C: Reading multiple values with scanf and saving them into an array

Question

I found an answer to the first part of my question (how to read multiple values with scanf) but it doesn't seem to work for me (I think it's because of putting the values into an array and maybe also because I'm checking if the values given are 6 ints for sure):

I am writing a program that stores co-ordinates of 4 triangles in an array. Each line has 6 values and stores co-ordinates of one triangle. I want to read 6 co-ordinates at one time and do this operation for 4 triangles separately.

int tab[4][6];

for (int i = 0; i < 4; i++){
    while (scanf("%d %d %d %d %d %d", &tab[i][0], &tab[i][1], &tab[i][2], &tab[i][3], &tab[i][4], &tab[i][5]) != 6){
        printf("Error, try again: ");
        while (getchar() != '\n'){}
    }
}

So for example if first triangle's co-ordinates are (2,1), (5,6), (2,7), then I want to type in: "2 1 5 6 2 7" and as a result I want it to fill the first line of the array with the said numbers in the order I typed them in.

Obviously it doesn't work, the program stops working (not finishes the work, it stops) after the first line is given.

I get this error after debugging (after giving first line): "Unhandled exception at 0x0FDCC28C (msvcr120d.dll) in xxx.exe: 0xC0000005: Access violation writing location 0xCCCCCCCC."

How to fix it?

minimal reproducible example? This isn't really enough for one. — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Mar 4, 2018 at 22:16
Couldn't reproduce. Please post a Minimal, Complete, and Verifiable example. — MikeCAT
– MikeCAT, Commented Mar 4, 2018 at 22:16
Can you post the actual error message of the compiler? Code seems OK, perhaps you are not including stdio.h? — Pablo
– Pablo, Commented Mar 4, 2018 at 22:17

Anders Cedronius · Accepted Answer · 2018-03-04 23:38:43Z

4

You need to subtract the pointer i when detecting input error like this for example ->

#include <stdio.h>
int main(int argc, const char * argv[]) {
    int tab[4][6];
    for (int i = 0; i < 4; i++){
        printf("Enter 6 values \n");
        int retVal=scanf("%d %d %d %d %d %d", &tab[i][0], &tab[i][1], &tab[i][2], &tab[i][3], &tab[i][4], &tab[i][5]);
        if (retVal == 6) {
            printf("You did enter -> %d %d %d %d %d %d\n",tab[i][0],tab[i][1],tab[i][2],tab[i][3],tab[i][4],tab[i][5]);
        } else {
            printf("Error entering values.. (Enter numbers). \n");
            while (getchar() != '\n'){}
            i--;
        }
    }
    return 0;
}

edited Mar 4, 2018 at 23:38

answered Mar 4, 2018 at 22:45

Anders Cedronius

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2 Comments

Ania Over a year ago

Thank you very much, this is exactly what I've been looking for! But why does this work and my version doesn't? I have always been using similar algorithm to check for mistakes and it always worked.

chux Over a year ago

Infinite loop on end-of-file.

chux · Accepted Answer · 2018-03-04 23:20:15Z

-1

Unclear why OP's code failed without posting input used and prior code.

How to fix it?

Use fgets() to read a line of user input. Avoid mixing scanf() with fgets() in prior code. Then parse the buffer. Use " %n" at the end to look for success and extra text.

int tab[4][6];
char buf[6*12 * 2];  // Use a buffer twice expected max needs

for (int i = 0; i < 4; i++) {
  while (1) {
    if (fgets(buf, size  buf, stdin) == NULL) {
      return "Failed to read enough data"; // Handle end-of-file in some fashion
    }
    int n = 0;
    sscanf(buf, "%d%d%d%d%d%d %n", 
        &tab[i][0], &tab[i][1], &tab[i][2], &tab[i][3], &tab[i][4], &tab[i][5], &n);
    if (n > 0 && buf[n] == 0) {
      break;  // Success!
    }
    printf("Error - bad input, try again: ");
  }
}

edited Mar 4, 2018 at 23:20

answered Mar 4, 2018 at 23:07

chux

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