Iterating over a string input

Question

I am trying to iterate over a string to see if the input is a whole number.

This is the code:

 for (int i = 1; i < strlen(buffer); i++) //Checking that each character of the string is numeric
    {
        if (!isdigit(buffer[i]))
        {
            valid = false;
            break;
        }
    }

    if(valid == false)
    {
        printf("Invalid input!");
    }

    else
    {
        num = atoi(buffer);
        printf("The number entered is %d", num);
    }

The problem is that even if the input is correct, let us say 2, the output message is still "Invalid input!".

I am very frustrated right now. I have tried using the atoi function, the strtol function and other methods so that I can validate a number and ensure that:

1) it is not a letter 2) it is a whole number

Please help me. I have been trying to solve this problem for over 2 hours.

the isdigit is a method provided by the ctype.h library. I did not create it myself — Matthew
– Matthew, Commented Mar 5, 2013 at 18:07
This is obviously not too localized. The code snippet is obvious, the output is unexpected though. — Lion
– Lion, Commented Mar 5, 2013 at 18:11
Reading man pages helps. As well as learning how to use a debugger. — alk
– alk, Commented Mar 5, 2013 at 18:14

md5 · Accepted Answer · 2013-03-05 18:05:42Z

8

Since fgets reads a line, a '\n' (which is not a digit) may remain at the end of your string. You can remove it:

#include <string.h>

char *pend = strrchr (buffer, '\n');

if (pend != NULL) *pend = '\0';

answered Mar 5, 2013 at 18:05

md5

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5 Comments

Matthew Over a year ago

Thank you so much Kirilenko :))) It solved the problem completely. What is the code doing exactly. Is it replacing \n with \0?

Matthew Over a year ago

Thank you so much. So pend is a data pointer to the null-terminating character and if pend is not null (null-terminating character found), it replaces it with \0?

md5 Over a year ago

It replaces the last newline character of the string with a \0. If no newline character is found, dereferencing pend would lead to undefined behavior, so we avoid such case.

Matthew Over a year ago

Thanks again :) Sorry if I ask such questions but I am kind of new to C.

md5 Over a year ago

@Matthew: No need to be sorry. If the question is well-asked, it's a pleasure to answer. ;-)

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