C Fundamental Practice Problems: Complete Guide

C Fundamental Practice Problems: A Comprehensive Tutorial

Learning C is not about memorizing syntax—it’s about building muscle memory through practice. This tutorial is a structured practice guide covering the fundamental concepts every C programmer must master: variables, data types, input/output, format specifiers, operators, conditional statements, and loops.

Each section explains the concept, then walks through a representative practice problem with a complete, portable C99/C11 solution and expected output.

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Conceptual Introduction & Fundamentals

Why Practice Fundamentals?

Fundamentals are the foundation of every C program, whether it’s 20 lines or 20,000. Weak fundamentals show up as:

• Off-by-one errors in loops.
• Format specifier mismatches causing undefined behavior.
• Buffer overflows from unsafe input functions.
• Silent type conversion bugs in arithmetic.
• Incorrect edge-case handling in conditionals.

The Fundamental Topics

Topic	Purpose
Variables	Named storage for values
Data Types	Size and interpretation of values
Input / Output	Communicating with the user
Format Specifiers	Controlling how data is printed or read
Operators	Arithmetic, relational, logical, bitwise
Conditional Statements	Branching logic (if, switch)
Loops	Repetition (for, while, do-while)

Difficulty Progression

• Easy: Printing, variables, simple arithmetic, I/O.
• Medium: Control flow, digit manipulation, number theory basics.

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Syntax & Core Mechanics

1. Variables and Data Types

c

int age = 25; double price = 19.99; char grade = 'A';

Use int for whole numbers, double for decimals, char for single characters. Prefer double over float for computation; use long long when values may exceed int.

2. Input with fgets and Parsing

Never use scanf("%s", ...) or gets for strings. Prefer fgets plus strtol/strtod for robust parsing.

c

char buffer[64]; if (fgets(buffer, sizeof(buffer), stdin) == NULL) return 1; int value = (int)strtol(buffer, NULL, 10);

3. Output with printf

Match the specifier to the type:

Type	Specifier
int	%d
unsigned int	%u
long long	%lld
double	%f, %lf, %g
char	%c
String	%s
size_t	%zu
Pointer	%p

4. Operators

c

int a = 7, b = 3; int q = a / b; /* 2 — integer division */ int r = a % b; /* 1 */ double f = (double)a / b; /* 2.333... */

Use casting to force floating-point division.

5. Conditionals and Loops

c

if (a > b) { /* ... */ } else if (a == b) { /* ... */ } else { /* ... */ } for (int i = 0; i < n; i++) { /* ... */ } while (cond) { /* ... */ } do { /* ... */ } while (cond);

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Full Practical Code Example with Explanation

Below are eight representative problems covering the fundamentals, each a self-contained C99 program with verified output.

Problem 1: Print “GeeksforGeeks” with Quotes

Goal: Print a string including literal double quotes.

c

#include <stdio.h> int main(void) { /* Escape double quotes with backslash */ printf("\"GeeksforGeeks\"\n"); return 0; }

Expected output:

text

"GeeksforGeeks"

Notes: \" inside a string literal produces a literal ".

Problem 2: Swap Two Numbers (Without Temp for Integers)

Goal: Swap a and b.

c

#include <stdio.h> int main(void) { int a = 5, b = 7; /* Arithmetic swap (works for ints; watch overflow) */ a = a + b; b = a - b; a = a - b; printf("a = %d, b = %d\n", a, b); return 0; }

Expected output:

text

a = 7, b = 5

Notes: For large values, prefer a temporary variable or XOR swap. XOR is safe for equal values but not for aliasing.

Problem 3: Convert double to int

Goal: Truncate a floating-point number.

c

#include <stdio.h> int main(void) { double d = 3.99; int i = (int)d; /* explicit cast truncates toward zero */ printf("%d\n", i); return 0; }

Expected output:

text

3

Notes: (int) truncates; round() from <math.h> rounds to nearest.

Problem 4: Last Digit of a Number

Goal: Extract the last decimal digit.

c

#include <stdio.h> #include <stdlib.h> int main(void) { int n = 12345; int last = n % 10; /* Works for non-negative; see notes for negatives */ printf("%d\n", last); return 0; }

Expected output:

text

5

Notes: For negative n, % in C99 yields a non-positive remainder. Use abs(n) % 10 or n % 10 + 10 (mod 10) for a non-negative digit.

Problem 5: Compare Two Numbers

Goal: Print which of two numbers is greater.

c

#include <stdio.h> int main(void) { int a = 10, b = 20; if (a > b) printf("%d is greater\n", a); else if (b > a) printf("%d is greater\n", b); else printf("Both are equal\n"); return 0; }

Expected output:

text

20 is greater

Problem 6: FizzBuzz (1 to N)

Goal: Print Fizz, Buzz, or FizzBuzz per divisibility rule.

c

#include <stdio.h> void fizzbuzz(int n) { for (int i = 1; i <= n; i++) { if (i % 15 == 0) printf("FizzBuzz "); else if (i % 3 == 0) printf("Fizz "); else if (i % 5 == 0) printf("Buzz "); else printf("%d ", i); } printf("\n"); } int main(void) { fizzbuzz(15); return 0; }

Expected output:

text

1 2 Fizz 4 Buzz Fizz 7 8 Fizz Buzz 11 Fizz 13 14 FizzBuzz

Notes: Check i % 15 == 0 first, otherwise the individual cases shadow the combined one.

