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10.21 Foundations of Programming and Process Management in Computing.

intro-to-computer-science-gtiit-02-2024.pdf

Running ./hello (1/3): typing the program name

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Running ./hello (2/3): program copied from disk to memory

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Running ./hello (3/3): execute program from memory

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Programs and Processes

  • Once a program is copied into the main memory of the computer and ready to be executed, we no longer call the program a program but a process.
  • A single program stored on disk, if executed several times, can create several processes running at the same time.
  • Commands ps​ and jobs​ show all current processes in terminal.
  • Command ps aux​ shows all the processes currently running in the system (many of them are from the operating system itself).

  • Each process has a number (PID, process ID). You can try and terminate some process using the kill​ command followed by the PID of the corresponding process.

  • If this is not enough (for instance, to terminate vi), use kill -9​.

  • ​top​ can list all the processes whether it is running and sleeping

​htop​ is beautiful

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Controlling processes in the Linux terminal

  • CTRL+C: terminates the process that is currently executing.

  • CTRL+Z: suspends (pauses) the current process. After that:

  • Command fg​ resumes program execution in the foreground.

  • Command bg​ resumes program execution in the background.
  • ​ps​ and jobs​ show the processes of the current terminal.
  • Try with the commands sleep​ and watch​. Such as sleep 10​

​man sleep​

​man ls​

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Back to C programming: putchar()​

  • ​putchar()​ is a function similar to printf()​ but instead of taking a string constant, it takes a character constant.
  • Character constants are written 'c'​ where c​ is some character.
  • Careful: "c"​ is a string constant (a string with only one character).

  • Examples:

  • ​putchar('a');​

  • ​putchar('\n');​

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Use of printf()​

  • Input: printf("Color %s, Number %d, Float %4.2f", "red", 123456, 3.14);​
  • Output: Color red, Number 123456, Float 3.14​

Format Specifiers:

  • ​%d​, %i​: prints an integer.
  • ​%f​: prints a floating-point number.
  • ​%s​: prints a string.
  • ​%c​: prints a character.
  • ​%%​: prints one %​ character.
  • ​\n​: prints a newline.

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Comments

  • A comment is some text placed in the source code, that is ignored during the compilation process (not treated as normal source code).
  • Useful to provide explanations/descriptions for developers.
  • Single-line comments start with the //​ characters until the end of the line.
  • Multi-line comments start with /*​ and end with */​.

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Exercise: what is the output of these programs?

main() {
    printf("GTI");
    /* printf("STU"); */
    printf("IT\n");
}

/* Output: GTIIT */

main() {
    /* printf("Ho"); */
    printf("\nCho");
    /* printf("la"); */
    putchar('o');
    /* printf("!\n"); */
    printf("se W");
    /* printf("man"); */
    printf("isel");
    putchar('y');
    printf("\n");
}

/* Output: 
Cho
ose Wisy
*/

main() {
    printf("Hey\n");
}

/* Output: Hey */

Variables

A variable is the combination of the following:

  • a name (or identifier) that you choose
  • an address in the memory of the computer

  • a value stored at that address

  • value may change during program execution

  • a type, that specifies how much space this value occupies in memory, and how to interpret this value (signed/unsigned...)

To use a variable, you need first to declare it.

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Variable declaration

A variable declaration is a statement of the following shape:

​type name;​

or:

​type name = value;​

The name should start with a lowercase letter. It may contain uppercase letters, numbers, underscore symbols. For instance:

​a, b, c, x10, state, tagName, inputStr, ...​

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Types

For now let us consider two types of the C language: int​ and char​.

  • The int​ type uses 4 bytes in memory, it holds values from –2,147,483,648 to +2,147,483,64 (if considered signed) or from 0 to 4,294,967,295 (if considered unsigned).
  • The char​ type uses 1 byte in memory, it holds values from -128 to 127 or from 0 to 255.
main() {
    int time = 100;
    char c = 50;
    ...
}

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Assignments: changing a variable's value

An assignment is a statement of the form:

​lvalue = expression;​ Where lvalue​ (as in "left value") can be the name of a variable, but cannot be a constant.

The assignment changes the value of the lvalue​ variable to the value of expression​.

main() {
    int time = 100;
    printf("%d", time);     // will print 100

    time = 60;
    printf("%d", time);     // will print 60
}

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Assignments from a constant or from a variable

main() {
    int time = 100;
    int another_time = 300;

    printf("%d", time);     // will print 100

    time = 60;              // assignment from a constant
    printf("%d", time);     // will print 60

    time = another_time;    // assignment from a variable
    printf("%d", time);     // will print 300
}

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A little riddle

main() {
    int time = 100;
    int another_time = 300;

    time = 60;
    another_time = 150;
    time = another_time;

    printf("%d", time);     // will print... ?
}

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About assignments

  • The assignment syntax is not symmetric: value gets copied from right to left.
  • You may find in some C code some assignments of the form:

​a = b = 10;​ * It is equivalent to doing b = 10;​ then a = b;​.

