Probably wrong use of memory-Segmentation fault

Question

When the 1st value I've assigned to the integer "number" gets assigned to the R array, my program crashes. It probably has something to do with the pointer I've created and I do not seem to be able to find what is the problem. If I omit this part of the code:

else{ while (k<=n/2){ temp2=R[k]; R[l-1]=temp2; R[k]=R[l-1];} l-=1; k+=1; }

, the program runs fine, even though it gives a different output. Below is the code:

#include <stdio.h> main(){ int i,n,number,temp,j,k=0,l,temp2; int *R; R=(int *)malloc(n*sizeof(int)); printf("Dwse mou to megethos tou pinaka se parakalw poly:\n"); scanf("%d", &n); for(i=0; i<n; i++){ printf("Dwse enan thetiko akeraio arithmo:\n"); scanf("%d", &number); R[i]=number; } for(i=1;i<n;i++){ temp=R[i]; j=i-1; while((temp>R[j])&&(j>=0)){ R[j+1]=R[j]; j=j-1; } R[j+1]=temp; } l=n; if (n%2==0) printf("Einai adynato na ginei h taksinomhsh:\n"); else{ while (k<=n/2){ temp2=R[k]; R[l-1]=temp2; R[k]=R[l-1];} l-=1; k+=1; } for(i=0; i<n; i++){ printf("R[%d]: %d\n", i, R[i]); } }

Thank you.

R=(int *)malloc(n*sizeof(int)); move to after scanf("%d", &n);. — BLUEPIXY
– BLUEPIXY, Commented Dec 30, 2016 at 18:06
you are allocating the array R using the variable n which is not initialized — younes zeboudj
– younes zeboudj, Commented Dec 30, 2016 at 18:07
Well, thank you very much and pardon me for making such kind of questions due to my noobishness. — Achilles
– Achilles, Commented Dec 30, 2016 at 18:11

Jens · Accepted Answer · 2016-12-30 18:07:30Z

2

int i,n,number,temp,j,k=0,l,temp2; int *R; R=(int *)malloc(n*sizeof(int));

What value does n have? It's uninitialized, so you invoke undefined behavior in C lingo. Anything may happen, and a crash is one thing.

answered Dec 30, 2016 at 18:07

Jens

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2 Comments

Achilles Over a year ago

Thank you! That was the problem indeed.

Jens Over a year ago

@Achilles Wonderful. The next step on Stack Overflow is to accept the answer that helped you most by clicking the green check mark to the left. This will earn you a few points of credit.

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