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pset3/find, i need some help

edX CS50x Intro to CS I
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``` /** * helpers.c * * Computer Science 50 * Problem Set 3 * * Helper functions for Problem Set 3. * * search 1. Linear search 2. Binary search * * sort 1. Selection sort */ #include // computer science 50 library #include // standard input-output library #include // mathematics library #include "helpers.h" // helpers header file /** * Returns true if value is in array of n values, else false. */ bool search(int value, int values[], int n) { // Searching algorithm /** 1. Linear Search, O(n) **/ /* // Compare each element in values with value for (int i = 0; i < n; i++) { if (values[i]== value) return true; // value found } return false; // value not found } */ // sort the values smallest to largest sort(values, n); while (n > 0) { /** 2. Binary Search, O(lb n) **/ printf("\nvalue = %d\t\tvalues = [", value); // value for (int i = 0; i < n; i++) // values[] printf("%d, ", values[i]); printf("\b\b]\n"); // printf("n = %d, ", n); // n int mip = n/2; int mid = values[mip]; // printf("mip = %d, ", mip); // mip printf("middle = %d" , mid); // mid int new_values[mip]; if (value == mid) return true; // value found else if (value < mid) { printf("\tValue is less than middle\n"); for (int i = 0; i < mip+1; i++) // new values[] new_values[i] = values[i]; search(value, new_values, mip); } else if (value > mid) { printf("\tValue is greater than middle\n"); for (int i = 0; i < mip; i++) // new values[] new_values[i] = values[mip+i+1]; search(value, new_values, mip); } return false; }; return true; } /** * Sorts array of n values. */ void sort(int values[], int n) { // Sorting algorithm /** 1. Selection sort, O(n^2) **/ for (int i = 0; i < n; i++) { int smallest = values[i]; // set smallest value int smallest_location = i; // set smallest location for (int j = i + 1; j < n; j++) // compare with other values { if (smallest > values[j]) // if there is a smaller number { smallest = values[j]; // change value smallest_location = j; // change location } } // put the beginning of the list where the smallest number was values[smallest_location] = values[i]; // swap smallest location // place it in the begining of the list values[i] = smallest; // swap smallest value // display /* printf("%d ", values[i]); // print values [hidden] */ } return; } ```
i have some serious bugs in the binary search algorithm ``` jharvard@appliance (~/Dropbox/pset3/find): ./generate 9 2 | ./find 19 haystack[0] = haystack[1] = haystack[2] = haystack[3] = haystack[4] = haystack[5] = haystack[6] = haystack[7] = haystack[8] = haystack[9] = value = 19 values = [1, 4, 5, 8, 10, 15, 17, 18, 19] middle = 10 Value is greater than middle value = 19 values = [15, 17, 18, 19] middle = 18 Value is greater than middle value = 19 values = [19, 1204366368] middle = 1204366368 Value is less than middle value = 19 values = [19] middle = 19 Didn't find needle in haystack. ```
``` jharvard@appliance (~/Dropbox/pset3/find): ./generate 9 2 | ./find 1 haystack[0] = haystack[1] = haystack[2] = haystack[3] = haystack[4] = haystack[5] = haystack[6] = haystack[7] = haystack[8] = haystack[9] = value = 1 values = [1, 4, 5, 8, 10, 15, 17, 18, 19] middle = 10 Value is less than middle value = 1 values = [1, 4, 5, 8] middle = 5 Value is less than middle value = 1 values = [1, 4] middle = 4 Value is less than middle value = 1 values = [1] middle = 1 Didn't find needle in haystack. ``` with this input it almost works, but i still can't get the right `return` value

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Other answers:

Consider a simple array where indices are the values: 0 1 2 3 4 5 We start with a length of 6. Half that to get 3. 0 1 2 <3> 4 5 If it is equal, we can return 3. If it is less, we simply change our length to 3 new values: 0 1 2 If is it greater, we shift out array pointer over by 3+1 = 4 new values: 4 5 _ _ _ _ We change our length to 6 - (3-1) = 2 4 5 Basic algorithm ``` bool search(int value, int values[], int n) { if (n < 1) { return false; } int mid = n/2; if (value < values[mid]) { return search(value, values, mid); } else if (value > values[mid]) { return search(value, values - (mid+1), n - (mid+1)); } return true; } ```
@wio you mean `We change our length to 6 - (3+1) = 2` right? also, doesn't minusing `(mid+1)` from `values` in the `else if` statement, only work because of the choice of array? I can't figure out how to run your code for some reason
Actually it should add it to values, not subtract it.
adding will return an array but it shifts what would be considered the 0 index by `(mid+1)`.
so `values+(mid+1)`
Thanks wio, i have managed to get my script to work now using the help of you script. ``` jharvard@appliance (~/Dropbox/pset3/find): ./generate 32 0 | ./find 52711 haystack[0] = haystack[1] = haystack[2] = haystack[3] = haystack[4] = haystack[5] = haystack[6] = haystack[7] = haystack[8] = haystack[9] = haystack[10] = haystack[11] = haystack[12] = haystack[13] = haystack[14] = haystack[15] = haystack[16] = haystack[17] = haystack[18] = haystack[19] = haystack[20] = haystack[21] = haystack[22] = haystack[23] = haystack[24] = haystack[25] = haystack[26] = haystack[27] = haystack[28] = haystack[29] = haystack[30] = haystack[31] = haystack[32] = value = 52711 values = [124, 1946, 2132, 3958, 7931, 7977, 8987, 9158, 9562, 10232, 16882, 17293, 17767, 18547, 22714, 22764, 23807, 25282, 29283, 31949, 37962, 39017, 43491, 49715, 50377, 52711, 55199, 55211, 56401, 57670, 59880, 63790] middle = 23807 Value is greater than middle value = 52711 values = [0, 25282, 29283, 31949, 37962, 39017, 43491, 49715, 50377, 52711, 55199, 55211, 56401, 57670, 59880, 63790] middle = 50377 Value is greater than middle value = 52711 values = [52711, 55199, 55211, 56401, 57670, 59880, 63790, 1204366368] middle = 57670 Value is less than middle value = 52711 values = [52711, 55199, 55211, 56401] middle = 55211 Value is less than middle value = 52711 values = [52711, 55199] middle = 55199 Value is less than middle value = 52711 values = [52711] middle = 52711 Value is equal to middle! Found needle in haystack! ```

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