Week 2: Repetition, Algorithms & Functions

You already know the basic language of C. Now you will make programs repeat work, trace state changes, solve mathematical number problems, and split larger tasks into reusable functions. Every chapter follows the same learning cycle: Introduction → When & Why → Example → Run it → Enter your observed output → Unlock detailed explanation → Activities → Practice Set → Project.

8chapters

56activities

16runnable examples

30assessment questions

Repetition & the while Loop

Understand why loops exist before learning to type their syntax.

1. Introduction

A loop is a control structure that repeats a block of code while a condition remains true. This matters because computers are excellent at doing the same precise operation thousands or millions of times. Instead of writing the same instruction again and again, you describe the repetition rule once.

2. When & Why

Use a loop when the same task must happen repeatedly: counting, processing a sequence, validating input, accumulating a result, or examining the digits of a number.

The four things to identify: starting state → condition → repeated body → update toward termination.

3. Example

while_01.coutput hidden

#include <stdio.h>

int main(void)
{
    int i = 1;

    while(i <= 5)
    {
        printf("%d\n", i);
        i++;
    }

    return 0;
}

Flexible: line breaks and extra spaces are ignored.

4. Example code explanation — after you run it

The explanation unlocks only after you enter the actual output. It covers symbols such as while, the condition, i++, printf, %d, \n, braces and return 0;.

5. Use Cases

Count until a limit
Read values until a sentinel such as 0
Validate input repeatedly
Process digits one by one

6. Activities

Practice Set:

Print 1–20
Print 20–1
Print multiples of 3 below 50
Print first 10 squares
Find 1+2+...+n
Count even values from 1 to n
Write a loop whose stopping condition depends on user input

Project: Number Sequence Generator — input a start and end value, print the sequence forward and backward.

while in Depth

Execution order, boundaries, sentinel values, and infinite loops.

1. Introduction

while checks its condition before every repetition. Therefore the body may execute zero times. That small fact becomes very important when writing validation and input-driven programs.

2. When & Why

Choose while when you can naturally say “keep doing this while this condition is true,” especially when the number of repetitions is not known in advance.

3. Example

while_02.coutput hidden

#include <stdio.h>

int main(void)
{
    int i = 10;

    while(i < 5)
    {
        printf("%d\n", i);
        i++;
    }

    printf("Done");
    return 0;
}

Flexible: line breaks and extra spaces are ignored.

4. Example — sentinel pattern

while_03.c

#include <stdio.h>

int main(void)
{
    int x = 1;
    while(x != 0)
    {
        printf("x = %d\n", x);
        x--;
    }
    return 0;
}

Flexible: line breaks and extra spaces are ignored.

Typical bugs: forgetting the update creates an infinite loop; choosing the wrong boundary creates an off-by-one error.

5. Use Cases

Input until a sentinel value
Running totals
Running maximum/minimum
Repeated validation

6. Activities

Practice Set:

Count to 100
Print multiples of 5
Count down from user input
Sum 1 to n
Read values until 0 and count them
Track the maximum value
Create and repair an infinite loop

Project: Basic Statistics Counter — read integers until 0 and report count, positive count, negative count and sum.

The for Loop

A compact loop form for counter-controlled repetition.

1. Introduction

A for loop puts initialization, condition and update together. It often makes a known repetition count easy to read.

2. When & Why

Use it when you naturally think “start here, continue while this is true, change the counter this way.”

3. Anatomy

for(initialization; condition; update)
{
    // repeated body
}

initialization
sets the starting state

condition
allows or stops the next iteration

update
moves the counter toward termination

body
the work repeated each iteration

4. Example

for_01.c

#include <stdio.h>

int main(void)
{
    for(int i = 1; i <= 5; i++)
    {
        printf("%d ", i);
    }
    return 0;
}

Flexible: line breaks and extra spaces are ignored.

5. Example — accumulator

for_02.c

#include <stdio.h>

int main(void)
{
    int sum = 0;
    for(int i = 1; i <= 5; i++)
    {
        sum = sum + i;
    }
    printf("%d", sum);
    return 0;
}

Flexible: line breaks and extra spaces are ignored.

6. Use Cases

Tables
Known iteration counts
Series and sums
Counting problems

7. Activities

Practice Set:

Print 1–10
Even numbers 2–50
Odd numbers 1–49
Multiplication table
Factorial
Sum of squares
Rewrite a while solution using for and compare readability

Project: Multiplication Table Generator — user chooses the number and limit.

do...while

Run once first, then decide whether to repeat.

1. Introduction

A do...while checks its condition after the body. Consequently, the body executes at least once.

2. When & Why

This makes it useful for menus, retry prompts and input that must be requested before you can know whether another attempt is necessary.

3. Example

do_01.c

#include <stdio.h>

int main(void)
{
    int i = 10;
    do
    {
        printf("%d", i);
        i++;
    }
    while(i < 5);
    return 0;
}

Flexible: line breaks and extra spaces are ignored.

4. Example — menu pattern

do_02.c

#include <stdio.h>

int main(void)
{
    int choice;
    do
    {
        printf("1 = Hello, 0 = Exit\n");
        scanf("%d", &choice);
    }
    while(choice != 0);
    return 0;
}

Flexible: line breaks and extra spaces are ignored.

5. Use Cases

Menus
Retry systems
Validation
“Ask at least once” interactions

6. Activities

Practice Set:

Write a do...while that runs once with a false condition
Ask until a positive number is entered
Build a 1/0 menu
Ask until a target number is entered
Convert a while program
Compare while vs do...while with a false first condition
Explain why menus often fit do...while

Project: Interactive Calculator Menu that repeats until Exit.

Nested Loops

Think in rows and columns; one loop can control repeated work inside another.

