* Print out Fahrenheit – Celsius Conversion Table */
int main ( )
{
int fahr;
/* Variable */
for (fahr = LOWER; fahr <= UPPER; fahr =fahr + STEP)
printf(“%3d,%6.1f\n”, fahr, (5.0/9.0)*(fahr – 32));
return 0;
}
10
For Statement
for (fahr = LOWER; fahr <=UPPER; fahr = fahr + STEP) {
statements within the loop;
}
next statement after the loop terminates;
for (A; B; C) - controls executing statement(s) within the loop
A is initialization (executed once when loop is started)
B is the loop test statement (when to stop looping)
C is a statement to execute at end of each loop
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Printf statement
printf (“%3d, %6.1f\n”, fahr, (5.0/9.0)* (fahr - 32));
First Argument to printf = “%3d, %6.1f\n”
%3d = integer format with 3 digits
%6.1f = floating point format with 6 digits and 1 decimal
\n = end of line character just as in “Hello World!”
Second Argument to printf = fahr
variable to print in %3d format
Third Argument to printf = (5.0/9.0)*(fahr – 32.0)
expression to calculate and print in %6.1f format
Note: For open book tests, remember K&R, Page 154!
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printf formats
• printf ("Values:∆%3d,∆%6.1f\n", fahr, (5.0/9.0)*(fahr-32));
• Where the characters specified by "∆" are what we write to
show a space explicitly. Quoted string "Value: . . ." is just
like "Hello, world", but %3d and %6.1f are special
placeholders, called conversion specifications.
• This means that the two expressions following the quoted
string, fahr, and (5.0/9.0)*(fahr-32), are to be printed
according to the prescription given.
• The table here would look like:
– Values:∆∆∆0,∆∆-17.8
– Values:∆∆20,∆∆∆-6.7
– Values:∆∆40,∆∆∆∆4.4
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printf formats
• Other characters in "Values: . . .", such as "," and "∆" are
printed literally.
• The %3d means that an integer is printed so as to take up 3
spaces, right adjusted -- "∆40", but no initial space for
"100" -- still have space before 100 because came after
"Values:∆".
• The %6.1f means to print a float number (floating point or
“double” by default, represented with a fractional part), with
a total of 6 spaces used up and 1 digit after the decimal
point: thus "∆-17.8" uses 6 spaces.
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Functions
• A function is a separate block of code that you
can call as part of your program
• A function executes and returns to next line after
you call it in your program
• Arguments within parentheses may be passed in
• Arguments are passed by value!
function_name (arguments);
• A return value may be passed back
return_value = function_name (arguments);
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Character I/O – getchar( )
• A standard function/macro defined in <stdio.h>
• No arguments are passed to “getchar ( )”
• getchar ( ) returns an int value from stdin
int c;
…
c = getchar ( );
• Next character input is returned as value of “c”
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Character I/O – putchar( )
• A standard function/macro defined in <stdio.h>
• Character to print to stdout is passed as argument
• There is no return value
int c;
…
putchar (c);
• Next character output is based on value of “c”
17
File Copying
# include <stdio.h>
int main ( )
{
int c;
c = getchar( );
while (c != EOF) {
putchar (c);
c = getchar ( );
}
return 0;
}
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File Copying - Alternative
# include <stdio.h>
int main ( )
{
int c;
while ( (c = getchar( ) ) != EOF) {
putchar (c);
}
return 0;
}
19
Redirecting stdin and stdout
• copy <hello.c >junk.c
Copies
FROM
the contents of existing file hello.c as stdin
TO
the contents of a new file junk.c as stdout
(Note: will overwrite any existing file junk.c)
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