从C中的文本文件读取整数

Question

I want to read a text file where image pixels values are stored in this fashion

234
23
176
235
107
187
201
128
147
...
....
..

I tried to read this text file in this manner

#include<stdio.h>
#define M 2500
int main()
{

unsigned  new_img[M];
unsigned char a;
FILE *input;
input=fopen("D:/trail.txt","r");
for(i=0;i<M;i++)
{
    a=fgetc(input);
    new_img[i]=(int)a; 
    input++;
}
....

....

...
return 0;
}

when I tried it to print it showing some random values. Also, all my pixels values are in the range of 0 to 255. But at the output screen I am getting values as large as 1 to 1 million

Answer 1

As per the man page of fgetc() , we can see

fgetc() reads the next character from stream and returns it as an unsigned char cast to an int, or EOF on end of file or error.

it reads character-by-character in lexicographical way, not based on the value .

Instead I suggest

• read a line using fgets()
• convert the string input to int using strtol()
• store it into the array.

Some other suggestions:

1. The recommended signature of main() is int main(void) .
2. Always check for the success of fopen() before using the returned pointer.
3. FWIW, fgetc() returns an int . You should collect the return value in an int variable, ideally.

_{Note: you need to take care of the trailing \\n read by fgets() and error checking for the validity of input.}

Answer 2

You are using fgetc() , which is not the right way to read integers, and also doing it wrong. Notice that its return type is int , since it can return the constant EOF . You're truncating the return value into an unsigned char , which makes no sense.

It reads characters , so for an input string of eg 234 , you would first see the character 2 , then 3 , then 4 , then the line feed character(s), and so on. Not what you want.

You should just use:

int pixel;

if(fscanf(input, "%d", &pixel) == 1)
{
  printf("read pixel %d\n", pixel);
}

This converts a decimal integer (which can consist of many individual characters), and writes the integer into pixel . If it fails, it won't return 1 .