如何將字符串劃分爲子字符串並分配另一個子字符串？

Question

我想把* eString分成子串。子應該是這樣的：

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}... 

其中y是爲EString的*元素，並y是這些元素的子串。例如，如果用戶期望密鑰長度爲5，則應該有（字符串大小/ 5）子字符串。 y_{1}必須包含每個劃分子字符串的第一個元素。那麼，我該如何實現呢？

#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; 
}