array modification in C

Question

Not sure what i'm doing wrong here. I'm trying to circularly shift elements of a char array left by one. It seems to work, but not persist. I assume it's a pointer issue, but I still don't get how pointers work so i'm not sure where to begin. Problem is, it only seems to change the value inside the for loop and reset afterwards.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void ip();

int main(){
    char d[10] = "010";
    char k[10] = "1010000010";
    initialPermutation(d, k);
    return(0);
}

void ip(char * data, char* key){
    int i;

    // P10(k1, k2, k3, k4, k5, k6, k7, k8, k9, k10) = (k3, k5, k2, k7, k4, k10, k1, k9, k8, k6)
    char P10[10] = {key[2],key[4],key[1],key[6],key[3],key[9],key[0],key[8],key[7],key[6]};

    // split P10 into two segments
    char left5[5] = {P10[0],P10[1],P10[2],P10[3],P10[4]};
    char right5[5] = {P10[5],P10[6],P10[7],P10[8],P10[9]};

    // pre shift binary string
    printf("Pre-shift: ");
    for(i=0;i<5;i++)
        printf("%c",left5[i]);
    for(i=0;i<5;i++)
        printf("%c",right5[i]);
    printf("\n");

    // circular shift left one bit (here is where the problem is)
    for(i=0;i<5;i++){
        char templ = left5[4-i];    
        left5[4-i] = left5[i];   
        left5[i] = templ;

        char tempr = right5[4-i];
        right5[4-i] = right5[i];
        right5[i] = tempr;
     }



    printf("Post- (outside for loop): ");

    for(i=0;i<5;i++)
        printf("%c",left5[i]);
    for(i=0;i<5;i++)
        printf("%c",right5[i]);
    printf("\n");
}

"1010000010" has 11 characters if count the null terminator so the string is not null terminated in initialization of char k[10] — ouah
– ouah, Commented Feb 9, 2014 at 22:14

Ola · Accepted Answer · 2014-02-09 22:27:52Z

1

Your loop is not shifting values, it is reversing the array twice. It swaps index 0 with index 4, then 1 with 3, then 2 with 2, then 3 with 1, and finally 4 with 0. At this point the array is exactly as when you started.

This code does an actual rotary left shift:

char tmp = left5[0];
for(i = 1; i < sizeof(left5); ++i){
    left5[i-1] = left5[i]
}
left5[4] = tmp;

If you actually declare the arrays one element too large you can write:

char left5[6] = {P10[0],P10[1],P10[2],P10[3],P10[4]};
left5[5] = left5[0]
for(i=0; i < 5; ++i){
    left5[i] = left5[i+1];
}

answered Feb 9, 2014 at 22:27

Ola

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1 Comment

cHam Over a year ago

That was the problem. Thanks

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