Use cases - Step-by-Step Execution

Concept Flow - Use cases

Start Program

↓

Define Use Case

↓

Implement Functionality

↓

Call Function in main()

↓

Observe Output

↓

End Program

This flow shows how a use case in C starts with defining a task, implementing it, calling it in main, and then observing the output.

Execution Sample

C

#include <stdio.h>

void greet() {
    printf("Hello, friend!\n");
}

int main() {
    greet();
    return 0;
}

This code defines a simple use case: greeting the user by printing a message.

Execution Table

Step	Action	Function Called	Output	Notes
1	Program starts	None		main() begins execution
2	Call greet()	greet		main calls greet function
3	Execute greet()	greet	Hello, friend!	greet prints message
4	Return to main()	None		greet finishes, back to main
5	Return 0 from main()	None		Program ends successfully

💡 Program ends after main returns 0

Variable Tracker

Variable	Start	After greet call	Final
None	N/A	N/A	N/A

Key Moments - 2 Insights

Why does the program print the message only after calling greet()?

Because the printf statement is inside greet(), which runs only when main() calls greet(), as shown in execution_table step 2 and 3.

What happens if we remove the greet() call from main()?

The message won't print because greet() never runs. The program would start and end without output, as no function prints anything.

Visual Quiz - 3 Questions

Look at the execution table, what is printed at step 3?

A Error message

B Hello, friend!

C No output

D greet function name

💡 Hint

Check the Output column at step 3 in the execution_table

At which step does the program return control back to main() after greet() finishes?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the Notes column for step 4 in execution_table

If we remove the greet() call from main(), what changes in the execution table?

A Step 5 changes to return 1

B Program crashes at step 2

C Step 3 output disappears

D Step 1 is skipped

💡 Hint

Refer to key_moments about what happens if greet() is not called

Concept Snapshot

Use cases in C show how to organize code for tasks.
Define a function for the task.
Call the function from main() to run it.
Output happens when function runs.
Program ends after main returns.

Full Transcript

This example shows a simple use case in C programming. The program starts in main(), which calls the greet() function. The greet() function prints a friendly message. After greet() finishes, control returns to main(), which then ends the program. If greet() is not called, no message prints. This flow helps beginners see how functions represent use cases and how calling them runs their code.

Practice

1. What is the main purpose of using a switch statement in C programming?

easy

A. To declare variables

B. To choose between multiple actions based on the value of a variable

C. To define a function

D. To repeat a block of code multiple times

Solution

1. Step 1: Understand the role of switch

   The switch statement is used to select one of many code blocks to execute based on a variable's value.

2. Step 2: Compare with other options

   Declaring variables, repeating a block of code multiple times, and defining a function are not the purpose of switch.

3. Final Answer:

   To choose between multiple actions based on the value of a variable -> Option B

4. Quick Check:

   switch selects code by value [OK]

Hint: Switch picks code based on variable value, like a menu choice [OK]

Common Mistakes:

• Confusing switch with loops
• Thinking switch declares variables
• Using switch to define functions

2. Which of the following is the correct syntax to start a switch statement in C?

easy

A. switch(variable) {

B. switch variable {

C. switch variable() {

D. switch { variable }

Solution

1. Step 1: Recall correct switch syntax

   The correct syntax requires the keyword switch followed by parentheses enclosing the variable, then an opening brace.

2. Step 2: Check each option

   switch(variable) { matches the correct syntax: switch(variable) {. Others miss parentheses or use wrong braces.

3. Final Answer:

   switch(variable) { -> Option A

4. Quick Check:

   Syntax needs parentheses around variable [OK]

Hint: Remember: switch(variable) with parentheses and braces [OK]

Common Mistakes:

• Omitting parentheses around variable
• Using wrong braces or brackets
• Adding parentheses after variable name

3. What will be the output of the following C code?

int x = 2;
switch(x) {
  case 1:
    printf("One\n");
    break;
  case 2:
    printf("Two\n");
    break;
  default:
    printf("Other\n");
}

medium

A. One

B. Other

C. Two

D. No output

Solution

1. Step 1: Identify the value of variable x

   The variable x is set to 2.

