How to divide a string to substring and assign another substring? - c

Question

I want to divide *eString to substrings. Substrings should be like that:
y_{1} = y_{1}y_{m+1}y_{2m+1}...
y_{2} = y_{2}y_{m+2}y_{2m+2}...
y_{m} = y_{m}y_{2m}y_{3m}...
where y is the element of *eString, and y is the substring of these elements.
For instance, if an user expects the key length which is 5, there should be (string size / 5) substrings. y_{1} has to contain the fist element of each divided substring. So, how can I implement this?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define ALPHA 26
char *ReadFile(char *);
int main(int argc, char *argv[])
{
double frequency[ALPHA] = {0};
int c = 0;
int keylen = 0;
int counter = 0;
double indexofCoincidence = 0,total = 0;
const char *eString = ReadFile("cipher.txt");
int len = 0;
if (eString) {
puts("The encrypted text is:");
puts(eString);
puts("");
len = strlen(eString);
printf("The length of text is %d\n",len);
}
puts("");
while(eString[c]!= '\0'){
if(eString[c]>= 'a' && eString[c]<='z')
frequency[eString[c]-'a']++;
c++;
}
puts("The letters frequencies are :\n");
for(c=0; c<ALPHA;c++){
if(frequency[c]!= 0)
printf("%c : %.3f\t",c+'a',(frequency[c]/len));
total += (frequency[c]*(frequency[c]-1));
}
indexofCoincidence = (total/((len)*(len-1)));
printf("\n\nIndex of Coincidence : %.3f\n",indexofCoincidence);
if(indexofCoincidence < 0.060){
printf("\nIt looks like randomly.\n");
}
printf("Enter the your expected key length : ");
scanf("%d",keylen);
printf("\n");
char *y;
while(counter != keylen)
{
for(int i = 0; i<(len/keylen);i++){
y[counter] = *eString();
}
counter++
}
return EXIT_SUCCESS;
}

Answer 1

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void){
char *eString = "The quick brown fox jumps over the lazy dog";
int keylen = 5;
int len = strlen(eString);
int y_len = (len + keylen) / keylen + 1;
int i,j;
char **y = malloc(keylen * sizeof(*y));
for(i=0; i < keylen; ++i){
y[i] = malloc(y_len * sizeof(**y));
}
char *p = eString;
i = j = 0;
while(*p){
y[i % keylen][j] = *p++;
y[i % keylen][j+1] = 0;
if(++i % keylen == 0)
++j;
}
//check print & deallocate
for(i = 0; i < keylen; ++i){
printf("y_{%d} : %s\n", i+1, y[i]);
free(y[i]);
}
free(y);
return 0;
}