How to use strncpy with a for-loop in C? - c

Question

I am writing a program which will take every 3 numbers in a file and convert them to their ASCII symbol. So I thought I could read the numbers into a character array, and then make every 3 elements 1 element in a second array, convert them to int and then print these as char.
I am stuck on taking every 3 elements, however. This is my code snippet for this part:
char arry[] = "073102109109112"; <--example string read from a file
char arryNew[16] = {0};
for(int i = 0; i <= sizeof(arryNew); i++){
strncpy(arryNew, arry, 3);
arryNew[i+3]='\0';
puts(arryNew);
}
What this code gives me is the first 3 numbers, fifteen times. I've tried incrementing i by 3, which gives me the first 3 numbers 5 times. How do I write a for-loop with strncpy so that after copying n chars, it moves to the next n chars?

Answer 1

You pass always the pointer to the beginning of the array, so you will always have the same result of course. You must include the loop counter to get at the next block:
strncpy(arryNew, &arry[i*3], 3);
Here you have a problem:
arryNew[i+3]='\0';
First of all, you don't need to set the null byte every time, because this will not change anyway. Additionally you will corrupt memory, because you use i+3 as the index so when you reach 14 and 15, it will write beyond the arrayboundary.
Your arrayNew must be longer, because your original array is 16 characters, and your target array is also. If you intend to have several 3char strings in there, then you must have 5*4 characters for your target, because each string also has the 0-byte.
And of course, you must also use the index here as well. The way it is written now, it will write beyond the array boundary, when i reaches 14 and 15.
So what you seem to want to do (not sure from your description) is:
char arry[] = "073102109109112"; <--example string read from a file
char arryNew[20] = {0};
for(int i = 0; i <= sizeof(arry); i++)
{
strncpy(&arryNew[i*4], &arry[i*3], 3);
puts(&arryNew[i*4]);
}
Or if you just want to have the individual strings printed then you can just do:
char arry[] = "073102109109112"; <--example string read from a file
char arryNew[4] = {0};
for(int i = 0; i <= sizeof(arry); i++)
{
strncpy(arryNew, &arry[i*3], 3);
puts(arryNew);
}

Answer 2

Making things a bit simpler: your target string doesn't change.
char arry[] = "073102109109112"; <--example string read from a file
char target[4] = {0};
for(int i = 0; i < strlen(arry) - 3; i+=3)
{
strncpy(target, arry + i, 3);
puts(target);
}
Decoding:
start at the beginning of arry
copy 3 characters to target
(note the fourth element of target is \0)
print out the contents of target
increment i by 3
repeat until you fall off the end of the string.

Answer 3

Some problems.
// Need to change a 3 chars, as text, into an integer.
arryNew[i] = (char) strtol(buf, &endptr, 10);
// char arryNew[16] = {0};
// Overly large.
arryNew[6]
// for(int i = 0; i <= sizeof(arryNew); i++){
// Indexing too far. Should be `i <= (sizeof(arryNew) - 2)` or ...
for (i=0; i<arryNewLen; i++) {
// strncpy(arryNew, arry, 3);
// strncpy() can be used, but we know the length of source and destination,
// simpler to use memcpy()
// strncpy(buf, a, sizeof buf - 1);
memcpy(buf, arry, N);
// arryNew[i+3]='\0';
// Toward the loop's end, code is writing outside arryNew.
// Lets append the `\0` after the for() loop.
// int i
size_t i; // Better to use size_t (or ssize_t) for array index.
Suggestion:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main() {
char Source[] = "073102109109112"; // example string read from a file
const int TIW = 3; // textual integer width
// Avoid sprinkling bare constants about code. Define in 1 place instead.
const char *arry = Source;
size_t arryLen = strlen(arry);
if (arryLen%TIW != 0) return -1; // is it a strange sized arry?
size_t arryNewLen = arryLen/TIW;
char arryNew[arryNewLen + 1];
size_t i;
for (i=0; i<arryNewLen; i++) {
char buf[TIW + 1];
// strncpy(buf, a, sizeof buf - 1);
memcpy(buf, arry, TIW);
buf[TIW] = '\0';
char *endptr; // Useful should OP want to do error checking
// TBD: test if result is 0 to 255
arryNew[i] = (char) strtol(buf, &endptr, 10);
arry += TIW;
}
arryNew[i] = '\0';
puts(arryNew); // prints Ifmmp
return 0;
}

Answer 4

You could use this code to complete your task i.e. to convert the given char array in form of ascii value.
char arry[] = "073102109109112";
char arryNew[16] = {0};
int i,j=0;
for(i = 0; i <= sizeof(arryNew)-2; i+=3)
{
arryNew[j]=arry[i]*100+arry[i+1]*10+arry[i+2]*1;
j++;
arryNew[j+1]='\0';
puts(arryNew);
}