2. Step 2: Match x with case labels

   The switch checks cases: 1 and 2. Since x is 2, it matches case 2 and prints "Two".

3. Final Answer:

   Two -> Option C

4. Quick Check:

   Value 2 matches case 2 [OK]

Hint: Match variable value to case label to find output [OK]

Common Mistakes:

• Ignoring break and falling through
• Choosing default when case matches
• Confusing case numbers

4. Identify the error in this switch statement:

int n = 3;
switch(n) {
  case 1:
    printf("One\n");
  case 2:
    printf("Two\n");
    break;
  default:
    printf("Default\n");
}

medium

A. Missing break after case 1 causes fall-through

B. Variable n is not declared

C. Default case must be first

D. Switch cannot use integers

Solution

1. Step 1: Check for break statements

   Case 1 lacks a break, so after printing "One", execution continues to case 2.

2. Step 2: Understand consequences of missing break

   This fall-through causes both "One" and "Two" to print when n is 1, which is usually unintended.

3. Final Answer:

   Missing break after case 1 causes fall-through -> Option A

4. Quick Check:

   Always add break to prevent fall-through [OK]

Hint: Add break after each case to stop fall-through [OK]

Common Mistakes:

• Forgetting break after cases
• Thinking default must be first
• Believing switch can't use integers

5. You want to write a switch statement that prints "Weekday" for cases 1 to 5, "Weekend" for 6 and 7, and "Invalid" for any other number. Which code snippet correctly implements this?

hard

A. switch(day) {
if(day >= 1 && day <= 5) {
printf("Weekday\n");
} else if(day == 6 || day == 7) {
printf("Weekend\n");
} else {
printf("Invalid\n");
}
}

B. switch(day) {
case 1:
printf("Weekday\n");
case 2:
printf("Weekday\n");
case 3:
printf("Weekday\n");
case 4:
printf("Weekday\n");
case 5:
printf("Weekday\n");
case 6:
printf("Weekend\n");
case 7:
printf("Weekend\n");
default:
printf("Invalid\n");
}

C. switch(day) {
case 1 ... 5:
printf("Weekday\n");
break;
case 6 ... 7:
printf("Weekend\n");
break;
default:
printf("Invalid\n");
}

D. switch(day) {
case 1: case 2: case 3: case 4: case 5:
printf("Weekday\n");
break;
case 6: case 7:
printf("Weekend\n");
break;
default:
printf("Invalid\n");
}

Solution

1. Step 1: Understand valid switch syntax for multiple cases

   Cases can be grouped by listing them one after another without breaks to share code.

2. Step 2: Evaluate each option

   switch(day) { case 1: case 2: case 3: case 4: case 5: printf("Weekday\n"); break; case 6: case 7: printf("Weekend\n"); break; default: printf("Invalid\n"); } correctly groups cases 1 to 5 and 6 to 7, with breaks and a default. switch(day) { case 1: printf("Weekday\n"); case 2: printf("Weekday\n"); case 3: printf("Weekday\n"); case 4: printf("Weekday\n"); case 5: printf("Weekday\n"); case 6: printf("Weekend\n"); case 7: printf("Weekend\n"); default: printf("Invalid\n"); } lacks breaks causing fall-through. switch(day) { case 1 ... 5: printf("Weekday\n"); break; case 6 ... 7: printf("Weekend\n"); break; default: printf("Invalid\n"); } uses invalid syntax (case 1 ... 5). switch(day) { if(day >= 1 && day <= 5) { printf("Weekday\n"); } else if(day == 6 || day == 7) { printf("Weekend\n"); } else { printf("Invalid\n"); } } uses if inside switch without case labels, making the body unreachable and invalid for switch logic.

3. Final Answer:

   Groups cases by listing consecutive cases for shared execution with breaks -> Option D

4. Quick Check:

   Group cases by listing them, add breaks [OK]

Hint: Group cases by listing them; use breaks to separate [OK]

Common Mistakes:

• Using invalid range syntax in case
• Missing breaks causing fall-through
• Trying to use if inside switch