Common mistakes with input and output
Comments and questions to John Rowe.
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| Wrong
| Right
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printf("Number of atoms?\n");
scanf("%d\n", &j);
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printf("Number of atoms?\n");
scanf("%d", &j);
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| Since scanf() is an intelligent function: it
skips over white space such as spaces and new-lines unless we
actively stop it. Such as the next mistake...
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For example, putting a comma after "%d" when reading two integers:
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| Wrong
| Right
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printf("Numbers required?\n");
scanf("%d, %d", &j, &k);
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printf("Numbers required?\n");
scanf("%d %d", &j, &k);
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| Note, this isn't always wrong
(for example we may be reading from a file with a fixed
data format), but we should avoid it if possible.
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| Wrong
| Right
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scanf("%d", &j);
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printf("Number of atoms?\n");
scanf("%d", &j);
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| Again, this isn't always wrong but in general it's
much better to tell the user what they are supposed to be entering.
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For variety we show a situation where we choose to warn of
the failure to open a file rather than exit the program:
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| Wrong
| Right
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outfile = fopen("helloworld.txt", "w");
fprintf(outfile, "Hello, world\n");
fclose(outfile);
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outfile = fopen("helloworld.txt", "w");
if ( outfile != NULL ) {
fprintf(outfile, "Hello, world\n");
fclose(outfile);
}
else {
fprintf(stderr, "Could not open helloworld.txt\n");
}
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You only need to worry about these if you have not
followed our advice...
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Forgetting the ampersand
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| Wrong
| Right
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printf("Please enter the mass\n");
scanf("%lg", mass);
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printf("Please enter the mass\n");
scanf("%lg", &mass);
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Not using %lg to read a double
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| Wrong
| Right
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printf("Please enter the mass\n");
scanf("%g", &mass);
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printf("Please enter the mass\n");
scanf("%lg", &mass);
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