Introduction to Computers and Programming - Class 2

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Introduction to Computers and Programming

Class 2

Introduction to C

Professor Avi Rosenfeld

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For Those Who Missed it…

Course home page is at

www.cs.nyu.edu/courses/fall01/V22.0002-001/index.htm Syllabus is accessible from the home page There will be seven homeworks, two midterms and a final There will be OPTIONAL homeworks given more

frequently Office hours are on Wednesday, 8:30-9:30 A.M., room 419

CIWW, and by appointment

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Style vs. Syntax

Syntax – elements that are needed Grammar

Style – elements that improve human comprehension Comments, indenting, and other elements

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A Sample Program

/* our first program in C */#include <stdio.h>/* start of the main program */int main() {

printf( “Hello World!\n” );return 0;

} /* end program */

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Escape Characters

The backslash (\) is called an escape character

Indicates that printf is supposed to do something unusual

When encountering a backslash, printf looks to the next character and combines it with the backslash to form an escape sequence

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Other escape sequences

\thorizontal tab

\aalert- will make the computer beep

\\prints a backslash character in a printf statement

\” prints a double quote character in a printf statement

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Printing Several Lines

One printf statement can print several lines by using newline characters. e.g. printf(“Welcome\nto\nC!\n”);

But that’s a stylistic horror! A better way:printf( “Welcome \n”

“to \n”

“C!\n” );

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Printing Long Lines

Can use two or more printf statements

(note the first has no newline)

printf( “The quick brown fox jumped ” );

printf( “over the lazy dog.\n” );

This will print one line like so:

The quick brown fox jumped over the lazy dog.

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Why does this work like this?

Because C ignores most white space characters in your editor (spaces, tabs, carriage-returns (“enters”), etc.)

C is looking for the \n character to tell it when to go to the next line

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Declaration

Creating an identifier and associating it with a type and (behind the scenes) a location in memory. A couple of examples: int integer1; int integer1, integer2, sum;

integer1, integer2 and sum are all variables

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Variables

Variables hold data values which can change

They must be declared with a data type and a name, immediately after a left brace, before they can be used

A variable name in C is any valid identifier

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Data Types

C (as well as many other programming languages) are very sensitive to data type.

The int family short, long

The float family Double

Characters (strings)

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Identifier

An identifier is a series of characters consisting of letters, digits and underscores “_” that does not begin with a digit or include several special characters such as math signs, $, and others

Can be any length but only the first 31 characters are required to be recognized by ANSI C compilers

Keep identifiers 31 characters or less for portability and fewer problems

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C Is Case Sensitive

Upper case and lower case letters are different in C

E.g., lower case a1 and capital A1 are different identifiers

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Good Programming Practices

Choose meaningful variable names to help make a program self-documenting (fewer comments will be needed).

First letter of an identifier used as a variable name should be a lower case letter.

Multiple-word variable names can help make a program more readable.

Use mixed-cases to help make the word stand out. E.G. totalCommissions.

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Printf Example #3

#include <stdio.h>int main(){

int num1 = 3, num2 = 2;printf("%d plus %d equals %d\n", num1, num2,

num1+num2);return 0;

}

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Conversion Specifiers

%d – integer %f – float %c – character %s - string

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Arithmetic in C

The C arithmetic operators are + for addition

- for subtraction

* for multiplication

/ for division, and

% for modulus

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Binary Operators

Operators that take two operands e.g. “+” is a binary operator and “a + b” has two

operands (a and b)

Note integer division will yield an integer result e.g. 5 / 2 = 2 (not 2 1/2) and 17 / 5 = 3

modulus is the remainder after an integer division e.g. 5 % 2 is 1 and 17 % 5 is 2

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Big (Very Common) Error

Divide by Zero e.g. x = y / 0

Normally undefined by computer systems and generally results in a fatal error

Usually shows up at run time

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Rules of Operator Precedence

C evaluates arithmetic expressions in a precise sequence determined by the rules of operative precedence

Expressions within parentheses are evaluated first (highest level of precedence) For nested or embedded parentheses, the

expression in the innermost pair is evaluated first

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Rules of Operator Precedence cont’d

Multiplication, division and modulus operations are evaluated next If more than one evaluated from left to right

Addition and subtraction operations are evaluated last If more than one evaluated from left to right

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Parentheses

Are your friends Are your really good friends Because with them you can ensure expressions are

evaluated as you expect Can avoid mistakes with operator precedence (one

less thing to think about) e.g. y = m * x + b ; y = (m * x) + b; e.g. y = a * b * b + c * b – d; y = ((((a * b) * b) + (c * b)) – d);