Problem 7: Multiplication Table

Goal: Print an n × n multiplication table.

c

#include <stdio.h> int main(void) { int n = 5; for (int i = 1; i <= n; i++) { for (int j = 1; j <= n; j++) { printf("%4d", i * j); } printf("\n"); } return 0; }

Expected output:

text

 1 2 3 4 5 2 4 6 8 10 3 6 9 12 15 4 8 12 16 20 5 10 15 20 25

Problem 8: Sum of N Numbers with Safe Input

Goal: Read n integers from stdin, sum them.

c

#include <stdio.h> #include <stdlib.h> #include <errno.h> #include <limits.h> int readInteger(const char *prompt, int *outValue) { char buffer[64]; char *endPtr; long val; printf("%s", prompt); fflush(stdout); if (fgets(buffer, sizeof(buffer), stdin) == NULL) return -1; errno = 0; val = strtol(buffer, &endPtr, 10); if (endPtr == buffer || (*endPtr != '\n' && *endPtr != '\0')) return -1; if (errno == ERANGE || val < INT_MIN || val > INT_MAX) return -1; *outValue = (int)val; return 0; } int main(void) { int n; if (readInteger("How many numbers? ", &n) != 0 || n <= 0) { fprintf(stderr, "Invalid count.\n"); return 1; } long long sum = 0; for (int i = 0; i < n; i++) { int x; char prompt[40]; snprintf(prompt, sizeof(prompt), "Number %d: ", i + 1); if (readInteger(prompt, &x) != 0) { fprintf(stderr, "Invalid number.\n"); return 1; } sum += x; } printf("Sum = %lld\n", sum); return 0; }

Sample run:

text

How many numbers? 3 Number 1: 10 Number 2: 20 Number 3: 30 Sum = 60

Notes: Uses long long for the sum to prevent overflow; strtol for robust parsing.

Bonus Problem: Medium — Count Digits of an Integer

c

#include <stdio.h> #include <stdlib.h> int countDigits(long long n) { if (n == 0) return 1; if (n < 0) n = -n; int count = 0; while (n > 0) { count++; n /= 10; } return count; } int main(void) { printf("%d\n", countDigits(0)); /* 1 */ printf("%d\n", countDigits(12345)); /* 5 */ printf("%d\n", countDigits(-9876)); /* 4 */ return 0; }

Expected output:

text

1 5 4

Bonus Problem: Medium — Leap Year

c

#include <stdio.h> int isLeapYear(int year) { /* A year is leap if divisible by 4 but not 100, unless divisible by 400 */ return (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0); } int main(void) { int years[] = {1900, 2000, 2024, 2023, 2100}; for (int i = 0; i < 5; i++) { printf("%d → %s\n", years[i], isLeapYear(years[i]) ? "Leap" : "Not Leap"); } return 0; }

Expected output:

text

1900 → Not Leap 2000 → Leap 2024 → Leap 2023 → Not Leap 2100 → Not Leap

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Common Pitfalls, Edge Cases & Memory Considerations

1. Integer Division Truncation

5 / 2 is 2, not 2.5. Cast one operand.

c

double r = (double)a / b;

2. Format Specifier Mismatch

printf("%d", 3.14) is undefined behavior. Match specifiers to types.

3. Negative Remainders

In C99, -7 % 3 == -1, not 2. Use (n % m + m) % m for a non-negative result.

4. Off-by-One in Loops

for (i = 0; i <= n; i++) iterates n+1 times. Use < unless you need inclusive bounds.

5. Unsafe Input

Never use gets (removed in C11). Prefer fgets with size limits.

6. Uninitialized Variables

Reading before assignment is undefined behavior.

c

int x; printf("%d\n", x); /* BUG */

7. Signed/Unsigned Confusion

Mixing signed and unsigned in comparisons yields surprising results.

c

int a = -1; unsigned int b = 1; if (a < b) { /* may be FALSE due to conversion */ }

8. Overflow

int overflows for large sums. Use long long.

c

long long sum = 0;

9. Character Classifications

Use <ctype.h> functions for portability.

c

if (isdigit((unsigned char)c)) { /* ... */ }

Cast to unsigned char before passing.

10. Memory Considerations

• Fundamental programs rarely allocate memory.
• When they do, prefer dynamic allocation for large inputs.
• Free everything you malloc.
• Never return pointers to stack variables.

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Summary & Best Practices

Fundamentals are the platform on which all C skill is built. Practice them until they become reflexive.

Key Takeaways

• Variables and types define what can be stored.
• Format specifiers define how data is printed/read.
• Operators and control flow define how programs compute.
• Safe input requires fgets + strtol/strtod.
• Every bug you encounter at scale is a fundamentals bug in disguise.

Best Practices

1. Compile with -Wall -Wextra -pedantic -std=c11 on every exercise.
2. Match format specifiers to types.
3. Use long long for sums; double for decimals.
4. Prefer fgets over scanf("%s", ...).
5. Cast before division when you need floating-point results.
6. Initialize all variables.
7. Handle negative remainders explicitly.
8. Cast char to unsigned char for <ctype.h> functions.
9. Test boundary cases: zero, negatives, maxima, empty input.
10. Practice daily — fundamentals are perishable without repetition.

Recommended Practice Progression

• Easy: Print GeeksforGeeks; Global Variable; Swap Numbers; Convert double to int; Input Integer; Print with Space/Without/Separator/New Line; Be Precise; Print with Quotes; Last Digit; Compute a/b; Compare Two Numbers; Sign Check; Big Number; Sum of N; Multiplication Table.
• Medium: AP Term; GP Term; Even Odd Game; Greatest of Three; Leap Year; FizzBuzz; Calculator; First Digit; Count Digits; Days Before n Days; Prime Factorization; Next Prime.

Work through each exercise with the templates and safety rules above. By the time you finish, fundamentals become second nature.