Issues with atoi() when converting numbers in a string to an integer

Question

I have been running into this funny issue with using atoi() in C for an assignment. Im gonna paste a bit of the code I have so far...

#include <stdlib.h> #include <stdio.h> #include <string.h> #include <ctype.h> #define _CRT_SECURE_NO_WARNINGS char RimDiameter[15]; char TireCode[15]; int x; puts("Enter the code on the Tire"); // P215/65R15 95H gets(TireCode); strncpy(RimDiameter, TireCode+8, 2); // TireCode = P215/65R15 95H, RimDiameter = 15 int x = atoi(RimDiameter); // where I'm having issues if(x >=14 && x <=22) { printf("%c is valid\n", x); } else { printf("%c is invalid\n", x); }

I'm using strncpy() to help grab the numbers I need in the string and copy them to RimDiameter. I should have '15' stored in RimDiameter but once I use atoi(RimDiameter) I get 0 stored in x instead of '15' which is needed for my if/else statements to work properly.

Anyone have any idea why I would be getting this issue?

I've added code block formatting around your code; can you please do this yourself in the future? Then the double-spacing is not required. Also, proper indentation would help a lot. — Nate Eldredge
– Nate Eldredge, Commented May 1, 2021 at 17:39
Also, you should carefully read the documentation of strncpy; its behavior is not what most people expect. See for instance stackoverflow.com/questions/1453876/… — Nate Eldredge
– Nate Eldredge, Commented May 1, 2021 at 17:43
And %c is the wrong format specifier to print a decimal integer; you probably want %d. — Nate Eldredge
– Nate Eldredge, Commented May 1, 2021 at 17:44
strncpy() does not automatically terminate the string: do it yourself...strncpy(RimDiameter, TireCode+8, 2); RimDiameter[2] = 0; — pmg
– pmg, Commented May 1, 2021 at 17:44

Chris Dodd · Accepted Answer · 2021-05-01 18:42:19Z

This is a place where sscanf is really useful:

int p, r, d, h; char TireCode[256]; fputs("Enter the code on the tire: ", stdout); if (!fgets(TireCode, sizeof(TireCode), stdin)) { fprintf(stderr, "Unexpected EOF"); } else if (sscanf(TireCode(" P%d /%dR%d%d", &p, &r, &d, &h) == 4) { if (d >= 14 && d <= 22) { printf("%d is valid\n"); } else { printf("%d is invalid\n"); } } else { fprintf(stderr, "Input is not in the expected format"); }

You can try multiple different patterns of letters/numbers to see which one matches with an if...else if...else if... chain

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