C programming for Engineers Lcc compiler – a free C compiler available on the web. lcc-win32/ Some instructions

C programming for Engineers

Lcc compiler – a free C compiler available on the web.

http://www.cs.virginia.edu/~lcc-win32/ Some instructions

What are variables?

A named area in the computer memory, intended to contain values of a certain kind (integers, real numbers, etc.)

They contain the data your program works with.

They can be used to store data to be used elsewhere in the program.

In short – they are the only way to manipulate data.

Variables in memory

5

int my_int = 5;

double my_double = 3.5;

3.5

my_int

my_double

Variables in memory

5

Whenever we write the variable name (my_int), we ask to read the value of that variable

If we write &variable_name, we ask for the address of that variable

3.5

my_int

my_double

/* Get a length in cm and convert to inches */#include <stdio.h>

int main(){

double cm, inches;

printf("Please enter length in centimeters: ");scanf("%lf", &cm);

inches = cm / 2.54;printf("This is equal to %g inches\n", inches);

return 0;}

Example

Declaring variables in C

Before using a variable, one must declare it.

The declaration first introduces the variable type, then its name.

When a variable is declared, its value is undefined.

double cm, inches;

Example: variable declarations

int i; char c; float f1, f2; float f1=7.0, f2 = 5.2; unsigned int ui = 0;

Variable naming rules

Letters, digits, underscores i CSE_5a a_very_long_name_that_isnt_very_useful fahrenheit

First character cannot be a digit 5a_CSE is not valid!

Case sensitive CSE_5a is different from cse_5a

Data types in C

char – a single byte character. int – an integer number – usually 4 bytes. float – a single precision real number –

usually 4 bytes. double – a double precision real number –

usually 8 bytes.

char

int

float

double

Data types in C

short int (or just short) – an integer number, usually 2 bytes (rarely used).

long int (or just long) – an integer number, 4 or 8 bytes (rarely used).

long double - a double precision real number – usually 8 bytes (rarely used).

unsigned vs. signed

That example again


int main(){

double cm, inches;

printf("Please enter length in centimeters: ");scanf("%lf", &cm);


return 0;}

printf and scanf

printf – prints to the screen. Can also accept variables and print

their values. scanf – gets values from the standard

input and assigns them to variables.

printf can print variable values

printf("z=%d\n", z); The sequence %d is a special

sequence and is not printed! It indicates to printf to print the

value of an integer variable written after the printed string.

scanf gets input from the user

scanf("%lf", &cm);

This statement waits for the user to type in a double value, and stores it in the variable named ‘cm’.

To get 2 doubles from the user, use –scanf("%lf%lf", &var1, &var2);

prinft/scanf conversion codes

A %<conversion code> in the printf/scanf string is replaced by the respective variable.

%c – a character %d – an integer, %u – an unsigned integer. %f – a float %lf – a double %g – a nicer way to show a double (in printf) %% - the ‘%’ character (in printf)

One last time


int main(){

double cm, inches;

printf("Please enter length in centimeters: ");scanf("%lf",&cm);


return 0;}

Exercise

Write a program that accepts as input - The Dollar-Shekel exchange rate An integer amount of dollars

and outputs - The equivalent amount in Shekels

Solution

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

double shekels, xchange;int dollars;

printf("Enter the US$-NIS exchange rate: ");scanf("%lf", &xchange);printf("Enter the amount of dollars: ");scanf("%d", &dollars);shekels = dollars * xchange;printf("%d dollars = %g shekels\n", dollars, shekels);

return 0;}

Char is also a number!

A char variable is used to store a text character: Letters. Digits. Keyboard signs. Non-printable characters. But also small numbers (0 to 255 or -128

to 127).

Text as numbers

Every character is assigned a numeric code.

There are different sets of codes: ASCII (American Standard Code for

Information Interchange) – most common. EBCDIC – ancient, hardly used today. Maybe others.

We will use ASCII. The ASCII table.

More about character encoding

Most of the time, you don't care what the particular numbers are.

The table above shows only 128 characters (7 bits). Some are non-printable.

Extended ASCII code contains 256 characters.

More about character encoding

ASCII code 0 (NULL character) is important – we will see it again.

Note contiguous sets of numbers, upper case and lower case characters.

