Array manipulation in C - c

Question

I am like 3 weeks new at writing c code, so I am a newbie just trying some examples from a Harvard course video hosted online. I am trying to write some code that will encrypt a file based on the keyword.
The point is each letter of the alphabet will be assigned a numerical value from 0 to 25, so 'A' and 'a' will be 0, and likewise 'z' and 'Z' will be 25. If the keyword is 'abc' for example, I need to be able to convert it to its numerical form which is '012'. The approach I am trying to take (having learned nothing yet about many c functions) is to assign the alphabet list in an array. I think in the lecture he hinted at a multidimensional array but not sure how to implement that. The problem is, if the alphabet is stored as an array then the letters will be the actual values of the array and I'd need to know how to search an array based on the value, which I don't know how to do (so far I've just been returning values based on the index). I'd like some pseudo code help so I can figure this out. Thanks

Answer 1

In C, a char is an 8-bit integer, so, assuming your letters are in order, you can actually use the char value to get the index by using the first letter (a) as an offset:
char offset = 'a';
char value = 'b';
int index = value - offset; /* index = 1 */

Answer 2

This is hard to answer, not knowing what you've learned so far, but here's a hint to what I would do: the chars representing letters are bytes representing their ASCII values, and occur sequentially, from a to z and A to Z though they don't start at zero. You can cast them to ints and get the ascii values out.
Here's the pseudo code for how I'd write it:
Cast the character to a number
IF it's between the ascii values of A and Z, subtract it from A
ELSE Subtract it from the ASCII value of a or A
Output the result.
For what it's worth, I don't see an obvious solution to the problem that involves multidimensional arrays.

Answer 3

char '0' is the value 48
char 'A' is the value 65
char 'a' is the value 97
You said you want to learn how to search in the array:
char foo[26]; //your character array
...
...
//here is initialization of the array
for(int biz=0;biz<26;biz++)
{
foo[biz]=65+biz; // capital alphabet
}
...
...
//here is searching 1 by 1 iteration(low-yield)
char baz=67; //means we will find 'C'
for(int bar=0;bar<26;bar++)
{
if(foo[bar]==baz) {printf("we found C at the index: %i ",bar);break;}
}
//since this is a soted-array, you can use more-yield search algortihms.
Binary search algortihm(you may use on later chapters):
http://en.wikipedia.org/wiki/Binary_search_algorithm

Answer 4

The use of a multidimensional array is to store both the lower case and upper case alphabets in an array so that they can be mapped. An efficient way is using their ASCII code, but since you are a beginner, I guess this example will introduce you to handle for loops and multidimensional arrays, which I think is the plan of the instructor as well.
Let us first set up the array for the alphabets. We will have two rows with 26 alphabets in each row:
alphabetsEnglish[26][2] = {{'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'},
{'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'}};
Now we can map elements of both cases.
int main()
{
int c,i,j;
char word[10];
printf("Enter a word:");
scanf("%s",word);
c=strlen(word);
printf("Your word has %d letters ", c);
for (i = 0; i < c; i++) //loop for the length of your word
{
for (j = 0; j <= 25; j++) //second loop to go through your alphabet list
{
if (word[i] == alphabetsEnglish[0][j] || word[i] == alphabetsEnglish[1][j]) //check for both cases of your alphabet
{
printf("Your alphabet %c translates to %d: ", word[i], j);
}
}
}
return 0;
}

Answer 5

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int *conv(char* str){
static const char* table = "abcdefghijklmnopqrstuvwxyz";
int size, *ret, *p;
if(NULL==str || *str == '\0') return NULL;
size = strlen(str);
ret=p=(int*)malloc(size*sizeof(int));
while(*str){
char *pos;
pos=strchr(table, tolower(*str++));
*p++ = pos == NULL ? -1 : pos - table;
}
return ret;
}
int main(void){
char *word = "abc";
int i, size = strlen(word), *result;
result = conv(word);
for(i=0;i<size;++i){
printf("%d ", result[i]);//0 1 2
}
free(result);
return 0;
}