1. Introduction

A nested loop is a loop inside another loop. The outer loop can represent rows or larger units; the inner loop can represent repeated work inside each row.

2. When & Why

Use nested loops for grids, patterns, tables and problems with two dimensions of repetition.

3. Example

nested_01.c

#include <stdio.h>

int main(void)
{
    for(int row = 1; row <= 3; row++)
    {
        for(int col = 1; col <= 4; col++)
        {
            printf("* ");
        }
        printf("\n");
    }
    return 0;
}

Flexible: line breaks and extra spaces are ignored.

4. Example — multiplication grid

nested_02.c

#include <stdio.h>

int main(void)
{
    for(int i = 1; i <= 3; i++)
    {
        for(int j = 1; j <= 3; j++)
        {
            printf("%d ", i * j);
        }
        printf("\n");
    }
    return 0;
}

Flexible: line breaks and extra spaces are ignored.

5. Use Cases

Patterns
Tables
Grid-like data
All pair combinations

6. Activities

Practice Set:

3×5 star rectangle
4×4 number square
Increasing triangle
Decreasing triangle
Print row/column coordinates
5×5 multiplication grid
Predict total inner-loop executions before running

Project: Pattern Generator — build at least three distinct patterns and one of your own.

Number Logic with Loops

Connect mathematics, integer arithmetic and repetition.

1. Introduction

One of the best ways to learn loops is to use them for digit algorithms. Two very useful integer operations are n % 10 to obtain the last digit and n / 10 to remove the last digit when using integer arithmetic.

2. When & Why

This pattern lets one loop process every digit of a positive integer. It is the basis of digit sum, digit count, reversal and palindrome problems.

n % 10
last digit

n / 10
remove last digit

accumulator
a variable that builds a result over iterations

dry run
trace the state on paper before executing

3. Example — digit sum

number_01.c

#include <stdio.h>

int main(void)
{
    int n = 4827;
    int sum = 0;
    while(n > 0)
    {
        sum = sum + n % 10;
        n = n / 10;
    }
    printf("%d", sum);
    return 0;
}

Flexible: line breaks and extra spaces are ignored.

4. Example — reverse

number_02.c

#include <stdio.h>

int main(void)
{
    int n = 1234;
    int rev = 0;
    while(n > 0)
    {
        rev = rev * 10 + n % 10;
        n = n / 10;
    }
    printf("%d", rev);
    return 0;
}

Flexible: line breaks and extra spaces are ignored.

5. Use Cases

Digit count
Digit sum/product
Reverse
Palindrome
Largest/smallest digit
Even/odd digit counts

6. Activities

Practice Set:

Count digits
Sum digits
Product digits
Reverse
Palindrome
Largest digit
Count even and odd digits

Project: Number Analyzer — return multiple facts about one integer.

Functions

Move from “one long main” to reusable, named pieces of logic.

1. Introduction

A function packages a task into a named unit. This reduces duplication and lets you reason about a big problem as smaller problems. Functions are a foundation of modular programming.

2. When & Why

Use a function when a task is logically separate, repeated, or easier to test when given a name.

Term	Meaning
Definition	The function's code.
Call	Requests that the function execute.
Parameter	Named input in the function definition.
Argument	Actual value passed in a call.
Return value	Value sent back to the caller.

3. Example — no parameters, no return value

function_01.c

#include <stdio.h>

void greet(void)
{
    printf("Hello from a function!");
}

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

Flexible: line breaks and extra spaces are ignored.

4. Example — parameters and return value

function_02.c

#include <stdio.h>

int add(int a, int b)
{
    return a + b;
}

int main(void)
{
    int answer = add(7, 5);
    printf("%d", answer);
    return 0;
}

Flexible: line breaks and extra spaces are ignored.

5. Use Cases

Reusable calculations
Validation helpers
Readable program structure
Testing one task at a time

6. Activities

Practice Set:

Create a no-parameter function
Call it multiple times
Function with one parameter
Function with two parameters
Function that returns a value
Arithmetic function set
Refactor an old program into functions

Project: Modular Calculator — add, subtract, multiply, divide plus a clean main().

Integration & Real Problem Solving

Combine loops, conditions and functions without turning main into a mess.

1. Introduction

Programming becomes useful when concepts work together. A menu can use a loop; its choices can use conditions; calculations can be delegated to functions; repeated input can use a loop; testing can verify each operation.

2. When & Why

Use decomposition when a problem contains multiple distinct tasks. Before coding, identify inputs, outputs, subtasks, loop points and decision points.

3. Example architecture

main()
 ├── showMenu()
 ├── add(...)
 ├── factorial(...)
 ├── reverseNumber(...)
 └── isEven(...)

4. Example — loop + function + condition

integration_01.c

#include <stdio.h>

int isEven(int n)
{
    return n % 2 == 0;
}

int main(void)
{
    for(int i = 1; i <= 5; i++)
    {
        if(isEven(i))
            printf("%d is even\n", i);
        else
            printf("%d is odd\n", i);
    }
    return 0;
}

Flexible: line breaks and extra spaces are ignored.

5. Activities

Practice Set:

Write a menu-driven calculator
Make each operation a function
Use a loop for repeated menu choices
Add even/odd and factorial utilities
Add reverse and palindrome utilities
Create a test plan with normal and edge cases
Debug one intentionally broken version

Week 2 Capstone: C Utility Toolkit — a menu-driven program containing several utilities, organized with functions and repeated with a suitable loop.

Final Combined Week 2 Quiz

Tests concepts, output prediction, tracing, debugging, loop choice, functions, and problem solving.

Do the output questions without running the code first. Then use your compiler afterward to verify your reasoning.

C from Zero · Week 2 · Local progress is stored only in this browser.