CS113 Introduction to C

CS113Introduction to C Instructor: Ioannis A. VetsikasE-mail: vetsikas@cs.cornell.edu

Administrative Issues • Note: All of this information is on web site! • Office hours: TBA • Prerequisite: CS100 or equivalent • Assignments once a week (4 total), due on Friday • Turn in printout of program + email it to me • I might give quizzes • Grades on Assignments: On a scale from 0-4. 4 = Perfect, 3=Very good, 2 = Good, 1 = Incomplete and/or significant errors, 0=no effort at all • You get an S if all assignments are completed with score of >= 2, and decent performance on quizzes • Failing to complete any assignment will probably result in a U. • If you have a problem and cannot complete the assignment on time let me know; I will allow one late assignment at most

More Course Details • Textbooks: I recommend purchasing Kernighan & Ritchie • Kelley&Pohl optional • Compiler: Codewarrior is available on most campus lab, but any compiler will do • I’ll be using “gcc” for compiling your programs, but any correct ANSI C program will run on any compiler! • Collaboration: You may discuss programs with others, but all code submitted must be your own.

Why learn C? • “Least common denominator” – good building block for learning other languages • Subset of C++, similar to Java • Closeness to machine allows one to learn about system-level details (e.g., C only has call-by-value) • Portable – compilers available for most any platform! • Very fast (almost as fast as assembly) • C/C++ are languages of choice for most programmers

The canonical “first C program” #include <stdio.h> void main() { printf( “Hello, world!\n” ); } • All programs run from the ‘main’ function • ‘printf’ is a function in the library “stdio.h” • To include library functions use “#include” • All programs use library functions

That wasn’t too hard, let’s try another! #include <stdio.h> void main() { int x = 1, y; int sum; y = 3; sum = x + y; /* adds x to y, places value in variable sum */ printf( “%d plus %d is %d\n”, x, y, sum ); }

Comments • Any string of symbols placed between the delimiters /* and */. • Can span multiple lines • Can’t be nested! Be careful. • /* /* /* Hi */ is an example of a comment. • /* Hi */ */ is going to generate a parse error Keywords • Reserved words that cannot be used as variable names • OK within comments . . . • Examples: break, if, else, do, for, while, int, void • Exhaustive list in book

Identifiers • A “token” (“word”) composed of a sequence of letters, digits, and underscore (“_”) character. (NO spaces.) • First character cannot be a digit • C is case sensitive, so beware (e.g. printfPrintf) • Used to give names to variables, functions, etc. • Identifiers such as “printf” normally would not be redefined; be careful • Only the first 31 characters matter Constants • 0, 77, 3.14 examples. • Strings: double quotes. “Hello” • Characters: single quotes. ‘a’ , ‘z’ • Have types implicitly associated with them… • 1234567890999 too large for most machines…

Simple Data Types • void • Integer types (signed or unsigned): char, short int, int, long int • char is an 8 bit (=1 byte) number • Floating-point types: float, double, long double • No boolean types • Use 0=False and anything else(usually 1)=True

Operators and “Punctuators” • These are special characters with particular meanings, potentially depending on context • + - * / are the usual arithmetic operators, and can be applied to integer types as well as floating-number types • % is “mod operator”: a % b is equal to the remainder when a is divided by b, and returns a value in the range 0…(b – 1). We won’t worry about what happens when either a or b is non-positive. • % character has different meaning in printf( “%d”, a ); • Punctuators include parentheses, braces, semicolons…

Initializers • int j=1; • char c=‘a’; • Declare type of variable and assign a value to it Assignment • a=b=c=1; • Same as a=(b=(c=1)); • The whole expression c=1 has value equal to 1 • The whole expression b=c=1 has value equal to 1 • Also a=b=c=d+1; • Whole expression c=d+1 has value d+1 etc. • But cannot write a=b=c+1=d+1 • Left hand side of any assignment must be “lvalue”; c + 1 is not a valid lvalue

Compound Statement { definitions-and-declarations (optional) Statement-list } • Used for grouping as function body and to restrict identifier visibility • Note: no semicolon after closing bracet • But every statement in C must be followed by ;

Summing the numbers 1 thru 10. #include <stdio.h> void main() { int i = 1, sum = 0; while( i <= 10 ) { sum = sum + i; i = i + 1; } printf( "Sum = %d\n", sum ); } for( i = 1; i <= 10; i++ ) { sum = sum + i; } Alternative to while loop! Better, I think.

for and while • for (init; cond; expr1) expr2; equivalent to: init; while (cond) {expr2; expr1;} • Init and expr1 can be any number of assignments while expr2 can be any statement Some special operators provided by C • Instead of writing i = i+1 one can write i += 1 • In general var op= expr means var = var op expr • Especially for var += 1 and var -= 1 one can write: var++ and var-- respectively

++ , the tricky guy int a = 10; printf( “%d”, a++ ); int a = 10; printf( “%d”, ++a ); • Two variants: before a variable and after. • I prefer personally not to mix in ++ or -- as part of a more complicated expression • Another complication: int a = 10; printf( “%d %d”, a++, a++ ); • This complication not limited to the ++, -- operators, but occur with any operator with side effects

Precedence of Operators • You may have learned about this in the third grade: • 1 + 2 * 3 has the value of 1 + (2 * 3) • If we want the addition to be performed first, must parenthesize: (1 + 2) * 3. • We say that * has a higher precedence than +. • See K&R page 53. Associativity of Operators • Question number two: What about operators at the same precedence level? For instance, * and / ? • Is 12 / 6 * 2 equal to (12 / 6) * 2, or 12 / (6 * 2) ? • It’s the first: these operators are left associative (as are most) • Moral of story: I say parenthesize when in doubt.

Input – Output #include <stdio.h> void main() { int x = 7, y; char id; printf( “Enter a letter, by which I will identify you\n” ); scanf( “%c”, &id ); printf( “Now enter a number!\n” ); scanf( “%d”, &y ); printf( “%c, if nothing else, I know this:” , id); printf( “%d times %d is %d\n”, x, y, x*y ); }

Input & Output functions • Use printf, putchar etc. to write to the screen • Use scanf, getchar etc. to read from the keyboard or console • More on this on Monday

Your Homework • Covered most of chapters 1&2 of K&R and some of chapter 3. • Write a “hello world” program and some other simple programs in the programming environment which you plan to use to make sure that you are comfortable • Start looking at the homework; most of the knowledge you need to complete it has been presented (the rest on Monday)

CS113 Introduction to C