Study Guide 1

Exams will be closed book. Lab coding challenges will be closed-internet.

Topics:

  • UNIX commands

  • Basic C programs

  • Data types and sizes

  • Function stacks

  • Arrays

Write C programs to check your answers. Or you can ask a TA or instructor to give feedback on your responses.

Exercises

1) What sizes are each of the variables below? If the variable is a pointer, give the size of the memory that the pointer points to. If the variable is a struct, give the minimum size.

A)
struct Snack {
  char name[32];
  int quantity;
  float cost;
};

B)
struct Snack snacks[2];

C)
struct Snack* snacks = malloc(sizeof(struct Snack) * 10);

D)
char message[16];

E)
int values[2];

Hint: To check your answers, use the sizeof() function.

2) Consider navigating directories from the command prompt in UNIX. Suppose you are in your home directory.

  • How would you create a subdirectory with name foo?

  • How would you see the contents of your .bashrc file?

  • How would you list the contents of foo?

  • How can you create a file in the directory foo called test.c?

  • How can you compile a c program?

3) Suppose Willa is in their home directory, /home/willa. Write commands to produce the following directory structure. Willa contains two sub-directories, A ad B, and one file hello.c. B contains a sub-directory C and file test.txt. Below is a visualization of it where child directories are indented.

\home\willa
  A
  B
    C
    test.txt
  hello.c

4) When Willow runs her program from the command line, she gets the following error? What is happening and how can she fix it?

$ ls
Makefile  fortune  fortune.c  hello  hello.c
$ hello
Command 'hello' not found

5) The following program crashes. What is the problem and how can we fix it?

#include <stdio.h>

int main() {
  int* value = NULL;
  int a = 4;

  printf("value is %d\n", *value);
}

6) The following program crashes. What is the problem and how can we fix it?

#include <stdio.h>
#include <string.h>

void initialize(char text[]) {
  strcpy(text, "pina collada");
}

int main() {
  char str1[5];
  initialize(str1);
  printf("%s\n", str1);
  return 0;
}

7) What is the output of this program?

#include <stdio.h>

int arg_modifier(int x, int *y);

int main() {
    int val1, val2, ret;

    printf("Enter a value: ");
    scanf("%d", &val1);
    printf("Enter another value: ");
    scanf("%d", &val2);

    // pass val1 by value and val2 by pointer:
    printf("before call: val1 = %d val2 = %d\n", val1, val2);
    ret = arg_modifier(val1, &val2);
    printf("after call:  val1 = %d val2 = %d ret = %d\n", val1, val2, ret);

    return 0;
}

int arg_modifier(int x, int *y) {
    printf("  in arg_modifier:      x = %d *y = %d\n", x, *y);
    *y = *y + x;
    x = x + 5;
    printf("  leaving arg_modifier: x = %d *y = %d\n", x, *y);

    // Draw function stack here
    return x;
}

8) Draw the state of the function stack and heap for the program above in arg_modifier. Assume the user entered the values 6 and 1. Draw the stack at the point in the execution right before the return from the arg_modifier function, and consider the following questions:

  • Where are variables val1 and val2 located on the stack?

  • What value does each parameter get?

  • What variables are in scope of arg_modifier?

9) Consider the following code. Find and fix the errors in the associated stack diagram.

float foo(float* vals, int n)
{
  float x = 0.0;
  for (int i = 0; i < n; i++)
  {
    x += vals[i];
  }
  return x;
}

void main()
{
  float heights[3] = {6.1f, 5.2f, 4.9f};
  float a = foo(heights, 3);
  float b = a / 3;
  // draw stack here
  printf("%.2f\n", b);
}
studyguide01 badstack

10) Draw the directory hierarchy that corresponds to the following commands

$ pwd
/home/alinen
$ mkdir A
$ cd A
$ mkdir Z
$ touch talk.c
$ cd ..
$ touch listen.c
$ cd
$ touch sing.c

11) Consider the following directory structure

root
> home
>> ren
>>> A
>> stimpy
>>> B
>>> C
>>>> hello.txt

  • Draw the directory hierarchy above as a tree.

  • What is the absolute path of hello.txt?

  • If we are in the directory A, what is the relative path of hello.txt?

  • If we are in the directory B, what is the relative path of hello.txt?

Coding

  • Implement a program, ask_matrix.c, that asks the user for a number of rows and columns, creates a 2D matrix, and then asks the user for its values.

Enter a number of rows: 3
Enter a number of cols: 4
Please enter a value for matrix[0][0]: 0
Please enter a value for matrix[0][1]: 1
Please enter a value for matrix[0][2]: 2
Please enter a value for matrix[0][3]: 4
Please enter a value for matrix[1][0]: 5
Please enter a value for matrix[1][1]: 6
Please enter a value for matrix[1][2]: 7
Please enter a value for matrix[1][3]: 8
Please enter a value for matrix[2][0]: 9
Please enter a value for matrix[2][1]: -1
Please enter a value for matrix[2][2]: -2
Please enter a value for matrix[2][3]: -3
Here is your matrix!
0 1 2 4
5 6 7 8
9 -1 -2 -3

  • Write code to reverse an array

$ ./reverse_numbers
Enter the size of the array: 4
Enter a number: 2
Enter a number: 4
Enter a number: 6
Enter a number: 8

The array in reverse order is: 8 6 4 2

  • Write code that implements a power function (for positive integer exponents only). A call to your function should compute \(base^{exp}\) and might look like:

result = power(base, exp);

  • Write code that implements a function void removeLetter(char* str, char letter) that removes all occurrences of letter from str. Your function should modify the contents of str. (To check your work, implement your program and test it from main).

  • Write code that implements a function isPalindrome that returns 1 if a given string is a palindrome and 0 otherwise. Your function can be iterative or recursive. Test your function from main.

  • Write a program, countsort.c, that asks the user for a series of digits and then prints the digits in sorted order. Your program should count the number of times each digit appears in the input and then use these counts to print the sorted list.

$ ./countsort
Enter digits: 034567139432
0 1 2 3 3 3 4 4 5 6 7 9
$ ./countsort
Enter digits: 3212
1 2 2 3

Requirements/Hints:

  • You can assume that inputs will be less than 31 characters

  • Your program should not have memory errors and have good style.

  • You cannot assume that inputs will be digits only

  • Use the ASCII values of the input to convert from a character to an integer.

  • Your algorithm should be O(n)

  • Write a program that takes a list of words as command line arguments and outputs the largest one.