Example of char as both a character and a small number

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

char i = 'b';printf("i as a character is %c\n", i);printf("i as an integer is %d\n", i);printf("The character after %c is %c\n",

i, i + 1);

return 0;}

Another example

/* Get the position of a letter in the abc */#include <stdio.h>

int main(){

char letter;

printf("Please enter a lowercase letter\n");scanf("%c", &letter);printf("The position of this letter in the abc is %d\n",

letter - 'a' + 1);

return 0;}

Exercise

Write a program that accepts as input – A lowercase letter

and outputs – The same letter in uppercase

(e.g., if the input is ‘g’, the output should be ‘G’)

Solution

/* Convert a letter to uppercase */#include <stdio.h>

int main(){

char letter;

printf("Please enter a lowercase letter\n");scanf("%c", &letter);printf("This letter in uppercase is %c\n", letter - 'a' + 'A');

return 0;}

Arithmetic operators

An operator is an action performed on something (e.g. constants, variables).

That “something” is called an operand. Common operators:

Assignment = Addition + Subtraction - Multiplication * Division / Modulo %

Example

Arithmetic operators on variables - digits.c

int main(){

int sum, num;

sum = 0;printf("Enter 3-digits number\n");scanf("%d", &num);

/*extract the first digit*/sum = sum + num % 10;num = num / 10;

Let’s see how it works

1350

num

sum5198

Arbitrary numbers (garbage)

int main(){

int sum, num;




1350

num

sum5198

int main(){

int sum, num;




1350

num

sum0

int main(){

int sum, num;




1350

num

sum0

int main(){

int sum, num;




369

num

sum0

int main(){

int sum, num;




369

num

sum0

int main(){

int sum, num;




369

num

sum9

int main(){

int sum, num;




369

num

sum9

int main(){

int sum, num;




36

num

sum9


/*extract the second digit*/sum = sum + num % 10;num = num / 10;

/*extract the third digit*/sum = sum + num % 10;

printf("The digits sum is %d\n", sum);

return 0;}

36

num

sum9





return 0;}

36

num

sum15





return 0;}

36

num

sum15





return 0;}

3

num

sum15





return 0;}

3

num

sum15





return 0;}

3

num

sum18

When operands of two different types are involved in an operation, the operand of the ‘weaker’ type is promoted to the other type (int → float → double).

The result of the operation is of the higher The result of the operation is of the higher type.type.

When the operands are of the same type, When the operands are of the same type, the result is of that type as well.the result is of that type as well.

Operations with different types

For example - 3 + 4 = 7 3.0 + 4 = 7.0 3 / 4 = 0 !!! 3.0 / 4 = 0.75


Sometimes it is desirable for a variable of one type to be considered as belonging to another in an operation

We say the variable is cast to the new type. The casting operator is of the form: (type) For example, (float)i casts the variable

i to a float.


#include <stdio.h>

int main(){

int a=1, b=2;

printf("%d / %d = %d\n", a, b, a/b);printf("%d / %d = %g\n", a, b, (float)a / b);

}

Casting variables

Example – find what’s wrong

#include <stdio.h>

int main(){

int a = 10; int b = 20;

printf("The average of %d and %d is %d\n", a, b, (a + b) * (1 / 2));

return 0;}

Will this work?

#include <stdio.h>

int main(){

int a = 10; int b = 20;

printf ("The average of %d and %d is %d\n", a, b, (a + b)*(1.0 / 2));

return 0;}

The unsigned qualifier

Normally, the last bit of a variable serves as a sign bit.

In unsigned variables, it has the same role as an ordinary bit.

Because of the extra bit, the range of possible values is doubled – but only for positive numbers.

For example, unsigned chars can range from 0 to 255 while (signed) chars range from -128 to 127.

To declare a variable as unsigned, add the ‘unsigned’ keyword before its type. For example –unsigned int ui;

Overflow

Happens when a variable gets assigned a value that is outside of its range

This is equivalent to saying that the number of bits required to encode the value exceeds the number of bits in the variable

The value of the variable will usually be non-sense

Overflow – An example

#include <stdio.h>

int main(){

int iA = 1000;int iB = 1000000;int iC = 3000000;int iD = 5000000;printf ("%d * %d = %d\n", iA, iB, iA*iB);

printf ("%d * %d = %d\n", iA, iC, iA*iC); printf ("%d * %d = %d\n", iA, iD, iA*iD);

return 0;}

Thank you

See you next time!

Documents

C programming for Engineers Lcc compiler – a free C compiler available on the web. lcc-win32/ Some instructions