LecLabs 2.3 on Pointers


Intro to Programming Languages.

[ ← Module List ]

This module dives into creating and using pointers in C with a focus on their use in c-strings.


Module Content


Challenges

Lecture video on Stack pointers

Objective

The program will indentify non-printable characters and display them in a readable format.

CTRL-Click Here for Follow Along Video

Requirements

  1. Check for non-printable characters contains_non_printable(char *str): checks if the string contains any non-printable characters.
    • Takes a pointer to c-string as input
    • Loop through each character in the string
      • Use the isprint function from ctype.h to check if each character is printable.
      • Returns 1 if any non-printable character is detected in the string
    • Otherwise return 0
  2. Print non-printable characters print_non_printable_hex(char *str): prints non-printable characters in the format \xXX and prints them in red using ANSI escape codes.
    • Takes a pointer to a c-string as input
    • Loop through each character in the string
    • If a character is non-printable
      • Print its hexadecimal value in the format \xXX using red text.
      • Use ANSI escape codes \033[31m to start red color and \033[0m to reset color.
      • The printf should use this formatter "\033[31m\\x%02X\033[0m"
    • Else, print the character
  3. The main function
    • If no argument is provided, printf("Usage: %s \n", argv[0]); and return 1
    • Retrieve the input string from the command-line arguments.
    • Use print_non_printable_hex to check for non-printable characters.
    • If non-printable characters are found, use print_non_printable_hex to print the string with non-printable characters highlighted.
    • Otherwise, print the string as is.
  4. Example
    • If the input string is "Hello\x01\x02\x03World!", the output should be:
      • 
    Hello\x01\x02\x03World!
    with \x01, \x02, and \x03 displayed in red.
    • If the input string is "Hello World!", the output should simply be:
      • 
    Hello World!

Steps to Complete

  • Implement the requirements
  • Compile and test the program
  • Create 2 user test cases, use the desciption in the test case to guide the creation of them.
  • Run /challenge/tester
  • Get flag

Lecture video on Examples Single Value

Lecture video on Examples of Ptr to Array

Set character values using pointers.

Set strings using pointers.

Lecture video on strstr video

Using the strstr function to find a string within another string.

Objective

Write a C program that takes a filename and a word as command-line arguments. The program will read the file and count the occurrences of the specified word.

CTRL-Click Here for Follow Along Video

Requirements

  1. Create a function that counts the occurrences of the specified word in each line
  2. Take two command-line arguments: the name of the file and the word to count, if both are not specified it should give a usage message.
  3. Open the file
  4. Read each line in the file in a loop using fgets
    • Call the occurrence counter function
    • Add the number of occurrences to a running total
  5. Output the results using "The word '%s' occurs %d times in the file\n"

Example Usage


./word_count filename.txt crazy
The word 'crazy' occurrs 18 times in the file

Steps to Complete

  • Implement the requirements
  • Compile and test the program
  • Run /challenge/tester
  • Get flag

Lecture video on strtok video

Using the strtok function to extract delimited values (i.e., tokens) from a string.

Objective

The program will read a line of text from the user and print out the number of times each word appears in the input.

CTRL-Click Here for Follow Along Video Part 1

CTRL-Click Here for Follow Along Video Part 2

Requirements

  1. Change string to have all lowercase letters to_lowercase(char *str)
    • Loop through the string and change all the letters to lowercase
  2. Search for words already found find_word(char words[][MAX_WORD_LENGTH], int size, char *word)
    • Search through all the words already found and see if a match exists with the current word
    • If a match is found, return the index of the match
    • If no match is found then return -1
  3. The main function
    • Provided input (for user input), found_words (a matrix that holds the found words), counts(an array that holds counts at an index)
    • Prompt user to enter a sentence
    • Receive the sentence via standard input using fgets
    • If the last character of the input is a newline, then remove it
    • Use strtok to tokenize the input based on space (" ")
    • While strtok finds a new word continue to loop through the words in the sentence
      • Call find_words with the current word and the found words
      • If the return is -1, copy the current word to the found_words array and increase count for the max index
      • If the return is not -1, then increase the associated value in counts
      • Increase word_count
    • Print out the counts for each word

Steps to Complete

  • Implement the requirements
  • Compile and test the program
  • Create 1 user test case, use the desciption in the test case to guide the creation of the test case.
  • Run /challenge/tester
  • Get flag

Lecture video on strchr video

Lecture video on memset


30-Day Scoreboard:

This scoreboard reflects solves for challenges in this module after the module launched in this dojo.

7-Day | 30-Day | All-Time

Rank Hacker Badges Score