Programming with C Fundamentals

What is the basic structure of a C program?

#include <stdio.h>

int main(void)
{
	//Declare variables
	//Input
	//Program Logic (loops, conditions)
	//output
	return 0;
}

What does #include <stdio.h> do?

It allows the program to use input/output functions like:

printf()
scanf()

How do you declare variables in C?

int x;
float y;
char c;

Variables store data in the program.

When should variables be declared?

At the top of a function, usually right after {.

What does printf() do?

Prints output to the screen.

printf("Hello");

What does scanf() do?

Reads input from the user.

scanf("%d", &x);

& means the address of the variable.

Common format specifiiers

%d ->	integer
%f ->	float
%c -> 	character

What does #define do?

Creates a constant value.

#define SIZE 200

Used often with arrays.

What arithmetic operators are used in C?

+ addition
- subtraction
* multiplication
/ division
% modulus (remainder)

When do you use % (modulus)?

To check divisibility.

if (num % 2 == 0)

Checks if a number is even.

When do you use an if statement?

When the program must make a decision.

if (x > 0)
{
	printf("Positive");
}

What does an if-else statement look like?

if (condition)
{
	//code if true
}
else
{
	//code if false
}

Common relational operators

== equal (comparison)
!= not equal
> greater than
< less than
>= greater or equal
<= less or equal

When do you use a for loop?

When you know how many times the loop should run.

Example:

for (i=0; i<n; i++)
{
	printf("%d", i);
}

Often used with arrays.

Structure of a for loop

for (initialization; condition; update)
{
	statements;
}

Example:

for (i=0; i<10; i++)

When do you use a while loop?

When a loop runs while a condition is true, and the number of repetitions is not known.

Example:

while (x>0)
{
	printf("%d", x);
}

Structure of a while loop

while(condition)
{
	statements;
}

When do you use a do-while loop?

When the loop must run at least once.

Example:

do
{
	printf("Hello");
}
while(x>0);

What does a break do?

Immediately exits a loop/

Example:

if (x == 5)
{
	break;
}

What is an array?

A collection of variables of the same type stored together.

Example:

int numbers [10];

What are valid array indexes?

If array size is:

int arr[10];

//Indexes are 0 to 9

How do you read values into an array?

for(i = 0; i<n; i++)
{
	scanf("%d", &arr[i]);
}

How do you print values from an array?

for (i=0; i<n; i++)
{
	printf("%d", arr[i]);
}

How do you search for a number in an array?

for (i=0; i<n; i++)
{
	if (arr[i] == key)
	{
		// found it
	}
}

What is a function?

A reusable block of code that performs a specific task.

Example:

int square(int x)
{
	return x * x;
}

What is a function prototype?

Declares the function before main.

Int square(int x);

How do you call a function?

result = square(5);

When do you use a switch statement?

When selecting one option from multiple choices.
Often used for menus.

Structure of a switch statement

switch(x)
{
	case 1:
		statements;
	case 2:
		statements;
		break;
	default:
		statements;
}

Why is break important in a switch?

without break, the program continues executing the next cases.

What are common causes of compile errors? (Hint: 6 common mistakes)

• missing ;

• missing }

• wrong variable name

• using variable before declaring it

• forgetting & in scanf

• mismatched function names

What should you do first when given a lab exam prompt?

• Write main structure

• Declare variables

• Write input section

• Write loops/logic

• Write output section

• Compile and fix errors

What is a flag variable?

A variable used to remember whether something happened during a loop.

int found = 0;

if(arr[i] == key)
{
	found = 1;
}

often used when searching for arrays

When should you use a flag variable?

Use a flag when you need to track:

• if a value was found

• if a condition occurred in a loop

• if a number is prime

Example:

• Searching Arrays

• Prime Checking

• Validating input

What does i++ and ++i mean?

i++ is a post-increment operator, which increments the value of ‘i’ but returns the original value that ‘i’ held before being incremented. Equivalent to i = i + 1;

++i is a pre-increment operator, which increments the value of ‘i’ immediately and returns the increments value.

What does i- - and - -i mean?

i- - is a post-increment operator, which decrements the value of ‘i’ but returns the original value that ‘i’ held before being decremented. Equivalent to i = i - 1;

- -i is a pre-increment operator, which decrements the value of ‘i’ immediately and returns the decrement value.

What does continue do?

Skips the rest of the current loop iteration and goes into the next iteration.

if(x < 0)
{
	continue;
}

Difference between break and continue?

Break exits the loop completely

Continue skips to the next iteration

What is a nested loop?

A loop inside of another loop.

for(i = 0; i < n; i++)
{
	for(j = 0; j < m; j++)
	{
		printf("%d", arr[i][j]);
	}
}

Often used for:

• comparing arrays

• pattern printing

• searching combinations

When are nested loops commonly used?

• comparing two arrays

• checking all combinations

• matrix-style problems

• searching multiple lists

How do you sum elements of an array?

sum = 0;

for(i = 0; i < n; i++)
{
	sum = sum + arr[i];
}

How do you calculate average of an array?

average = sum / n;

After computing the sum.

How do you find the largest number in an array?

largest = arr[0];

for(i=1; i<n; i++)
{
	if(arr[i] > largest)
		largest = arr[i];
}

How do you find the smallest number in an array?

smallest = arr[0];

for(i=1; i<n; i++);
{
	if(arr[i] < smallest)
		smallest = arr[i];
}

How do you search for a number in an array?

for(i = 0; i < n; i++)
{
	if(arr[i] == key)
	{
		printf("Found");
	}
}

What is linear search?

Checking each element of the array one by one until the value is found.

How do you reverse an array?

Swap elements from the front and back.

Example logic:

temp = arr[i];
arr[i] = arr[n-1-i];
arr[n-1-i] = temp;

When are menu-driven programs used?

When the user chooses options from a list.

Example:

1. Add
2. Subtract
3. Exit

Usually uses switch + loop

What is the typical structure of a menu program

do
{
	print menu
	scanf choice

	switch(choice)
	{
		case 1:
		    ...
		    break;
		case 2:
		    ...
		    break;
	}
} while(choice != exit);

What type must a case label be?

A constant value.

Example:

case 1:
case 2:
case 3:

Expressions are not allowed

What causes infinite loops?

• condition never becomes false

• update statement missing

• incorrect loop condition

Example mistake:

for(i = 0, i < 10;)

Basic logic for checking if a number is prime

1. Assume the number is prime

2. Try dividing from 2 to n-1

3. if divisible → not prime

Example:

for(i = 2; i < n; i++);
{
	if(n % 1 == 0)
	    not prime
}

What does pass-by-value mean?

Functions receive a copy of the variable, not the original.

Changes inside the function do not affect the original value.

How do you pass an array to a function?

void process(int arr[], int size)

Call:

process(arr, n);

What does tracing code mean?

Following the program step-by-step to determine what it prints

Often used with:

• loops

• arrays

• counters

How should you approach a coding lab problem?

1. Read prompt carefully

2. identify needed tools (loops, arrays, switch)

3. Write program structure

4. Add logic step by step

5. Compile frequently

Most common pattern in array problems

read n
read array
loop through array
apply condition
print result

Standard loop for processing an array

for(i = 0; i < n; i++)
{
	// work with arr[i]
}

Structure of a function

returnType functionName(parameters)
{
	//statements
	return value;
}

Example:

int square(int x)
{
	return x * x;
}

Structure of an if-else statement

if(condition)
{
   statements;
}
else
{
   statements;
}