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Expressions

An expression can be built from:

  • constants
  • variables
  • operators, including arithmetic and logic operators
  • function calls (we will see them later)

Expression examples

  • ​10​
  • ​'c'​
  • ​varName​
  • ​10 + 20​
  • ​(30 * 5) / (44 - 24)​
  • ​(a * 5) / (44 - b) + 'c'​
  • ​45 % 10​
  • Operators +​, -​, *​ (multiplication), /​ (division), %​ (modulo) are arithmetic operators
  • An expression has a value, calculated from its constants, variables, and operators.

The Division (/) and Modulo (%) operators

  • An expression of the form x / y​ has as value the quotient of x​ by y​:

  • ​100 / 10​: value is 10

  • ​100 / 15​: value is 6
  • ​10 / 100​: value is 0

  • An expression of the form x % y​ has value the remainder of the division of x​ by y​:

  • ​100 % 15​: value is 10

  • ​123 % 10​: value is 3

Riddle

main() {
    int x = 100;
    int y = 300;
    x = y + 200;
    y = x / 5;
    x = x + y;
    printf("%d", x);  // will print... ?
}

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Riddle

main() {
    int a = 5;
    int b = 7;
    int c = 8;
    a = b - c;
    b = a - c;
    c = a * b;
    printf("%d", c);  // will print... ?
}

Expressions in printf​ arguments

main() {
    int x = 100;
    int y = 300;
    y = y / 5;
    x = x + 40;
    printf("%d", x + y - 100);  // will print... ?
}

scanf()​

  • The scanf()​ function makes our programs pause to get user input.

  • The user types a value and presses ENTER to confirm.

  • The received value is stored in a variable.
  • Like printf()​, it works with a string argument that contains placeholders, but it is a little trickier.

  • Here is the syntax that we will use to ask for an integer and store it in the variable x​:

  • ​scanf("%d", &x);​

  • Notice the &​ before the variable name.

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scanf()​ in action

main() {
    int age;
    printf("Please enter your age.\n");
    scanf("%d", &age);
    printf("Your age is %d\n", age);
}

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scanf()​ in action, again

main() {
    int age, year;
    printf("Please enter your age.\n");
    scanf("%d", &age);
    printf("Please enter the current year.\n");
    scanf("%d", &year);
    printf("You were born in the year %d\n", year-age);
}

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Comparison operators

Comparison operators enable you to compare the values of two expressions:

  • ​EXPR > EXPR​
  • ​EXPR >= EXPR​
  • ​EXPR < EXPR​
  • ​EXPR <= EXPR​
  • ​EXPR == EXPR​
  • ​EXPR != EXPR​

These boolean expressions, or conditions, can be used in the context of conditional statements and loops.

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if statement

...

if (CONDITION)
    [STATEMENT]
[NEXT STATEMENTS]

...

if (CONDITION) {
    [STATEMENTS]
}
[NEXT STATEMENTS]

...

image​

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if statement example

main() {
    int age;
    printf("Please enter your age.\n");
    scanf("%d", &age);
    if (age < 18) {
        printf("You are minor.\n");
    }
    else {
        printf("Your age is %d.\n", age);
    }
}

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if-else statement

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if (CONDITION)
    [STATEMENT]
else
    [STATEMENT]
[NEXT STATEMENTS]

...

if (CONDITION) {
    [STATEMENTS]
} else {
    [STATEMENTS]
}
[NEXT STATEMENTS]

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if-else statement example

main() {
    int age;
    printf("Please enter your age.\n");
    scanf("%d", &age);
    if (age < 18) {
        printf("You are minor.\n");}
    else {
        printf("You are major.\n");}
    printf("Your age is %d.\n", age);
}

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Favor if-else over consecutive if’s

// Not great:
if (age < 18) {
    printf("You are minor.\n");}
if (age >= 18) {
    printf("You are major.\n");}
printf("Your age is %d.\n", age);

// Much better:
if (age < 18) {
    printf("You are minor.\n");}
else {
    printf("You are major.\n");}
printf("Your age is %d.\n", age);

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Lecture Summary

  • Linux processes
  • Variables, assignment
  • ​putchar()​ and printf()​
  • ​scanf()​
  • Arithmetic operators
  • Comparison operators
  • if and if-else statements