12.5 Tutorial 8

intro-to-computer-science-gtiit-tutorial-08-2024.pdf

Introduction to Computer Science

Tutorial 8: Randomness, Arrays

1. Randomness

1.1 Fixing a mistake

This program is supposed to print a sequence of 10 pseudorandom numbers, but it does not work as intended. What is the issue and how can it be solved?

main() {  
    int i;  
    for (i = 0; I < 10; i++) {  
        srand(time(NULL));  
        printf("%d ", rand());  
    }  
}

Correction:

srand will generate a sequence based on time, but if put it in the loop, the times vary very little, then the sequence is same, "469958 6545324 345676786 364567...."

then we will get the same random number by rand() because it returns the value from the sequence, then we will get "469958" again and again

Thus we need put the srand outside

main() {  
    int i;  
    srand(time(NULL));
    for (i = 0; i < 10; i++) {  
        printf("%d ", rand());  
    }  
}

1.2 Implementing a Password Generator

Write a program that prints n (n is provided by the user) passwords according to the following rules:

Each password is composed of 10 characters.
To generate a character, randomly choose a category, then randomly choose a character inside that category.
There are 3 categories of characters: a-z, A-Z, and 0-9. All three categories are mandatory (at least one character for each category exists in a valid password).

Here is an example of the desired output of one execution of the program (each execution of the program should yield a different output):

9E15GJ42f2  
64iR47215p  
oLaFXILG90  
0H632gjMKW  
S1MQE068v7  
pAKSwNIgr7  
8N3946D2mR  
8314J12nDI  
X63QG4b6jr  
5f274U76E4

#include<stdio.h>
#include<stdlib.h>
#include<time.h>

//48 57 0-9
//65 90 A-Z
//97 122 a-z

int main() {
    srand(time(NULL));
    int n;
    printf("Please enter the number of password needed:");
    scanf("%d", &n);
    int ch;
    int category;//0 = Number, 1 == Uppercase letter, 2 == Lowercase letter
    for(int i = 1; i <= n; i++) {
        for(int j = 1; j<= 10; j++) {
            category = rand() % 3;
            if(category == 0) {
                ch = rand() % 10 + 48;
            }
            else if(category == 1) {
                ch = rand() % 26 + 65;
            }
            else {
                ch = rand() % 26 + 97;
            }
            putchar(ch);
        }
        printf("\n");
    }
    return 0;
}

2. Output of Programs with Arrays

For each program, write down what would be its output when executed

2.1

main(){  
  int i;  
  int a[5] = {10,20,30,40,50};  
  a[1] = 33;  
  a[4] = 66;  
  for(i=0; i<5; i++)  
    printf("%d ", a[i]);  
}

2.2

main(){  
  int i=1, j=2;  
  int a[5] = {10,20,30,40,50};  
  a[i+j] = 33;  
  a[j*2] = 66;  
  for(i=0; i<5; i++)  
    printf("%d ", a[i]);  
}

2.3

main(){  
  int i;  
  int a[5] = {10,20,30};  
  a[4] = a[2];  
  for(i=0; i<5; i++)  
    printf("%d ", a[i]);  
}

2.4

main(){  
  int i;  
  int a[5] = {10,7,13,8,15};  
  int b[5] = {0};  
  for(i=1; i<4; i++)  
    b[i] = a[i+1];  
  for(i=0; i<5; i++)  
    printf("%d %d ",a[i], b[i]);  
}

3. Completing and Visualizing Programs with Arrays

You are given programs that are almost complete. You need to add statements in the parts indicated by comments to fulfill each corresponding task.

It is recommended to copy the provided programs to Python Tutor and fill the missing instructions to visualize the execution of the programs.

You may need to add an #include to use the NULL constant, like:

#include <stdlib.h>

3.1

The following program declares two arrays a and b; b is initialized with zeros, and a is filled with random values from 0 to 9.

In the indicated place, add statements to the main() function so that the contents of array a[] is copied into array b[].

#define T 4

main() {  
  int a[T];  
  int b[T] = {0};  
  int i;  
  srand(time(NULL));  
  for(i=0; i<T; i++)  
    a[i] = rand() % 10;  
  // modify from here  

  // modify to here  
}

3.2

The following program declares two arrays a and b, and fills them with random values. It also declares a third array, c, initialized with zeros.

Add statements to the main() function so that the array c[] receives, at each position i, the value a[i] + b[i].

#define T 4

main() {  
  int a[T];  
  int b[T];  
  int c[T] = {0};  
  int i;  
  srand(time(NULL));  
  for(i=0; i<T; i++){  
    a[i] = rand() % 10;  
    b[i] = rand() % 10;  
  }  
  // modify from here  

  // modify to here  
}

3.3

The following program fills an array a with random values from 1 to 5.

Modify the main() function so it calculates the product of the elements of array a[], into a variable called product.

#define T 4

main() {  
  int a[T];  
  int i;  
             // declare a new variable here  
  srand(time(NULL));  
  for(i=0; i<T; i++)  
    a[i] = 1 + rand() % 5;  
  // modify from here  

  // modify to here  
}

3.4

The following program fills an array a with random values from 1 to 5.

Modify the main() function so it finds the maximum value of the array a[] and stores it into a variable named max. Be careful with what you use as the initial value of max.

#define T 4

main() {  
  int a[T];  
  int i;  
             // declare a new variable here  
  srand(time(NULL));  
  for(i=0; i<T; i++)  
    a[i] = 1 + rand() % 5;  
  // modify from here  

  // modify to here  
}

3.5

Consider the program below.

Modify the main() function so that the contents of array a[] gets copied into array b[] in reverse order.

For instance, if a[] contains {3,6,0,0,2}, b[] should contain {2,0,0,6,3} after the statements you will add.

#include<stdio.h>
#include<stdlib.h>
#include<time.h>

#define T 4

int main() {  
    int a[T];  
    int b[T] = {0};  
    int i;  
    srand(time(NULL));  
    for(i=0; i<T; i++) {
        a[i] = rand() % 10;  
  // modify from here  
        b[T-i-1] = a[i];
    }
    for(i = 0; i < T; i++) {
        printf("%d\n", a[i]);
    }
    printf("\n");
    for(i = 0; i < T; i++) {
        printf("%d\n", b[i]);
    }
    return 0;
  // modify to here  
}

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