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Pseudocode

Pseudocode. A way to make programming easier Start with a verbal description of what the program is supposed to do! Slowly transform it into C, defining variables as you go. Exercise - example. Write a program that gets a string from the user and checks whether or not it is a palindrome

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Pseudocode

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  1. Pseudocode • A way to make programming easier • Start with a verbal description of what the program is supposed to do! • Slowly transform it into C, defining variables as you go

  2. Exercise - example Write a program that gets a string from the user and checks whether or not it is a palindrome Example for a palindrome: abbcbba (Hint: use strlen…)

  3. Exercise – solution step 1 • Read a string from the user • If it is a palindrome, print so • End the program

  4. … and in C /* This program checks whether a given string is a palindrome*/ #include <stdio.h> #include <string.h> int main(void) { char str[101]; printf("Enter a string\n"); scanf("%100s",str); if (isPalindrome(str) == 1) printf("The string is a palindrome!\n"); else printf(“The string is NOT a palindrome!\n”); return 0; }

  5. Exercise – solution step 2 • Implementing the function… • Ask myself – how do I know abbcbba, for example, is a palindrome? • One way – when I read the letters from left to right, it’s the same as when I read from right to left • A possible solution: copy first string in reverse, and compare to the original

  6. … and in C intisPalindrome(char str[]) { char reverse[101]; COPY str IN REVERSE TO reverse if str AND reverse ARE IDENTICAL return 1; else return 0; }

  7. How to copy a string in reverse? • Start at the end and work your way backwards • What variables would we need? • One for the next place to copy to • One for the next place to copy from

  8. … and in C intisPalindrome(char str[]) { char reverse[101]; int from, to; to = 0; for (from = END OF str; from>=0; from--) { reverse[to] = str[from]; to++; } …

  9. … and in C intisPalindrome(char str[]) { char reverse[101]; int from, to; to = 0; for (from = strlen(str); from>=0; from--) { reverse[to] = str[from]; to++; } …

  10. … and in C intisPalindrome(char str[]) { char reverse[101]; int from, to; to = 0; for (from = strlen(str)-1; from>=0; from--) { reverse[to] = str[from]; to++; } …

  11. … and in C intisPalindrome(char str[]) { char reverse[101]; int from, to; to = 0; for (from = strlen(str)-1; from>=0; from--) { reverse[to] = str[from]; to++; } reverse[to] = ‘\0’; …

  12. Continuing the function … if str AND reverse ARE IDENTICAL return 1; else return 0; }

  13. Continuing the function … if (strcmp(str, reverse) == 0) return 1; else return 0; }

  14. Great! • But what if we want to not use a second array? • Can we ‘read’ from the start and from the end at the same time? • And what if we want to ignore non-letters and letter-case? • For example – Madam, I’m Adam should would be a palindrome

  15. Solution • palindrome.c • advanced_pal.c

  16. Exercise • Implement the function void my_strcat(char s1[], char s2[]); • Takes two strings, and copies s2 at the end of s1 • Write a program that takes two strings from the user and prints their concatenation

  17. Solution strcat.c

  18. String in string • Write a function that takes two strings, str1 and str2, and returns the index of the first time str2 occurs in str1, or -1 if that doesn’t happen • For str1 = “abcde”, str2 = “bc”, return 1 • For str1 = “abbcc”, str2 = “rg”, return -1

  19. Solution – step 1 • Ask yourself – how do you know where str2 occurs in str1? • For example – • str1 = “abbccba”, str2 = “bc” • How do you know you shouldn’t return 0? • How do you know you shouldn’t return 1? • How do you know you should return 2?

  20. Solution – pseudocode 1 int strstr(char str1[], char str2[]) { for each location in str1 if str2 occurs in str1 starting in that location return that location if we didn’t find str1 anywhere return -1;

  21. Solution – pseudocode 2 int strstr(char str1[], char str2[]) { int i; for (i=0; i<strlen(str1); i++) { if str2 occurs in str1 starting in i return i; } if we didn’t find str1 anywhere return -1;

  22. Solution – pseudocode 3 int strstr(char str1[], char str2[]) { int i; for (i=0; i<strlen(str1); i++) { if (occursAtI(str1, str2, i)==1) return i; } if we didn’t find str1 anywhere return -1;

  23. Solution – pseudocode 4 int strstr(char str1[], char str2[]) { int i; for (i=0; i<strlen(str1); i++) { if (occursAtI(str1, str2, i)==1) return i; } return -1;

  24. Solution – pseudocode 4 int occurs(char str1[], char str2[], int index) { run over str2 iffor every letter there’s a corresponding letter in str1, starting at index, return 1; otherwise return 0; }

  25. Solution – pseudocode 5 int occurs(char str1[], char str2[], int index) { run over str2 for each letter if there’s no corresponding letter in str1, starting at index return 0; if we found corresponding letters for ALL letters in str2 return 1; }

  26. Solution – pseudocode 6 int occurs(char str1[], char str2[], int index) { int i; for (i=0; i<strlen(str2); i++) { if there’s no corresponding letter in str1, starting at index return 0; } if we found corresponding letters for ALL letters in str2 return 1; }

  27. Solution – pseudocode 7 int occurs(char str1[], char str2[], int index) { int i; for (i=0; i<strlen(str2); i++) { if (str1[index+i] != str2[i]) return 0; } if we found corresponding letters for ALL letters in str2 return 1; }

  28. Solution – pseudocode 8 int occurs(char str1[], char str2[], int index) { int i; for (i=0; i<strlen(str2); i++) { if (str1[index+i] != str2[i]) return 0; } return 1; }

  29. Another exercise • Implement the following function: • Input – str1, str2, integer n • Output – 1 if str1 is at most n letters away from str2, 0 otherwise • isSimilar(“watar”, “woter”, 2) == 1 • isSimilar(“watar”, “woter”, 1) == 0 • If the strings are a different length return 0

  30. solution is_similar.c

  31. Could it be another one? :O • Implement the following function –int isInArray(int[] arr, int len, int val) • Should return 1 if arr contains val, 0 otherwise • Implement the following function –int differentNums(int[] arr, int len) • Should return how many distinct numbers are in arr – {1, 2, 4, 1, 5, 1} -> 4 • Write a program that takes in numbers from the user and returns how many distinct ones there were

  32. solution different_nums.c

  33. Last one • Implement the following function – • Input – two strings str1, str2 • Output – pointer to the first instance in str1 of any of the characters contained in • Hint - use strchr! • Write a program that accepts a string from the user and replaces all punctuation signs (,.;:!?) with spaces

  34. solution strcspn.c

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