Programming Essentials in C - Lecture 4

User Input scanf()

• scanf() is the most commonly used function to accept user input from the standard input stream.
• The scanf() function in C reads data from the standard input (usually the keyboard, called stdin) and stores it in variables.

Syntax of scanf()

int x;
scanf("%d", &x);  // ("format_specifier", &variable)

• "format_specifier": Tells the computer what type of input to expect (e.g., %d for integers, %f for floating-point numbers).
• &variable: The & symbol gives the address of the variable where the input will be stored.

Common Format Specifiers

Data Type	Format Specifier
Integer (int)	%d or %i
Floating Point (float)	%f
Double (double)	%lf
Character (char)	%c
String (char[])	%s

Example

int num;
float decimal;

printf("Enter an integer and a decimal: ");
scanf("%d %f", &num, &decimal);

printf("Integer: %d, Decimal: %.2f\n", num, decimal);

Capturing Strings with scanf

Code Example 1: Using scanf

char name[50];  // Empty string with space for 50 characters

printf("Enter your full name (using scanf): ");
scanf("%s", name);  // Reads the name you type
printf("Name captured with scanf: %s\n", name);

What Happens

• When you run this:
  • Type: John Doe and press Enter.
  • Output: Name captured with scanf: John
• Why?: scanf only grabs the first word (John) and stops at the space. It can't handle full names with spaces.

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Code Example 2: Using gets

char name[50];  // Empty string with space for 50 characters

printf("Enter your full name (using gets): ");
gets(name);  // Reads the name you type
printf("Name captured with gets: %s\n", name);

What Happens

• When you run this:
  • Type: John Doe and press Enter.
  • Output: Name captured with gets: John Doe
• Why?: gets takes everything you type, including spaces, until you press Enter. It works for full names!

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Explanation

• scanf: Acts like a tool that cuts your input at spaces and only keeps the first piece.
• gets: Acts like a tool that grabs your whole sentence, spaces included, until you press Enter.

Conditional Statements

If

• Use the if statement to run a block of code if a condition is true.

Example

int num = 4;

if (num % 2 == 0) {
  printf("Even\n");
}

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Else

• Use the else statement to run a block of code if the condition is false.

Example: Check if a Number is Even or Odd

int num = 5;

if (num % 2 == 0) {
  printf("Even\n");
} else {
  printf("Odd\n");
}

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Else If

• Use the else if statement to check a new condition if the first one is false.

Example

int number;

printf("Enter a number: ");
scanf("%d", &number);  // Reads the number you type

if (number > 0) {
  printf("The number is positive!\n");
} else if (number < 0) {
  printf("The number is negative!\n");
} else {
  printf("The number is zero!\n");
}

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Short Hand If...Else (Ternary Operator)

• The ternary operator is a short way to write an if-else statement in one line. It uses three parts: a condition, a true result, and a false result.

Example

int time = 20;
// Prints "Good evening."
time < 18 ? printf("Good morning.\n") : printf("Good night.\n");

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Nested If Statements

• A nested if statement is an if inside another if. It checks one condition, then another.

Example: Checking if a Number is Positive and Even

int num;

printf("Enter an integer: ");
scanf("%d", &num);

if (num > 0) {  // Check if the number is positive
  if (num % 2 == 0) {  // Check if the number is even
    printf("The number %d is positive and even.\n", num);
  } else {
    printf("The number %d is positive but odd.\n", num);
  }
} else if (num < 0) {
  printf("The number %d is negative.\n", num);
} else {
  printf("The number is zero.\n");
}

Switch Statement

• The switch statement checks a value and runs code based on which case matches.
• It's like a menu: pick an option, and it does that one thing.

How It Works

• The switch checks one value.
• It compares it to each case.
• If a case matches, it runs that code.
• break stops the switch from continuing.
• default runs if no case matches (optional).

Rules for Switch Statement

1. The value in switch must be an integer or character.
2. Each case value must be a fixed number or character (no variables).
3. case only works inside switch.
4. break is optional. Without it, all cases after a match will run (called "fall through").

Example

int choice;

printf("Enter 1, 2, or 3: ");
scanf("%d", &choice);

switch (choice) {
  case 1:
    printf("You chose option 1.\n");
    break;
  case 2:
    printf("You chose option 2.\n");
    break;
  case 3:
    printf("You chose option 3.\n");
    break;
  default:
    printf("Invalid choice.\n");
    break;
}